DF Flashcards
Define “thermochemistry”
The study of energy and heat associated with chemical reactions
Define “exothermic”
A reaction that gives out energy and heats surroundings
What are the main features of an exothermic reaction?
What does an enthalpy profile for one look like?
Bond making
ΔH negative - products end up with less energy than reactants
Gives out energy to surroundings
Heats up surroundings
Reactants above products, AE arrow up, ΔH arrow down
Define “endothermic”
A reaction that takes in energy and cools the surroundings
What are the main features of an endothermic reaction?
What does an enthalpy profile for one look like?
Bond breaking
ΔH Positive - products end up with more energy than reactants
Takes in energy from surroundings
Cools surroundings
Reactants below products, both arrows up
Define “enthalpy change”
The energy transferred to/from the surroundings when the reaction is carried out in an open container
Why are standard conditions used when calculating enthalpy change?
They are set conditions which allows us to compare enthalpy changes
What are standard conditions?
Pressure: 1 atmoshpere (101kPa/1.01Nm-2)
Temp: 298K
Concs: 1.00moldm-3
State: Whatever state at 298K
How do you convert K to degrees C?
Minus 273
What are the 4 types of enthalpy change?
- Standard enthalpy change of a reaction
- Standard enthalpy change of formation
- Standard enthalpy change of combustion
- Standard enthalpy change of neutralisation
Define “standard enthalpy change of a reaction”
Enthalpy change when one mole of matter is transformed by a chemical reaction under standard conditions
Define “standard enthalpy change of a formation”
Enthalpy change when 1 mol of a compound is formed from its elements in their standard states, under standard conditions
Define “standard enthalpy change of a combustion”
Enthalpy change when 1 mol of a substance is completely burned in oxygen, under standard conditions
Define “standard enthalpy change of a neutralisation”
Enthalpy change when acid reacts with alkali to form 1 mol of water under standard conditions
Are all combustion reactions positive/negative and why?
Negative because they are all exothermic
What formula is used to measure enthalpy change?
E = mcΔT
E = energy transfered (KJmol-1)
m = mass (g)
c = specific heat capacity (Jg-1K-1)
ΔT = temp. change (K)
What is the specific heat capacity of water?
4.18Jg-1k-1
How does a bomb calorimeter work?
- Fuel is ignited electrically and burns oxygen inside the pressurised vessel
- Energy is transferred to the surrounding water
What is measured in a bomb calorimeter?
Temperature rise
How is the experiment conducted in a bomb calorimeter?
At a constant volume in a closed container
Why is the fuel in a bomb calorimeter burnt under pressure?
To increase the number of successful collisions leading to complete combustion of the sample
Why is a bomb calorimeter more accurate than a simple calorimeter?
Low heat losses
What is Hess’s Law?
ΔH for any reaction will be the same if the start + end conditions are the same
What is the formula for finding the enthalpy change of reaction from enthalpy changes of combustion?
Change in H1 = Change in H2 - Change in H3
What is the formula for finding the enthalpy change of reaction from enthalpy changes of formation?
Enthalpy change of reaction = sum of enthalpy changes of formation of products - sum of enthalpy changes of formation of reactants
Which ways do the arrows point in an enthalpy cycle for combustion?
Down
What is placed at the bottom in an enthalpy cycle for combustion?
Combustion products - CO2 + H2O usually
Which ways do the arrows point in an enthalpy cycle for formation?
Up
What is placed at the bottom in an enthalpy cycle for formation?
The elements that formed the compound reactants
Why is the amount of oxygen used in combustion not critical?
It is normally used in excess
Why are the enthalpy changes for elements always 0?
They have already been formed and do not change their amount so enthalpy change is 0
What is crude oil?
A thick black liquid consisting of a mixture of hydrocarbons
Explain the process of fractional distillation
- At the refinery, the crude oil is heated to vapourise it
- vapour passes into the distillation column
- Vapour rises up through the column, separating into different fractions
- as different ones condense into liquids and run off at different levels
Does density increase/decrease up a fractionating column?
Decrease
Does boiling point increase/decrease up a fractionating column?
Decrease
Does chain length increase/decrease up a fractionating column?
Decrease
Does volatility increase/decrease up a fractionating column?
Increase
Does flammability increase/decrease up a fractionating column?
Increase
Why does each fraction not have an exact boiling point?
Each one is a mixture of many different hydrocarbons
Why is organic chemistry named so?
Many carbon compounds are found in living organisms
What does organic chemistry include?
All carbon compounds except CO, CO2 and carbonates
How many electrons does carbon have in its outershell?
4
Why does carbon generally form covalent bonds?
If it achieved stability by becoming an ion it would be too highly charged (4+ or 4-)
What is catenation?
Carbon’s ability to form strong covalent bonds with itself to give chains and rings of itself
What does catenation lead to?
The limitless variety of organic compounds
Define “hydrocarbon”
Compounds containing only carbon or hydrogen
What is the general formula for hydrocarbons?
CxHy
Define “aromatic”
Compounds containing one/more benzene rings
Define “aliphatic”
Compounds not containing any benzene rings
Define “functional group”
Modifiers responsible for the characteristic chemical reactions of molecules
Define “saturated”
A compound with no bonds available for bonding
Define “unsaturated”
A compound containing bonds that can be broken open
Define “homologous series”
A group of compounds with the same functional group
Are alkanes aliphatic?
Yes
What is the general formula for alkanes?
CnH2n+2
Name the first ten alkanes
- Methane
- Ethane
- Propane
- Butane
- Pentane
- Hexane
- Heptane
- Octane
- Nonane
- Decane
What is the relationship between alkane chain length and tendency to become a solid and why?
As chain length increases, the tendency towards a solid also increases because the boiling point increases
What state are alkanes n= 1 - 4 in?
Colourless gases
What state are alkanes n=5 - 16 in?
Colourless liquids
What state are alkanes n=17+ in?
White waxy solids
Do alkanes mix well with water?
