Chapter 21 - Fuels & Heats of Reactions Flashcards
Organic chemistry
The study of the compounds of carbon
Hydrocarbon
A compound that only contains hydrogen and carbon
Name common sources of hydrocarbons
Coal, natural gas (methane) & petroleum
Fossil fuels
Fuels that were formed from the remains of plants and animals that lived millions of years ago
What are the three main types of hydrocarbons?
alkanes
alkenes
alkynes
Give the characteristics of alkanes
○ All atoms are linked together by single bonds
○ Names all end in -ane
○ All prepared in the same way
○ All have similar chemical properties
○ First four are gases, next twelve are liquids, rest are waxy solids
○ Are saturated compounds
Saturated compound
One in which there are only single bonds between atoms in a molecule
What is the general formula for alkanes?
CₙH₂ₙ₊ₙ
Homologous series
Defined as:
- a series of compounds of similar chemical properties
- showing gradations in physical properties
- having a general formula for its members
- each member having a similar method of preparation
- each member differing from the previous member by a CH₂ unit
Structural isomers
Compounds with the same molecular formula but different structural formulas
How do you name an Alkane?
- Identify the longest chain of carbon atoms
- Identify the functional group
- Number the carbon chain starting from the end that gives the substituents the lowest possible number
- Indicate the type and position of substituents, and if multiples exist we use prefixes like di, tri etc
- Name all substituents in alphabetical order
Give the characteristics of alkenes
- Contain at least one double carbon bond in a chain
- Are unsaturated
- Names end in ene
- First three are gases, next eleven are liquids, rest are waxy solids
- Boiling points increase as you go down the series
Unsaturated compound
One that contains one or more double or triple bonds between the atoms in the molecule
What is the general formula for alkenes?
CₙH₂ₙ
How do you name an alkene?
- Identify the longest chain of carbon atoms containing the c=c bond
- Identify the functional group
- Number the carbon chain starting from the end that is closest to the c=c bond
- Indicate the type and position of substituents, and if multiples exist we use prefixes like di, tri etc
- Name all substituents in alphabetical order
Explain how to prepare ethene
- Pour ethanol into a boiling tube and add glass wool
- Place a small piece of aluminium oxide (Al₂O₃) half way
- Gently heat
- Discard first tube and collect five more
- Lift tubing from water and turn off bunsen
What happens when the aluminium oxide (Al₂O₃) is heated during the preparation of ethene experiment?
- Bubbles of gas collect in the gas tube
- Aluminium oxide (Al₂O₃) becomes hot, heat reaches the ethanol which changes to vapour, passes over the hot aluminium oxide (Al₂O₃) and is dehydrated to form ethene
Why is the tubing lifted from the water at the end of the preparation of ethene experiment?
To prevent suck back → as air in the boiling tube cools, a vacuum is created and water is sucked vack to fill the space. The cold water could cause the hot glass to crack and break
What are the physical properties of ethene?
- Colourless gas
- Sweetish smell
- Insoluble in water
- Soluble in cyclohexane
Ethene burns with a…
…yellow luminous flame and is slightly smokey
Give the equation for the combustion of ethene in oxygen
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
What is observed when bromine water is added to a tube of ethene gas?
The colour changes from orange to colourless
What is observed when acifified potassium permanganate (KMnO4) is added to a tube of ethene gas?
Colour changes from purple to colourless
Give a use for ethene
Fruits produce ethene naturally to help them ripen.
Also used to make plastic e.g polythene
Give the characteristics of alkynes
- Contain a triple carbon carbon bond
- Are highly unsatutated
What is the general formula for alkynes?
CnH2n-2
What is ethyne used for?
Oxyacetylene welding and cutting
What is the equation for the preparation of ethyne?
CaC2 + 2H2O → Ca(OH)2 + C2H2
calcium carbide + water → calcium hydrocarbide + ethyne gas
Explain how to prepare ethyne
- Add a small amount of calcium carbide into a buchner flask using a spatula
- Set up apparatus so that the gas produced passes through acidified copper (II) sulfate (CuSO4) solution to remove impuritied
- Slowly add water to flask to get a steady and slow production of ethyne
- Collect a few tubes of the gas
- Do not pour down the drain
What impurities does calcium carbide contain, and what impurities does it cause ethyne to contain?
calcium sulfide
calcium phosphide
calcium nitride
hydrogen sulfide (H2S)
phosphine (PH3)
ammonia (NH3)
Is the reaction of calcium carbide and water in the production of ethyne exothermic or endothermic?
exothermic → the flask becomes warm
What are the physical properties of ethyne?
