OZ Flashcards
How do you convert from ppm to %?
Divide by 10,000
How do you convert from % to pmm?
Multiply by 10,000
In which two ways is light described as?
- A wave
- A particle
What is the equation for working out the speed of light?
Speed of light (c) = wavelength (λ) x frequency (v)
What is the speed of light?
3.00 x 10^8 ms^-1
What is the unit for wavelength?
m (metres)
What is the unit for frequency when working out the speed of light?
s^-1
What is the equation for working out energy using plank’s constant?
Energy (E) = Plank’s constant (h) x frequency (v)
What is the unit for energy?
J (Joules)
What is the unit for frequency when working out energy?
Hz (hertz)
What is Plank’s Constant?
6.63 x 10^-34 JHz^-1
What is the equation for working out energy with both speed of light and Plank’s Constant?
E = h x c/ λ
What does energy’s interaction with chemicals depend on?
- The type of chemical
- The amount of energy involved
What are the 4 possible outcomes when molecules absorb EM raditation?
Put them in order of decreasing energy (highest energy to lowest)
Electronic transmission between energy levels
Vibration of bonds
Rotation of whole molecule
Translation of whole molecule
Why do changes in rotational energy require a lower frequency of light on the EM spectrum to be absorbed than changes in vibrational energy?
Making molecules rotate requires less energy than making bonds within a molecule stretch
What energy and type of light on the EM spectrum does changing the electronic energy level correspond to?
- UV and visible
What energy and type of light on the EM spectrum does changing the vibrational energy level correspond to?
- Infrared
What energy and type of light on the EM spectrum does changing the rotational/translational energy level correspond to?
- Microwave
Why are the amounts of energy required for different energy changes given in ranges?
The size of the energy change depends on the strength of the substance’s bond
What three types of electronic changes occur when molecules absorb UV light (in order of increasing energy absorbed)?
- Electrons are excited to a higher energy level
- Photodissociation
- Ionisation
How are electrons excited to a higher energy level?
They absorb the UV energy and jump up
What is photodissociation?
Bond breaking caused by visible light/UV radiation
What is ionisation?
When an electron leaves a molecule turning it into an ion
What is bond fission?
The breaking of bonds
What is heterolytic fission?
Type of covalent bond breaking
Both electrons from a shared pair go to same species
Forms ions
Where is heterolytic fission common?
In polar bonds with a large difference in electronegativity
What is homolytic fission?
Type of covalent bond breaking
One electron from a shared pair goes to each atom
Forms radicals
What is a radical?
A species with one (or more) unpaired electron
Why do radicals have no overall charge?
They have the electronic structure they had before they shared their electrons
Why are radicals very reactive?
- Due to their unpaired electron
- Gives them a strong tendency to pair up again with another electron from another substance
What does the amount of energy needed to photodissociate depend on?
The bond enthalpy of the bond
What is initiation?
- The first step of the radical chain reaction
- Free radicals are produced
What is a propogation reaction?
when radicals react to produce new radicals to continue the reaction
What is termination?
- The final step of the radical chain reaction
- Two radicals react together to form a stable molecule
Is termination exothermic or endothermic and why?
Exothermic – energy is released as bonds are formed
Give the overall formula for the chlorine and hydrogen radical chain reaction.
H2 + Cl2 ==UV==> 2HCl
Give the initiation reaction equation for the chlorine and hydrogen radical chain reaction.
Cl2 + hv ===> Cl. + Cl.
How does the Cl2 molecule undergo photodissociation?
Sunlight provides enough energy to break the Cl-Cl bond
What type of fission is photodissociation?
Homolytic fission
Give the propagation reaction equations for the chlorine and hydrogen radical chain reaction.
Cl. + H2 ====> HCl + H.
H. + Cl2 ====> HCl + Cl.
Give the termination reaction equations for the chlorine and hydrogen radical chain reaction.
H. + H. ====> H2
Cl. + Cl. ====> Cl2
H. + Cl. ====> HCl
Give the overall formula for the methane and chlorine radical chain reaction.
CH4 + Cl2 ==UV==> CH3Cl + HCl
Give the initiation reaction equation for the methane and chlorine radical chain reaction.
Cl2 + hv ====> Cl. + Cl.
Give the propagation reaction equations for the methane and chlorine radical chain reaction.
- CH4 + Cl. ====> CH3. (methyl radical) + HCl
- CH3. + Cl2 ===> CH3Cl + Cl.
Give the termination reaction equations for the methane and chlorine radical chain reaction.
- Cl. + Cl. ====> Cl2
- CH3. + Cl. ====> CH3Cl
- CH3. + CH3. ====> C2H6
Give the products of the methane and chlorine radical chain reaction.