No they form two separate layers
Why do alkanes not mix well with water?
Water contains polar molecules that attract each other and prevent alkane molecules mixing with them
What is a cycloalkane?
A saturated hydrocarbon where the carbon atoms are joined in a ring
What is the general formula for alkanes?
CnH2n+2
Are cycloalkanes saturated?
Yes
Are alkenes saturated and why?
No - the carbon double bond can open up and bond with more hydrogens
What is the general formula for alkenes?
CnH2n
Are alkenes aliphatic?
Yes
What is the general formula for cycloalkenes?
CnH2n-2
Are cycloalkenes aliphatic?
Yes
Are cycloalkenes saturated?
No
What is the formula of benzene?
C6H6
How many carbons and double bonds does a benzene ring contain?
6 carbons and 3 double bonds
Why are benzene rings more stable (less reactive) than expected?
The double bond electrons are delocalised around the carbon ring, unattached to any particular carbon
Is benzene aliphatic?
No - aromatic
Is benzene saturated?
No
Is alcohol a hydrocarbon?
No
What is the general formula for alcohols?
CnH2n+1OH
What is the functional group for alcohols?
The hydroxl group (-OH) which can be attached to any carbon atom on the chain
What is a general formula?
A formula that can describe any member of a family of compounds
What is a molecular formula?
The actual number of atoms of each element in a molecule
What is the shortened structural formula?
A formula showing the atoms, carbon by carbon with the attached hydrogens and functional groups
What is the structural formula?
Shows how all the atoms are arranged and all the bonds between them
What is the skeletal formula?
Shows the bond between the carbon skeleton only, with any functional groups. The hydrogen and carbon atoms are not shown.
Why is giving systematic names important?
It allows for the full structural formula of a compound to be determined.
What is the formula of methyl?
CH3
What is the formula for all alkyls after methyl?
CH3 + CH2 for every carbon atom
What is the prefix for two identical sidechains?
Di
What is the prefix for three identical sidechains?
Tri
What is the prefix for four identical sidechains?
Tetra
Define “energy density”
The amount of energy obtained per kg of fuel when burnt
What is the equation for energy density?
(1000/relative molecular mass) x standard enthalpy change
Define “bond enthalpy”
The energy needed to break 1 mol of a bond in the gaseous state
Define “average bond enthalpy”
The energy needed to break 1 mol of a bond gaseous state, averaged over many different compounds
Is bond forming exothermic/endothermic?
endothermic
Is bond making exothermic/endothermic?
exothermic
What is the relationship between bond strength and bond enthalpy and why?
The stronger a bond, the more energy needed to break it so the higher its bond enthalpy
Explain the forces which determine a bonds length in covalent molecules
- The positive nucleui are attracted to the shared electrons so the atoms move together
- The two positive nucleui also repel each other, these forces become greater until the atoms stop moving together
- The distance between the 2 nucleui is the distance where the attractive and repulsive forces balance each other out : equilibrium bond length
What is the relationship between bond length and bond enthalpy and why?
The shorter the bond length, the higher the bond enthalpy as there is a stronger attraction between the two atoms in the covalent bond.
Order these types of bonds in order of increasing bond enthalpy:
- Double bond
- Single Bond
- Triple Bond
- Triple bond
- Double bond
- Single Bond
Why do triple bonds have a higher bond enthalpy than double and single bonds?
- More electrons are shared with more bonds so electron density between the two atoms is greater
- Therefore the positive nucleui are more attracted to the electrons and so the atoms move closer together
What is the formula for bond enthalpy change?
Bond enthalpy change = total amount of bonds broken + total amount of bonds made
What do all reactions initially need and why?
Energy to stretch and break bonds as some must be broken before product molecules can begin to form
Why do some reactions such as neutralisation need barely be heated before they start?
They need only a little energy with enough available in their surroundings of room temperature
Why do you not need to break all bonds before a reaction occurs?
Once a few bonds have been broken, new bonds start to form and this gives out enough energy to keep reactions going
Why do some reactions need continuous heating?
They are only slightly exothermic
Why is there some variation between experimental and theoretical bond enthalpies?
- The bond enthalpy value is not actual standard value, e.g as under standard conditions water is a liquid but when calculating bond enthalpies you must work in a gaseous state
- Bond enthalpies in theory are given out as averages of several different bonds from different compounds
Why are bond enthalpies useful?
They enable enthalpy changes to be measured when there is little specific data for the compound
Why is it difficult to measure bond enthalpy?
- There is often more than one type of bond in a compound
- Difficult to make measurements when everything is in the gaseous state.
Hence an average taken across several different compounds
How are bond enthalpies measured?
Indirectly using enthalpy cycles
Explain two problems with fractions from fractional distillation
- “Straight run” -fractions from primary distillation makes for poor petrol and need to be treated further
- Supply and demand - crude oil contains a surplus of the high boiling fractions e.g gas oil and not enough of the lower boiling fractions e.g gasoline for the demand
How does cracking solve the problem for the demand of petrol?
- Alkanes with larger molecules too large for use in petrol are broken down into alkanes with shorter chains that can be used
- Alkanes are also broken down into cycloalkanes, arenes and alkenes which are blended to produce high grade petrol
Name 4 types of cracking reactions
- alkanes —-> branched alkanes + branched alkenes
- alkanes —-> smaller alkenes + cycloalkanes
- cycloalkanes —-> alkenes + branched alkenes
- alkenes —–> smaller alkenes
Are the products of cracking predictable?
No, they are random, with the same molecule giving different cracking products
Without a catalyst, what would the cracking process need?
It would require extremely high temperatures and pressures and would take far longer, which would be more expensive
Give details of an experimental procedure to crack a hydrocarbon vapour over a heated catalyst (in the lab).
- If using a liquid alkane mixture: Place 2 cm depth of mineral wool into a boiling tube, then use a dropper pipette to add approximately 1 cm3 of liquid alkane mix. Rotate the tube to ensure that the liquid alkane mixture soaks into the mineral wool.