- Colourless gas
- Sweet smell
- Insoluble in water, but dissolves in organic compounds like propanone and cyclohexane
- Stored in steel containers as it is very explosive
Ethyne burns with a…
…luminous smoky flame and a lot of soot is produced
Give the equation for the combustion of ethyne in oxygen
2C2H2 + 5O2 → 4CO2 + 2H2O
What is observed when bromine water is added to a test tube of ethyne?
Colour changes from orange to colourless. This is evidence of unsatutation
What is observed when acifified potassium permanganate (KMnO4) is added to a tube of ethyne gas?
Colour changes from purple to colourless
Aliphatic compound
Organic compound that consists of open chains of carbon atoms and closed chain compounds (rings) that resemble them in chemical properties
Aromatic compounds
Compounds that contain a benzene ring in their structure
Give the characteristics of benzene
- Is highly unsaturated
- Is very unreactive
- All bond lengths are the same
- Each carbon has one electron which belongs to the whole molecule, which gives it extra stability
- Molecule is flat and all bond angles are 109.5°
What is the traditional name for methylbenzene?
toluene
Give the characteristics of methylbenzene
- Liquid at room temperature
- Is insoluble in water as it is non-polar and water is polar
- Iodine is soluble in methylbenzene but not water, as it is also non-polar
How are fossil fuels formed?
Layers of silt and mud covered bodies of dead sea creatures millions of years ago. Over the years the decay and pressure turned these bodies into oil and gas
Describe the appearance of crude oil
Thick black liquid that doesn’t burn easily and has an unpleasant odour
Fractional distillation
This is the heating of the crude oil and separating out of the different mixtures on the basis of their boiling points
Outline the process of fractional distillation
- Crude oil heated to ≈600°C
- Oil enters column partially vapourised
- Trays have holes called bubble caps
- Temperature drops as you move up the tower
- Substances with high boiling points (large hydrocarbons) come off as liquids at the bottom
- Substances with low boiling points (small hydrocarbons) come off as gases at the top
- Collected in trays, which are connected to an outlet from which the fraction is collected
List the fractions of crude oil from top to bottom
- Refinery gas (petroleum gas/LPG)
- Petrol gas (light gasoline)
- Naphtha
- Kerosene (paraffin)
- Diesel oil (gas oil)
- Lubricating oil
- Fuel oil
- Bitumen
What is refinery gas used for?
Bottled gas (propanone/butane)
What is petrol used for?
Fuel for car engines
What is naphtha used for?
Solvents and used in the petrochemical industry
What is kerosene (paraffin) used for?
Fuel for aircraft and stoves
What is diesel oil (gas oil) used for?
Fuel for road vehicles and trains
What is lubricating oil used for?
Lubricant for engines and machines, waxes and polishes
What is fuel oil used for?
Fuel for heating ships, power stations and heating plants
What is bitumen used for?
Road surfacing and roofing
Name the compounds added to refinery gas
It is odourless so compounds containing sulphur called mercaptans are added to give the characteristic smell so leaks can be detected
Lubricating oil, fuel oil and bitumen are called…
…residue fractions
How do engines work?
- In the internal combustion engine, a mixture of petrol and air is drawn into the cylinder
- Mixture is compressed and ignited by a spark
- The gases produced by the explosion drives the piston down
- This rotates the crankshaft causing the wheels to turn
Knocking
If the explosion in the engine occurs too early, it causes the pistons to vibrate and a metallic noise can be heard from the engine. This noise is called knocking. The early explosion is called by the petrol-air mixture exploding when being compressed rather than exploding due to the spark
Auto-ignition
Premature ignition (explosion) of the petrol-air mixture before normal ignition of the mixture by a spark takes place
Octane number
The octane number of a fuel is a measure of the tendency of the fuel to resist knocking
Name the reference hydrocarbons, give their octane numbers and the tendency they have to auto-ignite
2,2,4-trimethylpentane (iso-octane), very efficient, low tendency to auto-ignite, 100
heptane, very inefficient, prone to auto-ignition, 0
What are the factors affecting octane numbers?
- Length of chain - shorter the alkane chain, the higher the octane number
- Degree of branching - the more branched the chain the higher the octane number
- Straight chain vs cyclic structures - cyclic compounds have higher octane numbers than straight-chain compounds
What anti-knocking additive was added to fuel in the 1920s? What problems were associated with this additive?
Discovered a small amount of lead reduced knocking, e.g tetraethyl lead Pb(C2H5)4
But lead can damage health and catalytic converter in the car so it was removed from petrol
What are four ways to increase the octane numbers of fuels?
- Isomerisation
- Catalytic cracking
- Reforming / Dehydrocyclisation
- Adding oxygenates
What is isomerisation?