- Hydrogen chloride
- Ethane
- Chloromethane
Give 2 possible further side products of the methane and chlorine radical chain reaction.
- Dichloromethane
- Trichloromethane
Which 2 substances react to form dichloromethane?
Cl2 + CH3Cl
Which 2 substances react to form trichloromethane?
CH2Cl2 + Cl.
Why is oxygen called a “biradical”?
Two unpaired electrons
What is meant by the “rate of reaction”?
How quickly reactants are converted into products
Give the formula for calculating the rate of reaction.
Change in property/time taken
Give 5 ways change in property can be measured to determine rate of reaction.
Volume of gas evolved Mass change pH Colour change (colorimetry) Chemical analysis (i.e. taking samples, quenching, titration/colormetry)
Why is it important for scientists to measure rates of reactions?
- A reaction occurring too quickly can be dangerous
- A reaction occurring too slowly is impractical because it ties up equipment and people, which costs money
Give an example in which the volume of a gas is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- Gas is collected in a measuring cylinder
Give an example in which mass change is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- Mass lost of calcium carbonate can be recorded against time
Give an example in pH is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- HCl concentration falls so the pH of the reaction mixture also falls
What does a calorimeter do?
Measures change in a reaction’s colour
Give an example where calorimetery is used to determine rate of reaction.
- When zinc reacts with copper (II) sulphate, the blue coloration of copper sulphate decreases
- Measure the rate of this colour decreasing against time
Explain how chemical analysis can determine the rate of reaction.
- Involves taking samples of the reaction mixture at regular intervals
- Stopping reaction in a sample (quenching) before analysis
Give an example in which chemical analysis is used to determine rate of reaction.
- Iodine and propanone react in the presence of an acid catalyst
- The sample is extracted and quenched by adding sodium hydrogen carbonate, neutralising the acid
- Amount of iodine remaining is determined by titration
How do you plot a graph for the rate of reaction?
- Time is plotted on the x axis
- Change in property is plotted on the y axis
Give the conditions for a successful collision.
- Must collide in the right direction
- Must collide with correct orientation
- Must collide with the minimum amount of Kinetic energy required
What are liquid and gas particles always doing?
Always moving and colliding
Why is initial reaction the fastest?
- As the reaction continue, the concentration of reactants decreases
- Less frequent collisions between reactant particles
- Decreased rate of reaction
What does an increased rate of reaction mean in terms of collisions?
Increase in collisions in a given time between reacting particles
Define “activation energy”
The minimum amount of kinetic energy required by a pair of colliding particles before a reaction occurs
What is activation energy needed for?
To make or break bonds
What happens at the highest point of an enthalpy profile diagram?
Transition state:
- Old bonds stretch and break
- New bonds form
In an exothermic enthalpy profile are the reactants higher up or lower down than the products?
Higher
In an endothermic enthalpy profile are the reactants higher up or lower down than the products?
Lower
What does a Maxwell-Boltzmann Distribution graph show?
The distribution of different kinetic energies in a gas
What does the area under a Maxwell-Boltzmann Distribution graph represent?
The number of molecules
Are there more particles on the left or the right side of a Maxwell-Boltzmann Distribution graph?
On the right side
Where on a Maxwell-Boltzmann Distribution graph is the max number of particles with a specific energy?
In the middle on the highest peak on the graph
Where on a Maxwell-Boltzmann Distribution graph is the average energy shown?
In between the middle and Ea on the graph
How does increased temperature increase rate of reaction?
- Increased temperature, means particles on average have more kinetic energy and will move faster
- A greater proportion of molecules will have the activation energy and will react
- So there will be a higher frequency of collisions which involve energy equal to activation energy
- So there will be more successful collisions in a given time (e.g per second)
How does increased temperature change a Maxwell-Boltzmann Distribution graph?
- The peak of the curve is lower
- Graph is shifted to the right
- But same total area under the graph
Why is the peak of the curve shifted lower?
To balance out other areas of the graph increasing so there is a same total area under the graph
What happens to the most probable and average energy when temperature increases?
They also increase
How does the amount of particles with the greater than/same energy as activation energy?
More particles
How does increased concentration/pressure increase rate of reaction?
- Particles are closer together on average
- More particles per cm^3
- So they collide more often, leading to more successful collisions per second
How does increased concentration/pressure change a Maxwell-Boltzmann Distribution graph?
- Area under the graph is bigger because they are more molecules
- So same shape of graph but the curve is higher up
How does particle size increase rate of reaction?
- Powder is better than a lump
- Because there is a greater surface area making collisions more likely
How does a catalyst increase rate of reaction?