- Set up the apparatus as shown in the picture
- Use a spatula to place some of the catalyst into the middle of the reaction tube. Keep it separate from the alkane mixture + mineral wool. Make sure there is space above the catalyst for gases to pass freely over it.
- Heat the catalyst strongly with a Bunsen flame. Once it’s hot, gently warm the alkane mixture to produce alkane vapour.
- Move the Bunsen burner backwards and forwards between the catalyst and the alkane mixture until you have collected several tubes of gas.
Stopper the tubes for testing later. The first tube can be discarded because it will contain mainly displaced air from the apparatus - Do not allow water to suck back from the trough
- When finished, lift the delivery tube out of the water + leave the apparatus to cool before dismantling it in a fume cupboard
What temperature does catalytic cracking take place at?
450 - 500 degrees
What catalyst is used normally in catalytic cracking?
Zeolite
Where does cracking take place?
In a riser reactor: a 60m high vertical tube around 2m in diameter
What is the mixture in the riser reactor like?
A moving fluidised bed where solid particles flow like a liquid
Explain the process of catalyctic cracking in detail
1) The hot vapourised hydrocarbons and the zeolite catalyst are fed into the bottom of the tube and forced upwards by the stream
2) The mixture takes 2 seconds to flow from the bottom to the top of the tube
3) After the riser reactor, the mixture passes into a separator where the steam carries away cracking products leaving behind the solid catalyst
4) The catalyst goes into the regenerator where it takes 10 minutes for coke to burn off in the hot air blown into the regenerator
5) The catalyst is then re-introduced into the base of the riser reactor, ready to repeat the cycle
What does the mixture only taking 2 seconds to flow from the bottom to the top of the tube mean?
The hydrocarbons are in contact with the catalyst for only a very short period of time
Why does the coke need to be burnt off in the regenerator?
Coke forms on the catalyst surface during cracking making it eventually go inactive and unable to catalyse reactions
What does energy released from burning the coke do?
Heats up the catalyst, this energy is then transferred to the feedstock so that the cracking can occur without additional heating
Define “catalyst”
Substance which speeds up the rate of chemical reaction by providing an alternate reaction pathway with a lower EA
Is not chemically changed/used up at the end of the reaction
(Although may form intermediates or be poisoned)
Define “catalysis”
The process of speeding up a chemical reaction using a catalyst
Do catalysts undergo any permanent chemical change during a reaction?
No
Do catalysts change physically?
Yes, they crumble/become roughened
What does catalysts changing physically suggest?
That they take part somewhat in the reaction, although they can be regenerated
How much of a catalyst is usually required?
Only small amounts
What do catalysts affect in the reaction?
Its rate, not the amount of product formed
Do catalysts appear as reactants in the overall equation?
No
Define “homogeneous catalyst”
A catalyst which is in the same physical state as the reactants
Give one example of homogeneous catalysis
Enzyme catalysed reactions in cells take part in an aqueous solution
Define “heterogeneous catalyst”
A catalyst which is in a different physical state to the reactants
What does heterogeneous catalysis usually involve?
A mixture of gases/liquids in the presence of a solid catalyst, providing a surface for the reaction to take part on
What shape does a single carbon bond have?
Tetrahedral
What is the angle between two carbon atoms in a single covalent bond?
109.5 degrees
What shape does a carbon double bond have?
Trigonal Planar
What are the bond angles around the C=C bond?
Why?
All bond angles around C=C bond 120º
Because there are 3 groups of e- around each C atom - 2 single bonds + 1 double bond
These groups repel each other as far as possible
Do all the carbon atoms in an alkene compound lie in a planar shape?
No only those in the double bond e.g ethene is completely planar but in propene only the C=C unit is
How is a sigma bond formed?
When two s orbitals overlap in the space between two atoms
Why are sigma bonds very strong?
There is the highest possible electron density between the two positive nucleui as the two s orbitals overlap
How is a pi bond formed?
When two p orbitals overlap sideways
What does a pi bond consist of?
2 areas of negative charge, one above and one below the line of atoms
Why is a double bond not twice as strong as a single bond?
Because pi bonds are weaker than sigma bonds
Define an “electrophile”
A positive ion or molecule with a partial positive charge on one of the atoms
This causes it to be attracted to a negatively charged area
It will react by accepting a lone-pair to form a dative covalent bond
Define an “addition reaction”
Two or more molecules reacting to form a larger single molecule
Why does the C=C have a highly negative region?
The four electrons in the double bond of ethene give the region between the two atoms a higher than normal negative charge density
What can be used to test for unsaturation?
Shaking the compound with bromine water
Bromine - is decolorised if unsaturated.
Goes from orange to colourless
Write a formula for the electrophilic addition of bromine to an alkene
H2C=CH2 + Br2 ====> CH2BrCH2Br
Describe the first step of electrophilic addition with bromine (polarisation)
The bromine molecule moves closer to the negative C=C bond which repels electrons to the bottom of the Br-Br bond, polarising the molecule and creating a postively charged (an electrophile) Br atom
Describe the second step of electrophilic addition with bromine (carbocation)
An electron pair from the C=C bond attacks the positive bromine atom, forming a dative covalent bond, the carbon atom the electron pair came from becomes a carbocation
Describe the third step of electrophilic addition with bromine (new covalent bond)
A pair of electrons moves from the now negative bromide ion to the carbocation to form a new carbon-bromine covalent bond
What product is formed from the electrophilic addition of ethene with bromine?
1,2-dibromoethane
How does ethene react with hydrogen bromide solution and what is formed?
Readily at a room temperature in a polar solvent to form bromoethane
What occurs if HBr is added to an unsymmetrical alkene?
Two products are formed e.g with propene: 2-bromopropane and 1-bromopropane is formed
How do alkenes create alcohols?