Changing straight chained molecules into their branched equivalents
Explain the process of isomerisation
- The alkanes are heated in the presence of a suitable catalyst to break the chains apart
- Then they join back together again, when the product is more likely to be branched
- Any straight chains left are removed and sent over the catalyst again
Catalytic cracking
The breaking down of long-chain hydrocarbon molecules by the action of heat and catalysts into short chain molecules for which there is a greater demand
Give two benefits of catalytic cracking
- The short chain molecules produced in catalytic cracking tend to be highly branched, thus petrol containing them have a higher octane number
- The alkanes produced are important as feedstock for the petrochemical industry
What is reforming/dehydrocyclisation?
Using catalysts to form ring compounds, which have high octane numbers
Why is the addition of benzene to fuel strictly controlled?
Benzene is a carcinogen
Oxygenate
Name used by petroleum chemists to refer to any fuel containing oxygen in its molecules
What are the three main molecules containing oxygen that are added to petrol?
methanol
ethanol
MTBE (methyl tertiary butyl ether / 2-methoxy-2-methylpropane)
Why are oxygenates added to fuel?
- To increase the octane number of the fuel
- They give rise to very little pollution when burned
Why does the composition of fuel vary from season to season?
In winter, the fuel is more volatile (low boiling point) and in summer it is less volatile
What are the two main methods of producing hydrogen?
Steam reforming of natural gas
Electrolysis of water
Explain steam reforming of natural gas and give an equation
Methane is reacted with steam and a suitable catalyst
CH4 + H2O → 3H2 + CO
Give an equation for the electrolysis of water
H2O → H2 + ½O2
What is the most important use of hydrogen?
The manufacture of ammonia
3H2 + N2 → 2NH3
Give one advantage and one disadvantage of using hydrogen as a fuel
✔ It is environmentally friendly as the only by-product is water
✘ It can form an explosive mixture with air so it has to be stored and transported carefully
Thermochemistry
The study of heat changes
Exothermic
A reaction in which heat is liberated eg fuel burning
Endothermic
A reaction in which heat is taken in e.g amminium nitrate + ammonium chloride
Heat of reaction
The heat change that occurs when the number of moles of reactants indicated in the balanced equation for a reaction react completely
What is ΔH used to indicate?
It is used to indicate the heat change taking place when a chemical reaction occurs.
ΔH is negative if their is a heat loss in the system i.e exothermic
ΔH is positive is there is a heat gain in the system i.e endothermic
What are the rules for writing a thermochemical equation?
- The equation must be balanced
- Although the SI unit for energy is the joule, this is too small so we use the kJ
- If the moles in the equation changes, so too will the ΔH
- The physical states of the reactants and products is always given
- Reaction carried out at 25°
Heat of combustion
The heat change that occurs when one mole of a substance is completely burned in excess of oxygen
What tool is used to accurately measure heat of combustion?
Bomb calorimeter
How does a bomb calorimeter work?
- A fixed amount of the sample whose heat of combustion is to be measured is placed in a cucible inside the bomb
- Bomb is placed in a container of a known quantity of water
- Oxygen is pumped in excess/under pressure into the bomb
- Electric wires ignite the sample
- Rise in water temperature is recorded
- The calculation of m x c x Δt works out the heat of combustion of the sample
Kilogram calorific value
The kilogram calorific value of a fuel is the heat energy produced when 1kg of fuel is completely burned in oxygen
Bond energy
The average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
Heat of neutralisation
The heat change observed when one mile of H+ ions from an acid reacts with one mole of OH- ions from a base
What is the measure/unit for heat of neutralisation?
kJ mol-1
What is the average ΔH for the reaction of all strong acids with all stong bases?
-55 → -58 kJ mol-1
Outline the experiment to determine the heat of reaction of HCl with sodium hydroxide
- Place a known volume of HCl into a polystyrene cup
- Same volume of sodium hydroxide is placed in a second cup
- The volumes and concentrations are recorded
- Both solutions are allowed to stand until they are at the same temperature
- Record temperature
- Quickly add the NaOH solution to the HCl solution, taking care not to spill
- Immediately put lid on cup
- Stir continuously
- Record the max temperature
Is the reaction of NaOH and HCl endo or exothermic?
Exothermic (-)
Specific heat capacity
The amount of heat required to raise the temperature of 1kg of a substance by 1K (J kg-1 K-1)
What assumptions are made in the experiment to determine the heat of reaction of HCl with NaOH?
- We assume the plastic absorbs no heat
- We assume the acid and base have the same specific heat capacity as water (i.e 4060 J kg-1 K-1)
- We assume the density of the solution is the same as water (i.e 100cm3 of solution has a mass of 100g/0.1kg)
Heat of formation
The heat change that takes place when one mole of compound in its standard state is formed from its elements in their standard state
Standard state
A compound’s natural form at 25°C and at atmospheric pressure of 101 kPa
Hess’s Law
If a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage
Law of Conservation of Energy
Energy cannot be created or destroyed, but can only be converted from one form to another