- Provides an alternative pathway for a reaction
- with a lower activation energy
How does using a catalyst change a Maxwell-Boltzmann Distribution graph?
- Put the Ea line further up the graph
- Because at any given temperature, a larger proportion of particles collide successfully
Define homogeneous catalysis.
- Catalysis where the reactants and catalysts have the same physical state
How does homogeneous catalysis work?
- Intermediate compounds are formed
- These break down to form products and reform catalysts
What does the enthalpy profile for homogeneous catalysis look like?
- Two humps for each step (first=intermediate compounds formed, second=intermediate compounds breaking down)
- Combined activation enthalpy is lower than the same reaction without a catalyst
Where does photodissociation occur?
Stratosphere
What type of compound undergoes photodissociation to form radicals which contribute to the breakdown of ozone?
Haloalkanes
What occurs in the photodissociation of haloalkanes?
- High energy UV radiation is absorbed by the haloalkane
- This breaks the C-halogen bond homolytically to form 2 free radicals
What is the photodissociation of haloalkanes an example of?
Initiation
What does the ease of the photodissociation of haloalkanes depend on?
The carbon-halogen bond enthalpy
Why is the carbon-iodine bond more likely to break than the carbon-chlorine bond?
C-I has a lower bond enthalpy
Why are only a few haloalkanes broken down by UV in the troposphere?
Most of the high frequency UV has been absorbed by the ozone layer
Why are halogen radicals (e.g Cl.) bad for the atmosphere?
They are catalysts for the breakdown of ozone
Give the two steps in the homogenous catalysis of the breakdown of ozone by chlorine radicals.
- Cl.+ O3 ===> ClO. + O2
2. ClO. + O ===> Cl. + O2
What is the intermediate compound formed in the homogenous catalysis of the breakdown of ozone by chlorine radicals?
ClO. (Chlorine monoxide)
What is the overall reaction for the breakdown of ozone?
O3 + O ====> 2O2
Give the two steps in the homogenous catalysis of the breakdown of ozone by hydroxyl radicals.
- HO. + O3 ====> HO2 + O2
2. HO2 + O ====> HO + O2
What is the homogenous catalysis of the breakdown of ozone by chlorine radicals an example of?
Propagation
Why is homogenous catalysis of the breakdown of ozone by chlorine radicals particularly dangerous, despite there being a low concentration of Cl radicals?
- The reaction happens faster than ozone can be reformed
- Chlorine radicals are regenerated and can go on to degenerate more ozone
How is ozone formed?
Oxygen molecules photodissociate into oxygen atoms
O2 + hv → 2O
Ozone is formed when an oxygen atom combines with an oxygen molecule
O2 + O → O3
Where is ozone formed?
Stratosphere
Give the two equations for the formation of ozone.
- O2 + hv ====> O + O
2. O2 + O ====> O3
Give the equation for UV radiation reversing the formation of ozone.
O3 + hv ====> O2 + O
How is an equilibrium set up between the formation of ozone and its breakdown?
The ozone layer is constantly being replaced or destroyed by UV radiation either hitting ozone or oxygen molecules
Give the equation for the reversible reaction for ozone formation and destruction.
O2 + O –> O3
Why is ozone important?
It absorbs high energy UV radiation when it breaks down
What problems does ozone cause?
- Ages skin
- Mutates DNA in skin cells causing skin cancer
- Damages eyes
- Damages crops
Why is UV radiation still essential to humans?
Helps humans produce vitamin D
How is ozone formed in the troposphere?
Nitrogen dioxide and hydrocarbons from vehicle engines and power stations react in the sunlight to form ozone
How is photochemical smog formed?
Ozone and carbon particulates mixing
Why is photochemical smog harmful?
- Haziness and reduced visibility in the air
- Respiratory problems
- Eye irritation
Give the two steps in the homogenous catalysis of the breakdown of ozone by nitrogen monoxide radicals
- NO + O3 ====> NO2 + O2
2. NO2 + O ===> NO + O2
What is a haloalkane?
An alkane with at least one hydrogen atom replaced by a halogen
How do you name a haloalkane?
- Longest part of the carbon chain = last part of the compound’s name
- Add “chloro-“, “bromo-“, “iodo-“ or “fluoro-“ depending on which halogen is bonded to it
(If more than one list in alphabetical order) - Show the halogen position by adding numbers
- If more than one of the same halogen, use “di” for 2, “tri” for 3, “tetra” for 4
How does boiling point change down Group 7?
Increases down the group
Why does boiling point increase down Group 7?
- Atomic radius increases
- Number of electron shells increase
- More electrons
- Stronger instantaneous dipole-induced dipole bonds = harder to break so more energy needed to overcome
Define “electronegativity”
A measure of the ability of an atom in a molecule to attract the electrons in a covalent bond
How is electronegativity measured?