By hydrating them in the presence of an acid catalyst
Give the overall equation for the hydrolysis reaction of an alkene with water
H2C=CH2 + H20 ===H2SO4===> C2H5OH
Give the steps of the hydrolysis reaction of alkene with water and their formulas
1) Cold concentrated sulfuric acid reacts with an alkene in an electrophilic addition reaction
H2C=CH2 + H2SO4 ====> CH3CH2OSO2OH
(ethene) + (sulfuric acid) ====> (ethyl hydrogen sulfate)
2) Cold water is added and the product is warmed, it is hydrolysed to form an alcohol
CH3CH2OSO2OH + H2O ====> CH3CH2OH + H2SO4
(ethanol)
How can ethene be turned into ethanol?
Through the process of steam hydration
What type of reaction is the steam hydration of ethene?
A reversible one
Give the equation for the steam hydration of ethene
H2C=CH2 + H2O ⇌ C2H5OH
What kind of catalyst is used during the steam hydration of ethene?
A solid phosphoric acid catalyst absorbed onto solid silica
How can is the overall yield of the steam hydration of ethene increased from 5% to 95%?
By recycling unreacted ethene gas
How does ethene form ethane?
By reacting with hydrogen gas
Why is a catalyst needed in the hydrogenation of ethene?
To break the strong H-H bonds and form H atoms that can react with the alkane
What are the two catalysts that can be used in the hydrogenation of ethene?
- Platinum
- Nickel
What is the advantage of using nickel over platinum as a catalyst?
It is cheaper
What is the disadvantage of using nickel over platinum as a catalyst?
It is less efficient
What are the conditions if nickel used as a catalyst in the hydrogenation of ethene
150ºC + 5atm
Give the formula for the reaction of ethene with hydrogen?
H2C=CH2 + H2 ====> CH3CH3
Give one usage of hydrogenation
To make unsaturated fats and oils more saturated and therefore thicker
Define “polymer”
A long molecule made up from many small molecules (monomers)
They are saturated
Define “monomer”
A molecule which is the starting material for a polymer
Is unsaturated
Define “polymerisation”
Small molecules called monomers joining together to produce long chain polymers
What two types of polymers are there?
Homopolymers (aka A-A polymers)
Heteropolymers (aka A-B polymers)
How do homopolymers form?
If all monomer molecules are the same
What does a homopolymer look like if A is used to represent a monomer?
–A+A+A+A–===>–A-A-A-A–
How do heteropolymers form?
If two different monomers are used, the two different monomers alternate along the chain
What does a heteropolymer look like if A and B are used to represent the two different monomers?
–A+B+A+B–===>–A-B-A-B–
Give two examples of homopolymers
- Poly(ethene)
- Poly(choloroethene)
Give two examples of heteropolymers
- Polyamides e.g nylons
- Polyesters
Why can alkenes be used in addition polymerisation but alkanes cannot?
The alkene monomers contain C=C which open up and join together
Alkanes don’t have C=C
How do you write a polymer? (Both ways)
poly(insert monomer here)
-[repeating unit]-n (make sure you have the lines poking out of the brackets)
What is co-polymerisation?
When more than one monomer is used in addition polymerisation and they are both incorporated into the final polymer
What are the number of molecules in one mole of gas?
6.02x10^23 (Avagadro’s constant)
What is the rule for equal volumes of gases?
At the same temperature and pressure, they contain an equal number of molecules
Why do equal volumes of gases at the same temperature and pressure contain an equal number of molecules?
- The number of molecules in one mole of gas is the same
- The molecules in a gas are very far apart compared to the actual size of each molecule, so there is a negligible effect on the total volume
Define “molar volume”
Volume occupied by one mole of any gas at a particular temperature and pressure
What is the molar volume of any gas at RTP?
24 dm^3
What is the formula for calculating the number of moles in a gas?
volume/molar volume (they must be the same unit)
What is the ideal gas equation?
PV=nRT
What does the “P” stand for in the ideal gas equation and what are its units?
Pressure - pa (pascals)
What does the “V” stand for in the ideal gas equation and what are its units?
Volume - m^3
What does the “R” stand for in the ideal gas equation and what is it?
Gas constant - 8.314JK-1mol-1
What units of temperature are used in the ideal gas equation?
K
How could a gas syringe be used to calculate the volume of gas produced in a reaction?
By attaching it to the opening of a reaction vessel
What shows a reaction has finished?
When there is no further change in gas volume
What is the setback of a gas syringe?
It only shows total volume, not individual gas volumes
Define “isomers”
Molecules with the same molecular formula but with different arrangement of the atoms
What are the two types of isomerism?
- Structural isomerism
- Stereoisomerism
Define “structural isomers”
Molecules with the same molecular formula but with different structural formulae
Name the 3 types of structural isomer
Chain isomerism
- Position isomerism
- Functional group isomerism
What is chain isomerism?
The different arrangement arrangement of carbon atoms in a chain.
Chain lengths are different/carbons are in different places in the chain because of branching
What are the two ways carbon skeletons can be arranged?
- As a straight chain
- As branched chains
Are the chemical properties of chain isomers similar or different?
Similar
Are the physical properties of chain isomers similar or different?
Different
Give one physical property of chain isomers that differ and why.
Boiling point differs because of the changes in the shape of the molecule
The more ____ in an alkane, the more chain isomers it has.
Carbon atoms
Which alkanes have no chain isomers?
- CH4
- C2H6
- C3H8
What is position isomerism?
Functional group is situated in different positions on the carbon chain
Are the physical properties of position isomers similar or different?
Different
Are the chemical properties of position isomers similar or different?
Different
What is functional group isomerism?
Molecules having the same molecular formula but different functional group
Are the physical properties of position isomers similar or different?
Different
Are the chemical properties of position isomers similar or different and why?
Different as they belong to different homologous series
What are the two types of stereoisomer?
- E/Z isomerism
- Cis/Trans isomerism
Explain how E/Z isomers form in alkenes due to restricted rotation
- Alkanes don’t contain any double bonds, single bonds allow atoms to rotate freely so they cannot form isomers of each other
- Alkenes have at least one double bond, double bonds are rigid and don’t allow the atoms to rotate
- Alkenes still contain single bonds in their molecules which means atoms can rotate these
When is an alkene an E isomer?