- Using the Pauling Scale
- The higher the electronegativity value, the more electronegative the element
How does electronegativity change across a period?
Increases from left to right
Why does electronegativity increase across a period?
- Atomic cores are attracted to shared electrons in covalent bonds
- The two atoms bonded together have different sized cores
- The core of a smaller atom is closer to the shared electron and so exerts a stronger pull on them and is more electronegative
- Atomic radius decreases across a period
How does electronegativity change down a group?
Decreases down the group
How does electronegativity decrease down a group?
- Atomic cores are attracted to shared electrons in covalent bonds
- The two atoms bonded together have different core charges
- Shared electrons are attracted more strongly by the atom with the greater core charge
- Core charge decreases down the group
Why are molecules with similar/identical electronegativities non polar?
- Electrons sit midway between the two nuclei, at equal distance between them
- Because they are equally attracted to both nuclei
Why are molecules with different electronegativities polar?
- Bonding electrons are pulled more towards the more electronegative atom
- The electrons are spread unevenly so each atom has a partial charge
Define “dipole”
Difference in charge between two atoms caused by a shift in electron density in the bond
How does the difference in electronegativity between the two atoms affect the polarity of the bond?
The greater the difference in electronegativity, the greater the shift in charge, so the more polar the bond
What does a molecule need to have in order to be polar?
A permanent charge across the molecule
How must the polar bonds be orientated in a molecule for it to be polar?
Pointing in the same direction
How must the polar bonds be orientated in a molecule for it not to be polar and why?
- Pointing in the opposite direction
- The charges cancel each other out so there is no permanent charge across the molecule
What type of molecules are always non polar?
Symmetrical molecules
What is a permanent dipole?
Occurs if 2 neighbouring molecules both have a permanent dipole
If so, there will be an electrostatic attraction between the charges of the dipoles
What is an instantaneous dipole?
Arises due to the fact electrons in constant motion
At a particular time they may not be evenly distributed
Hence a dipole may arise due to temporarily uneven charges
What is an induced dipole?
Occurs if non-polar/unpolarised molecule next to a dipole
Dipole attracts/repels electrons in unpolarised molecule, creating a dipole in it as well
Can’t occur if no permanent or instantaneous dipoles
What are intermolecular bonds?
Forces between molecules
Why do stronger intermolecular bonds have higher melting points?
More energy needed to break them
Name 3 types of intermolecular bonds?
- Instantaneous dipole - induced dipole bonds
- Hydrogen bonding
- Permanent dipole - permanent dipole bonds
Where are instantaneous dipole - induced dipole bonds present?
In between all molecules
What are permanent dipole - permanent dipole bonds?
Weak electrostatic forces of attraction between polar molecules
What is the relationship between the length of a carbon chain and the alkane’s boiling point?
The longer the carbon chain the higher the hydrocarbon boiling point
Why do longer carbon chained alkanes have a higher boiling point?
Because longer chains increase the no. IDIDBs
What is the relationship between the “branchedness” of an alkane and its boiling point?
- The more branched the alkanes the lower the boiling point
Why do more branched alkanes have a lower boiling point?
- Branched alkanes cannot pack as closely together
- So there are less points of contact between molecules so less IDIDBs
- Molecular surface area of branched alkanes is also smaller so there are fewer IDIDBs
Describe a method for comparing strengths of intermolecular bonds between substances.
- Wrap a piece of filter paper around a thermometer’s bulb
- Dip it in one of the liquids to be tested
- Record the initial temperature
- Remove the thermometer and saturated filter paper from the liquid
- Leave them at room temperature and record the temperature again
- After 5 minutes, calculate the temperature change
- The greater the change in 5 mins, the faster the rate of evaporation, so the weaker the intermolecular bonds in the liquid
Why do molecules with hydrogen bonding have higher energy than those with instantaneous dipole-induced dipole bonds?
Hydrogen bonding is the strongest type of intermolecular bonding
What are the 3 requirements for hydrogen bonding?
- Large dipole between a hydrogen atom and a highly electronegative atom
- A second small hydrogen atom
- A lone pair of electrons the non hydrogen atom in the dipole
What 3 atoms are most commonly the highly electronegative non hydrogen atom bonded to hydrogen?
- Nitrogen
- Oxygen
- Flourine
Why does the non hydrogen atom bonded to the hydrogen atom in the dipole need to be highly electronegative?
To draw bonding electrons away from the hydrogen atoms in their bonds, polarising the molecule
Give 5 substances that undergo hydrogen bonding.
- Water
- Ammonia
- Hydrogen fluoride
- Alcohols (from the -OH group)
- Amines (from the -NH group)
How many hydrogen bonds per molecule can water form? What does this mean?
4
- Means water has a high boiling point
Why can water form 4 hydrogen bonds per molecule?
- The oxygen atoms posses 2 lone pairs of electrons
- Water has twice as many hydrogen atoms as oxygen atoms
How does hydrogen fluoride undergo hydrogen bonding?
- The hydrogen atoms have a strong positive charge as they are bonded to the highly electronegative fluorine atom
- This positive charge lines up with another fluorine atom’s lone pair
- The hydrogen and fluorine atoms can get very close and therefore attract strongly because the H atom is so small
Why does HF have a lower boiling point than water?
- Fluorine has 3 lone pairs
- But HF only has as many H atoms as F atoms so only 1/3 of the available lone pairs are used
Why does ammonia not form that many hydrogen bonds?
- Because there is only 1 lone pair on nitrogen
- So only 1 of the 3 hydrogen atoms can form hydrogen bonds
Why do substances with hydrogen bonding have a high viscosity?
- Hydrogen bonds between molecules are strong
- They cannot constantly break and reform
- So molecules cannot easily flow past each other
Why do substances with hydrogen bonding have a high solubility?
- They can form hydrogen bonds with water molecules
- Allows them to mix and dissolve
Define “nucleophile”
- A molecule or negatively charged ion
- with a lone pair of electrons it can donate to a positively charged atom
- to form a covalent bond
Describe the nucleophilic substitution reaction between a haloalkane and OH- ions
- The nucleophile OH- attacks the electron deficient carbon atom in the C-halogen polar bond
- The OH- donates 2 electrons to form a new dative covalent bond
- The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group)
Describe the nucleophilic substitution reaction between a haloalkane and water.
- The nucleophile H2O attacks the electron deficient carbon atom in the C-halogen polar bond
- The H2O donates 2 electrons to form a new dative covalent bond
- The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group) and an intermediate forms with an oxygen that has 3 bonds.
- This is unstable so one OH bond breaks and an alcohol is formed alongside the halide ion and H+ that has left
Describe the nucleophilic substitution reaction between a haloalkane and ammonia.
- The nucleophile ammonia attacks the electron deficient carbon atom in the C-halogen polar bond
- The ammonia donates 2 electrons to form a new dative covalent bond
- The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group) and an intermediate forms with a nitrogen that has 4 bonds.
- This is unstable so one NH bond breaks and a hydrogen is removed by an ammonia molecule
- This leaves an amine
What is an amine?
An organic compound based on ammonia but with 1 or more of the hydrogen atoms replaced by an alkyl group
What nucleophilic substitution reaction produces a haloalkane?
A nucleophilic substitution reaction between a halide ion (X-) and an alcohol
What is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of?
A strong acid
Why is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of a strong acid?
Gives the carbon atom to which the oxygen is attached in the alcohol a higher partial positive charge, allowing it to be more readily attacked by halide ions
What is the general formula for the nucleophilic substitution reaction producing a haloalkane from an alcohol?
ROH + HX ===> RX + H2O
What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond enthalpy of the haloalkane?
The further down Group 7, the lower the bond enthalpy
What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond polarity of the haloalkane?
The further down Group 7, the lower the bond polarity
Why are fluoroalkanes not useful in nucleophilic substitution reactions?
C-F is a very difficult bond to break because it has a high bond enthalpy
Which haloalkane bond is the easiest to break?
C-I bond
Is bond enthalpy or bond polarity the overriding factor in determining reactivity of haloalkanes? Why?
Bond enthalpy, because of experimental evidence
Describe a method for determining the reactivity of the haloalkanes.
- Put chloroalkane, bromoalkane and iodoalkane in 3 different test tubes
- Add silver nitrate solution and some ethanol as a solvent
- The silver halide compound formed is insoluble and so forms a precipitate
- Time how quickly this is formed with each haloalkane
- The precipitate forms the fastest with iodoalkane which means it has the weakest bond enthalpy
Give the general formula for the reaction of haloalkanes with silver.
Ag+ (aq) + X- (aq) ====> AgX (s)
What does PPM stand for?
Parts per million
Amount of gas particles in a sample containing 1 million particles
When might the concentration of gas in a mixture be given as percentage by volume?
When the gas is present in a high concentration
e.g. various gases that make up the air in the atmosphere
When might the concentration of gas in a mixture be given in ppm?
If the gas is present in a low concentration - less than 1% by volume
Why is high energy UV radiation bad for the skin?
What damage can it do?