When the hydrogens are across the double bond
When is an alkene a Z isomer?
When the hydrogens are on the same side of the double bond
When do we use the terms “E or Z isomer”?
Only when the two groups that are the same are hydrogens
What is an E isomer also known as?
Trans isomer
What is an Z isomer also known as?
Cis isomer
What is optical isomerism?
When two compounds are non superimposable mirror images of each other
When does optical isomerism usually happen?
A central carbon atom with four groups arranged around it
What property of alkenes allows E/Z isomers to form?
Restricted rotation around the C-C double bonds
What are the main pollutants from vehicle engines?
- Carbon dioxide
- Carbon monoxide
- Hydrocarbons
- Nitrogen Oxide
- Particulates
- Sulfuric Oxides
How is carbon dioxide formed from vehicle engines?
Complete combustion of hydrocarbons (e.g alkanes, alkenes, alcohols, cycloalkanes)
Why do hydrocarbons make good fuels?
Combustion is a very exothermic reaction
What problem does carbon dioxide cause?
The greenhouse effect
What is the greenhouse effect?
- Earth radiates IR into space
- Greenhouse gases absorb some of this and reflect it back
- The build up of CO2 causes an enhanced greenhouse effect, leading to global warming
What problems does global warming cause?
- Destruction of habitats e.g ice cap meltings, leading to species extinction
- Sea level rising
- Freak, stronger storms
How is carbon monoxide formed from vehicle engines?
Incomplete combustion of hydrocarbons (e.g alkanes, alkenes, alcohols, cycloalkanes) due to a limited air supply
What problem does carbon monoxide cause for humans and why?
- Poisoning and internal suffocation
- CO molecules are chemically and physically similar to oxygen molecules
- They can bind to the same sites on haemoglobin molecules in red blood cells as oxygen molecules
- Carbon monoxide molecules are better at binding to haemoglobin molecules
- So less oxygen can be carried around the body
How are hydrocarbons emitted from vehicle engines?
Engines don’t burn all the fuel molecules, so some come out as unburnt hydrocarbons
How is nitrogen oxide formed from vehicle engines?
High temperature and pressure in a car engine causes nitrogen and oxygen in the air to react together
How does nitrogen oxide contribute to the problem of acid rain?
It reacts with water and oxygen in the atmosphere to form nitric acid
What problems does nitrogen oxide cause?
- Acid rain
- Photochemical smog
What are particulates?
Small carbon particles below 2.5x10^-12 m
Where do particulates come from?
- By the incomplete combustion of hydrocarbons in engines
- Power stations
- Volcanoes
What problems do particulates cause?
Penetrate the body causing lung cancer + heart attacks
Where does sulphur dioxide come from?
- The burning of sulphur impurities in fuels
- Volcanoes
How does sulphur dioxide contribute to acid rain?
Sulphur dioxide gas enters the atmosphere and reacts with the moisture in it to form sulphuric acid
SO2+H2O==>H2SO3
What problems does acid rain cause?
- Breathing difficulties
- Corrodes limestone buildings and statues
- Kills forests
- Kills life in lakes
Define a “secondary pollutant”
A pollutant not released directly into the atmosphere
Instead formed from primary pollutants in the atmosphere
How is photochemical smog formed?
Primary pollutants e.g hydrocarbons and nitrogen oxides react in the presence of sunlight to form secondary pollutants e.g ozone
What way (other than catalytic converters) can carbon monoxide emissions be reduced?
Oxygenates can be added to petrol
Why is a catalytic converter described as being “3-way” (and what pollutants does it reduce)?
It reduces 3 pollutants:
- Carbon monoxide
- Hydrocarbons
- Nitrogen monoxide
What do catalytic converters consist of?
Platinum/rhodium on a porous support
Give the word equation for the reduction of carbon monoxide
2CO+O2====> 2CO2
Give the word equation for the reduction of heptane
C7H16 +11O2 ====> 7CO2 + 8H2O
Give the word equation for the reduction of nitrogen monoxide
2NO + CO =====> N2 + 2CO2
How do catalytic converters work?
They reduce harmful pollutants into less harmful ones
Why does reducing nitrogen monoxide not work in diesel engines?
They have a higher concentration of oxygen so any reducing agent would be oxidised
What type of catalyst is a catalytic converter in a car an example of?
Why?
A heterogeneous catalyst
The reactants are the exhaust fumes whilst the catalyst is a solid (finely-divided) metal (e.g. Pt/Rh)
What temperature would a platinum catalyst need to work?
240ºC
What temperature would a platinum alloyed with rhodium catalyst need to work?
150ºC
What type of fuel would a platinum rhodium alloy catalyst need to work and why?
Lead free fuel as lead would poison it
What two ways are there to remove nitrogen oxides in diesel engines?
- Reducing it by recycling exhaust gases through the cylinder, lowering temperature and thus the amount of NOx formed
- Using ammonia as a catalyst instead
Give the formula for using ammonia as a catalyst for reducing nitrogen oxides
4NO + 4NH3 + O2 ===> 4N2 + 6H2O
What do diesel particulate filters contain?
A variety of materials, the most common being ceramic
What methods can the government take to reduce overall pollution?
- Changing laws to reduce emissions e.g vehicles not allowed to pollute above a certain level
- Taxing pollution more highly e.g raise taxes on fuel
How can the population reduce overall pollution?
By changing their behaviour e.g car shares
What are the largest sources of air pollutants?
- Transport
- Industry
- Power generation
What 2 changes should be made to current fuel used to make it more environmentally friendly?
- Decrease in aromatic hydrocarbons used
- Decrease in butane content
What 4 things do aromatic hydrocarbons lead to?
- Higher amount of carbon monoxide emissions
- Higher amount of unburnt hydrocarbons
- Higher amount of nitrogen oxide emissions
- Cause cancer
What are the two bad qualities of butane?