Its wavelength/frequency corresponds to the energy required to break chemical bonds, such as DNA
Therefore it can damage genes and lead to skin cancer
or damage proteins and age the skin
What is the electromagnetic spectrum?
The range of wavelengths/frequencies over which electromagnetic radiation extends…
What do the surfaces of the Sun and Earth both emit?
EM radiation
How do you calculate the frequency needed to break a bond?
AKA energy needed to cause photodissociation
Convert kJ/mol into J/mol by x1000
Work out the min. energy needed to break a single bond by ÷NA
Divide energy by Planck’s constant (v = E ÷ h)
How do you calculate bond enthalpy from frequency?
Calculate energy needed to break 1 bond by using E = hv
x NA to work out energy needed to break 1mol of bonds
÷1000 to give answer in kJ/mol
Why is the energy required to cause electronic transmission/vibration/rotation/translation given as a range despite the fact the outcomes are quantised?
The amount of energy needed to do each of these things changes depending on the chemical/substance
What 3 changes to a compound/substance can electronic transmission cause?
Put them in order of decreasing energy requirement (i.e. from highest→lowest energy requirement)
Can cause:
Ionisation - Cl2 → Cl2+ + e-
Dissociation - Cl2 → Cl• + Cl•
Release of energy + return to original state
How does the effect visible light/UV radiation have on matter differ?
Both cause electronic transmission…
Visible light causes electrons to be excited to higher energy levels and some bonds break
UV causes electrons to be excited to higher energy levels and bonds break
What are the 2 ways covalent bonds can break?
What are the products of each type of breaking?
Homolytic fission - produces ions
Heterolytic fission - produces radicals
What are curly arrows used to show?
What are the 2 types and what do they denote?
Used to show the movement of electrons
Full (double-headed) shows movement of a pair of electrons
Half (single-headed) shows movement of a single electron
What is a radical chain reaction?
A reaction in which new radicals are formed at the end of one step
These radicals then continue/propagate the reaction
What are the 3 stages of a radical chain reaction?
Briefly describe each step
Initiation - radicals formed from a stable molecule
Propagation - radical reacts + process forms new radical
Often occurs in pairs - radical formed in 1st propagation step reacts again in 2nd.
Termination - 2 radicals collide to form stable molecule (non-radical)
What is the name of the mechanism/process by which a halogen atom can substitute a hydrogen atom in an alkane chain?
Radical substitution
What is the troposphere?
The layer of the atmosphere directly above Earth’s surface
What is the stratosphere?
The layer of the Earth’s atmosphere above the troposphere
What are the effects of ozone in the troposphere?
Photochemical smog and respiratory problems
What role does ozone play in the stratosphere?
Absorbs high-energy UV radiation from the Sun
This prevents the radiation reaching the Earth’s surface
High-energy UV can cause health problems such as skin cancer and cataracts
Which haloalkanes photodissociate most easily?
Why?
What implications does this have for the atmosphere?
Iodo- + Bromoalkanes photodissociate more easily than chloroalkanes
This is because the C-I and C-Br bonds are weaker than C-Cl bonds
Hence they can be broken down by lower frequency radiation found in the troposphere and do not reach the stratosphere
Fluroalkanes don’t photodissociate in the stratosphere because the C-F bond is too strong to be broken by the UV radiation present
Why can the Cl• radical present in the following porpagation steps of the depletion of ozone be described as a catalyst?
Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2
Because it enables the second propagation stage to take place by forming an intermediate but is reformed by the end of the overall reaction
It goes through the reaction cycle many times and is not used up/depleted
Describe how ozone is destroyed naturally in the stratosphere
Occurs when it absorbs high-energy UV raditation
O3 + hv → O2 + O
What is reaction kinetics?
The study of rates of reaction
What factors can affect rate of reaction?
Concentration Pressure Use of a catalyst Temperature Surface area Intensity of radiation
What is collision theory?
Theory that explains how the frequency of collisions between particles affects the rate of reaction
i.e. reactions only occur when particles collide with the minimum amount of kinetic energy (EA)
More collisions = higher/faster rate
How does increasing concentration/pressure affect the rate of reaction?
More particles available for collisions + in closer proximity to each other so there is a higher chance of them colliding
Therefore there is a higher chance of a successful collision occuring
(Provided the particles both have the minimum energy to react)
How does increasing temperature increase the rate of reaction?
At higher temps a greater proportion of colliding particles have the sufficient energy to react so more collisions have a greater energy than the EA
Therefore there is a higher liklihood of successful collisions occuring/more successful collisions occur
How does increasing the surface area of a solid affect the rate of reaction?
When a solid is more finely divided, there is a larger surface area for reactions to take place on
This means there is a greater frequency of collisions and thus also a greater frequency of successful collisions
Rate increases
How does the use of a heterogeneous catalyst affect the rate of reaction?