- It is volatile
- Emissions cause ozone formation
- Emissions cause photochemical smog
What are aromatic hydrocarbons and butane needed for?
To help petrol perform well in modern engines
What are the two more environmentally friendly hydrocarbon fuels that should be used?
- Liquified petroleum gas (LPG)/autogas
- Liquid natural gas (LNG)
What is LPG/autogas made from?
The distillation of crude oil
What is LPG/autogas made up of?
A mixture of propane and butane
What are the advantages of liquified petroleum gas (LPG)/autogas?
- Petrol vehicles can be easily converted to work with it
- Works in high performance engines
- Less CO2 emissions
- Less CO emissions
- Less unburnt hydrocarbons
- Less nitrogen oxide emissions
- Lower road and fuel taxes
What are the disadvantages of LPG/ autogas?
- LPG filling stations are rare
- It needs to be kept under pressure to be stored as a liquid
What is liquid natural gas (LNG) made of
Mainly methane
Where does liquid natural gas (LNG) come from?
Oil and natural gas fields
What is liquid natural gas (LNG) most suitable for and why?
Large vehicles, especially as it works in modified diesel engines
What are the advantages of liquid natural gas (LNG) compared to diesel?
- Less NOx emissions
- Less carbon monoxide emissions
What is the disadvantage of liquid natural gas (LNG) compared to diesel?
Cannot be liquified by pressure, must be cooled to -160 degrees c
What are the 3 types of biofuels?
- Ethanol
- Biodiesel
- Biogas
What is ethanol made from?
The fermentation of sugar from crops e.g maize
What is the advantage of using ethanol as an alternative fuel?
- Carbon neutral
- Less CO, SO2, and NOx than car engine, sugar cane
- high octane number (higher performance fuel)
What are the disadvantages of using ethanol as an alternative fuel?
- Highly flammable
- engine cannot run on it alone
- Land used to grow crops could be used to produce food instead, so food prices could go up
How is biodiesel made?
Chemically reacting fats and oils e.g vegetable oils/animal fats with an alcohol producing fatty acid esters (trans-esterifcation)
What are the advantages of biodiesel?
- It can be made from waste oil instead of using fossil fuel based oil
- Carbon neutral
- Biodegradable if spilled
- less CO, SO2, + particulates than disesel engine
What are the disadvantages of biodiesel?
- Higher NOx emissions than diesel engine
- not necessarily sustainable depending on energy source used to produce
What is biogas produced from?
The breakdown of organic waste matter
What two ways can hydrogen be used to generate energy?
- Burnt in a modified engine
- Used in a fuel cell
What do fuel cells used for?
To generate electricity on a small scale by converting chemical energy from a fuel into electrical energy
How is chemical energy generated in a fuel cell?
Through the chemical reaction with oxygen/an oxidising agent in electrochemical cells
Give the equation for the combustion of hydrogen.
2H2(g)+O2(g)====> 2H2O (g)
How is hydrogen obtained?
From seawater using the hydrolysis of water
Why is hydrogen an “energy carrier” rather than an “energy source”?
It takes energy to extract it
What are the advantages of using hydrogen as a fuel?
- Renewable - can be made from the electrolysis of water
- Can be used in internal combustion engines and fuel cells
- Water is the only product of combustion
What are the disadvantages of using hydrogen as a fuel?
- Less energy dense than petrol, less energy per gram so a larger amount of hydrogen is needed to get the mileage equivalent of petrol
- Highly flammable so not easy to store
- high-pressure tank needed to store it as a liquid (otherwise cannot be used)
What should future fuels aim to do?
- Reduce emissions + pollution and therefore global warming
- Be much more economical
Give 2 reasons the government needs energy security
- Increasing competition from other countries for energy sources means an increase in price
- Supplies from other countries could be disrupted due to political issues
Give 5 methods for achieving energy security,
- Encouraging the public and industry to become more energy efficient
- Continuing to make use of coal, oil and gas reserves
- Creating financial incentives to reduce CO2 emissions
- Using more renewable energy
- Creating research of new energy sources
Define system
The reactantd + products being measured (inside the reaction vessel)
Define surroundings
Everything else that’s not the system - outside the reaction vessel
If the energy released by forming bonds is greater than the energy required to break bonds, what type of reaction occurs?
An exothermic reaction
If the energy released by forming bonds is less than the energy required to break bonds, what type of reaction occurs?
An endothermic reaction
how can you measure enthalpy change
using calorimetry to measure temperature change of a reaction and then use q=mcΔT
What general formula can be used to calculate ΔH of a reaction?
ΔH = Hproducts - Hreactants
What is an alkane?
A saturated hydrocarbon
What are the 4 main features of alkanes?
Have the general formula CnHn+2
Have names ending in -ane
Are saturated - all bonds between Cs single
Are aliphatic - don’t contain benzene rings
How are alkanes named?
Name the longest carbon chain
Identify + name any alkyl side chains in alphabetical order
Use di, tri, tetra before the alkyl prefix is the side chains are identical
Show the position of any side chains using the lowest numbers possible
What is crude oil?
A mixture of many hundreds of different hydrocarbons
What is fractional distillation used to do?
Separate the different hydrocarbons within crude oil
What is the issue with using structural/skeletal formulae to represent molecules?
How is this problem solved?
Don’t accurately represent the 3D shapes of molecules
Wedges and dashed/dotted bonds are used
What do dashed/dotted bonds represent in the 3D structure of a molecule?
A bond in the direction behind the plane of the paper
What do wedges represent in the 3D structure of a molecule?
A bond in the direction in front of the plane of the paper
What are the 5 main features of a cycloalkane?
Have the general forumla CnH2n
Have names begining with cyclo-
Have names ending in -ane
Are saturated - all bonds betweens Cs are single
Are aliphatic
Have the general forumla CnH2n
Have names begining with cyclo-
Have names ending in -ane
Are saturated - all bonds betweens Cs are single
Are aliphatic
An unsaturated hydrocarbon containing 1+ C=C bonds
What are the 4 main features of alkenes?
Have the general forumla CnH2n
Have names ending in -ene
Are unsaturated - Have 1+ double bonds between C atoms in molecule
Are aliphatic - don’t contain benzene rings
How are alkenes named?
Have names ending in -ene
The number preceeding the -ene indicates the position of the double bond
If there is more than 1 double bond there will be more than 1 number. Di, tri, tetra will be used before the suffix if this is the case
Describe the bonding in an alkene
A C=C bond contains a sigma (σ) bond and a pi (π) bond.
What is a σ bond?
an area of increased e- density between the C atoms. Only contains 1 area of negative charge
Where are σ bonds found?
in single bonds
Where are π bonds found?
in double bonds (along with a σ bond)
What is electrophilic addition?
A reaction in which an electrophile joins onto an alkene/molecule with double/triple bond
No atoms are removed from the alkene/molecule it joins on to
When is a covalent bond said to be polarised?
If the electrons are unevenly distribute between the atoms
What is a carbocation?
A molecule containing a carbon that has a positive charge
It is formed as an intermediate in electrophilic addition
What is the experimental evidence for the mechanism of electrophilic addition?
If Cl- ions are present when ethene reacts with bromine, the molecule BrCH2CH2Cl forms as well as the expected BrCH2CH2Br
This is because both Cl- and Br- ions can attack the intermediate carbocation
I.e. when other anions present, next anion will always to remaining side not added to
What is the general rule that determines where the anion will add to the alkene?
Anion will always add (first) to the end with the least steric hindrance
i.e. end with least atoms/molecules in the way
What is the product and conditions for when HBr(aq) is used as an electrophile to react with ethene?
CH3CH2Br bromoethane
Aqueous solution, room temp + pressure
What are the conditions for when H2O is used as an electrophile to react with ethene?
(Hydration)
Phosphoric acid adsorbed onto silica catalyst, 300ºC + 60atm
If conc H2SO4 used then at 1atm + steam used
What are the conditions if platinum used as a catalyst in the hydrogenation of ethene
room temp + pressure
What are the products of addition polymerisation?
The polymer and no other products
How could the bonds in an addition polymer be described?
How are they different to those in the monomer used to make it?
Addition polymers are saturated molecules
The monomers used to make them are unsaturated
How are addition polymers named?
By putting the name of the monomer in brackets and prefixing with ‘poly’
e.g. poly(cholorethene)
HOWEVER the polymer itself is not an alkene
How are the repeating units of polymers conventionally drawn?
Only 2 carbons have bonds extending beyond the brackets.
Further carbons ‘branch’ above/below the 2 main ones so there’s never more than 2 ‘main’ carbons in the brackets
What is the atom economy for addition polymerisation?
100%
What is an elastomer?
A polymer that returns to its original shape after being deformed
Are soft + springy
What is stereoisomerism?
Same structural formula, different arrangement of atoms in space
Excludes any different arrangements due to the molecule rotating as a whole or rotating about particular bonds.
Not the same as structural isomerism!
What is needed for stereoisomerism to occur?
A double bond between 2 carbon atoms
2 different groups on each double-bonded carbon
(e.g. cannot have 2 different groups on only 1 carbon/end)
In terms of stereoisomerism, what does cis- mean?
Groups on the same side of the double bond
In terms of stereoisomerism, what does trans- mean?
Groups on opposite sides of the double bond
Why are stereoisomers able to exist?
Trying to rotate the C=C bond would break the pi bond as the p orbitals forming it wouldn’t line up any more.
This requires energy and only happens if the compound is heated strongly. There’s not enough energy at room temp for this to happen
What is the general formula for Hess’ Law?
ΔH1 = ΔH2 - ΔH3
What is ΔfH of an element in its standard state (under standard conditions)?
0
Because forming an element from the same element requires 0 energy
So you don’t need to include the value in Hess’ Law calculation
if ΔfH data is given, what will be on the bottom of the cycle and which way will the arrows be pointing?
If ΔcH data is given, what will be on the bottom of the cycle and which way will the arrows be pointing?
If ΔfH data is given, elements will be on the bottom and arrows will be pointing up
If ΔcH data is given, combustion products will apeear at the bottom and arrows will be pointing down
What is a chemical bond a balance between?
Attractive + repulsive forces
Define cracking
A reaction that breaks large molecules into smaller ones
Why is cracking done after fractional distillation?
Crude oil contains too many high bp fractions + not enough low bp fractions to meet demand
‘Straight-run’ gasoline from primary distillation is poor quality + needs refining further
How does a catalyst affect the enthalpy change of a reaction?
It doesn’t!
The enthalpy change remains the same
What is a catalyst poison?
How do they work?
Poisons adsorb to surface of catalyst more strongly than reactants
Prevents other reactants being adsorbed so the catalyst becomes less efficient
What can reduce the effectiveness of heterogeneous catalysts?
Catalyst poisons
Give an example of a catalyst poison (related to the DF unit…) and what it affects
Lead poisons the metal catalytic converters found in cars (e.g. Pt/Rh)
What properties must the surface of a heterogenous catalyst have in order to function/catalyse reactions?
Must not have a calatyst poison bound to it
Must have a large surface area in order to adsorb reactants.
Hence they are used in a finely-divided form, often on a porous support material
Describe diagrams to show the mechanism of heterogeneous catalysis
- Reactants adsorbed onto catalyst surface. This weakens intramolecular bonds
- Bonds are strained and broken
- New bonds form, creating the products of the reaction
- Products diffuse away from the catalyst surface, leaving it free for new reactants to adsorb to
Why are heterogenous catalysts able to provide an alternate reaction pathway with a lower EA?
They provide a surface on which a reaction may take place
This lowers the EA needed for a successful collision + means the reactants are more likely to collide (which will result in a successful reaction if they have the EA or above)
Give details of the tests you could perfom (in the lab) on the alkane vapour produced by cracking hydrocarbons in the lab
(i.e. What tests? What do they test for?)