Provides a surface where reacting particles may break and make bonds
Increases proximity of reactants + provides an alternate pathway with lower EA, making successful collisions/reactions more likely to take place
How do catalysts affect the rate of reaction?
Provide an alternate reaction pathway with a lower EA therefore increasing the chance of particles having the minimum amount of energy needed to successfully collide
Rate increases
How does increasing raditation intensity affect the rate of reaction?
(Photo)dissociation of bonds faster when intensity increases as bonds absorb more energy per second therefore more are broken
Increases rate
Wht general thing(s) do you need to measure in order to measure the rate of reaction
How quickly a reactant is used up or how quickly a product is formed
What is activation enthalpy?
The minimum kinetic energy required by a pari of colliding particles in order for a successful reacton to occur
What is a transition state?
The highest point on an enthalpy profile/point with highest energy on a reaction pathway
Between reactants + products forming
Where old bonds stretch and new ones start to form
What is an enthalpy profile?
Graph plotting enthalpy against the progress of a reaction
What are the main methods by which the rate of reaction can be measured experimentally?
Measuring vol. gas produced - done using gas syrine or displacement of water.
More gas produced per unit of time = faster rate
Measuring mass changes - reactions which give off gas involve mass changes. Bigger change in mass per unit of time = faster rate
Colorimetry - measures change of intensity as coloured chemical used up/produced. Measured using a colorimeter
pH changes - if acid/alkali used up/produced, can be measured. pH meter can be used to monitor change. Faster change = faster rate
What effect does increasing the temperature by 10ºC (roughly) have on the rate of reaction?
It roughly doubles
What is the Maxwell-Boltzmann distribution?
What does it show?
Shows the fraction of particles with certain kinetic energy
Comparing 2 different temperatures on the distribution shows that as temp increases so does the rate of reaction
What is needed in order for a reaction to take place?
Enough molecules with a combined kinetic energy that is higher than the Ea when they collide
Why do reactions take place faster at higher temperatures?
Because a larger proportion of the colliding particles have the minimum Ea needed to react
In Maxwell-Boltzmann distribution plots showing a catalysed and uncatalysed reaction OR reactions at 2 different temps, what is the same and what is different?
The area under the graphs is the same but the distribution is different
How do catalysts affect the enthalpy change of a reaction?
They don’t
It stays the same regardless of whether a catalyst is used or not
(Ea is affected)
What aspects of equilibrium do/don’t catalysts affect?
Don’t affect the position/composition of equilibirum
BUT do affect the rate at which it is reached
What did CFCs used to be used for/in?
Refrigerants + in air can units
Aerosol propellants
Blowing agents for expanded plastics
Dry cleaning solvents
What is a polar bond?
A bond in which there is an uneven distribution of charge between the 2 atoms
Results in 1 atom have a partially positive (𝛿+) charge and 1 atom have a partially negative (𝛿-) charge
What is the distribution of electrons like in a non-polar covalent molecule?
Electrons equally shared
Distance between each nucleus + electrons identical
Arrangement may also be due to equal electron affinity/electronegativity
What is the distribution of electrons like between a larger and smaller atom?
Shared electrons more strongly attracted to nucleus of smaller atom as its nucleus is closer due to electron shielding in the larger atom
What is the distribution of electrons like in atoms with different electron affinities/electronegativies?
The shared electrons are more strongly attracted to the nucleus with the larger charge + greater electronegativity
What is another name for a permanent dipole?
An overall dipole
If a molecule has symmetrical polar bonds, will it have an overall dipole?
No
If charges are arranged symmetrically around the central molecule then no overall dipole
However, electrons wil still be unevenly distributed in individual bonds
e.g. CCl4
Use bond shapes/VSEPR theory models to work it out
What are the 4 types of intermolecular bond?
Hydrogen bonds
Instantaneous dipole-induced dipole bonds
Permanent dipole-induced dipole bonds
Permanent dipole-permanent dipole bonds
Out of all of the types of intermolecular bonds, which is the strongest?
Hydrogen bonds
Which of the dipole-dipole bonds is the strongest?
Permanent dipole-induced dipole
What types of dipole-dipole attractions will be present in a molecule with an overall dipole?