Flammability: For solids + liquids, place a spatula end or a few drops of the substance into a combustion spoon* (or dip a glass rod in) and hold in a Bunsen flame.
For gases, use a lit splint to see if the gas is flammable.
Test with bromine water: For solids + liquids, place a spatula end/a few drops of the substance into a test tube. Use pipette approximately 1 cm3 bromine water in, stopper the tube, and shake to mix.
For gases, add approximately 1 cm3 of bromine water directly to the tube, stopper the tube, and shake to mix.
What equation can be used to give the number of moles of a gas at RTP?
Moles = vol (dm3) / 24.0
So vol = moles x 24.0
What are the 2 general methods that can be used to measure the volume of gases (evolved from a solution)?
Using a gas syringe
Using an inverted burette/measuring cylinder
What might you take into consideration when deciding whether to use a gas syringe or inverted measuring cylinder/burette to measure the volume of gas evolved from a solution?
What needs to be taken into account regardless (in relation to the gas evolved)?
Whether the gas is more/less dense than air - hence whether to use downward/upward delivery
If the gas is soluble in water
Regardless, the reaction may not go to completion, so the vol gas evolved may be lower than expected
Why might a gas syringe be used to measure volumes of gases?
Up to 100cm3 gas can be colleced
Can be measured to the nearest 1cm3
Can be used if gas soluble in water
Can be used for collection of all gases regardless of density
Why might an inverted burette/measuring cylinder be used to measure volumes of gases?
Burette can collect up to 50cm3 gas and measure to the nearest 0.1cm3
A measuring cylinder can collect greater volumes (although only measures to nearest 1cm3 like syringe)
Can be used for downward delivery if gas more dense than air
Describe how you would set up a reaction to measure the volume of gas evolved from a solution using an inverted measuring cylinder.
- Place a measuring cylinder filled with water upside down in a water trough so the water doesn’t escape from the measuring cylinder.
- Clamp the measuring cylinder in place so that it cannot topple over.
- Add a delivery tube to the bottom of the measuring cylinder so gas collected will go into cylinder.
- Add reactants to conical flask one after another and place a bung on top of the flask immediately after so no gas can escape.
- The volume of gas evolved can be measured by recording the amount by which the water in the cylinder has moved down.
Describe how you would set up a reaction to measure the volume of gas evolved from a solution using a gas syringe
- Attach the gas syringe to a delivery tube. Attach the other end to a bung
- Push the plunger in as far as possible (so it’s at 0, or record the starting point)
- Mix together the reactants, then quickly replace the bung so no gas escapes
- Record the volume of gas evolved once the reaction has gone to completion
What is a primary pollutant?
A pollutant released directly into the atmosphere
What is photochemical smog?
Smog containing mix of primary + secondary pollutants which absorb light energy and undergo chemical reactions, forming smog
What are the effects of photochemical smog?
- Produces ozone + other secondary pollutants/irritating chemicals
- Ozone is a greenhouse gas + damages lung tissue/immune system
- Haziness + reduced visibility
- Eye + nose irritation
- Difficulty breathing
- Harmful to plants + animals
- Damages substances with C=C bond (e.g. plastic + rubber)
Where do unburnt hydrocarbons come from and what are their implications?
from incomplete combustion
can react in sunlight to form ground level ozone which can damage lungs
where does carbon monoxide come from and what are their implications?
from incomplete combustion
it is toxic
where does carbon dioxide come from and what are their implications?
from combustion, it is a green house gas and causes global warming
where does NOx come from and what are their implications?
formed in engines under high pressure and temperatures when oxygen and nitrogen in the air react
causes ground level ozone and photochemical smog
where does SOx come from and what does it cause?
from burning hydrocarbons with sulfur impurities
causes acid rain
Which pollutants harm humans directly?
Particulates
CO
NO2
Which pollutants harm humans indirectly?
Unburnt hydrocarbons
CO2
SOx (acid rain)
NOx (when causing acid rain)
What are the 3 important reactions that occur in a catalytic converter?
Using oxygen to turn CO into CO2
Using oxygen to turn hydrocarbons into CO2 + H2O
Reacting NO with CO to form CO2 + N2
What does carbon neutral mean?
When burnt release as much CO2 as absorbed when (plant) growing/created
No net increase in production of CO2 - that absorbed balances that produced
A fuel can only be considered this if its production, trasportation, etc. are also carbon neutral
What does sustainable mean?
Able to be used without having a negative impact on future generations
What is a finite resource?
A resource that will run out
Is diesel a sustainable alternative fuel?
No - crude oil is running out
What are the benefits/drawbacks of using diesel as an alternative fuel?
Less CO2 produced than petrol engine, already sold at petrol stations
Produces more NOx + particulates than petrol engine, not sustainable
Is LPG/autogas a sustainable alternative fuel?
No - crude oil is running out
What are the benefits/drawbacks of using LPG/autogas as an alternative fuel?
Less CO, CO2, CxHy, + NOx than petrol engine, petrol engines easily converted to use it
Needs to be stored under pressure so it’s a liquid, not sustainable
What are the reaction conditions needed for cracking?
Hydrocarbon vapour over a heated catalyst
Why are heterogeneous catalysts greener than homogeneous?
They are easier to separate from the product and recycle
why is cracking done?
to make smaller hydrocarbons which are more useful
what is the difference between E/Z and cis/trans?
E/Z must contain 1 hydrogen on each carbon in the C=C
what is formed under electrophillic adition with:
bromine
hydrogen bromide
water
dibromo alkane
bromoalkane
alcohol
what are the conditions needed for electrophillic addition with water?
concentrated sulfuric acid and water
or
steam withh phosphoric acid catalyst at high temperature and pressure
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why is enthalpy change calculated experimentally smaller than the theoretical value?
heat is lost to the surroundings
in complete combustion
evaporation of fuel from the wick