Permanent dipole-permanent dipole
AND Instantaneous dipole-induced dipole
(And possibly H-bonds)
Describe an instantaneous dipole-induced dipole bond
If 2 neighbouring molecules don’t have a permanent dipole there will still be attraction between them
This is an id-id bond
Occurs between molecules even if permanent dipoles also present
Caused by constant motion of electrons. This means they may not be evenly distributed beween atoms at a given time, causing temporarily uneven charges and, therefore, a dipole
Explain how instantaneous dipole-induced dipole bonds arise
Electrons in a molecule are in continuous, random motion
At a particular moment in time they may be unevenly distributed
This creates an instantaneous dipole
The dipole induces a dipole on a neighbouring molecule, creating an… induced dipole
There is an electrostatic attrction between the two dipoles
Why do instantaneous dipole-induced dipole bonds continously break and re-form?
Because the electron distribution in molecules is constantly chaning due to the fact they are in continuous, random motion
What factors affect the strength of instantaneous dipole-induced dipole bonds?
The no. electrons in the molecule - more means greater chance of instantaneous dipole arrising
Distance between molecules - closer packing means greater electrostatic attraction
How does the number of electrons in a molecule affect the strength of IDIDBs?
How can this be used to explain the trend in boiling points of the halogens?
Molecules with more electrons have more chance of forming IDIDBs
The strength of IDIDBs also increases
Hence boiling points increase as you go down the halogens (Iodine = highest)
When drawing a diagram to show hydrogen bonding, what should you include?
The lone pair(s)
The partial charges on the atoms
The correct bond angle around the H𝛿+ atom
Dashed/dotted lines between the atoms that are hydrogen bonded
What are the requirements for hydrogen bonding to occur?
A large dipole between a H atom and a highly electronegative atom (resulting in a H𝛿+)
A small electronegative atom in the other molecule - N, O, or F
Lone pair on NOF that H can line up/bond with
Describe the shape and structure of ice with reference to H bonding
H bonds formed when water freezes gives ice a regular structure
The H bonds + covalent bonds around each O are arranged tetrahedrally
This arrangement of bonds around each O gives ice a very open structure
Hence, ice has a lower density than water and floats on it
What properties does H bonding give molecules?
High viscosity - for liquid to flow molecules must be able to move past each other, so frequent/easy breaking + forming of bonds required
Solubilty in water due to fact that H bonds can form between water + molecules of substance
What is the trend for boiling points in the halogens?
Why?
Boiling points increase with heavier halogen atoms + in molecules containing more halogen atoms
The larger the halogen/the more halogen atoms, the greater the overall number of electrons
This increases the number of instantaneous dipole-induced dipole bonds
This means the intermolecular bonds are stonger so more energy is needed to seperate the molecules from each other, hence they have higher boiling points
What is the trend in bond strength in the halogens?
Why?
C-Hal bonds become weaker as the size of the halogen atom increases
This makes the bond easier to break and the compounds become more reactive
Although the C-F bond is the most polar, fluroalkanes are very unreactive
This shows that it is bond strength rather than bond polarity that has the greatest effect on the reactivity of halogens (it is the determining factor in reactivity)
Describe the general reactivity of different haloalkanes
Reactivity increases down the group
Bond enthalpy decreases down group - needs to be broken for reaction due to smaller atomic radius
What is the general equation for the homolytic fission of haloalkanes?
R-Hal (+ hv) → R• + Hal•
What is the general formula for the heterolytic fission of haloalkanes?
R-Hal (+ hv) → R+ + Hal-
What is a carbocation?
An ion with a positively charged carbon atom
What are the conditions for homolytic fission (of haloalkanes?)
Gas phase with high temps OR the presence of UV radiation (e.g. in the stratosphere)
What is a substitution reaction?
A reaction in which one atom/group in a compound is replaced by another
What is the general equation for nucleophilic substitution reactions involving haloalkanes?
R-Hal + Nu- → R-Nu + Hal-
Describe the stages of nucleophilic substitution
- Nucleophile attacks electron deficient carbon atom in C-Hal bond
- Nucleophile donates its lone pair to C to form a dative covalent bond
- The carbon-halogen bond breaks heterolytically, with the halogen receiving 2 e- + forming a halide ion. This is the leaving group
How many steps is there to nucleophilic substitution if the nucleophile is an ion OR a neutral molecule?
For an ion - 1 step
For a neutral molecule - 2 steps
How can haloalkanes be synthesised?
By reversing nucleophilic substitution
What is a nucleophile substitution reaction involving what also called?
A hydrolysis reaction
Why is water able to act as a nucleophile?
Because it has 2 lone pairs on the oxygen atom
Why is ammonia able to act as a nucleophile?
Because it has a lone pair on the nitrogen atom
Describe the mechanism for the reverse nucleophilic substitution of haloalkanes
(Used to synthesize them)
The halide ion (X-) acts as the nucleophile
The reaction is done in the presence of a strong acid which provides a H+ for the O in the alcohol to bond with
This causes the oxygen to have a positive charge, so it more strongly attracts electrons from the C-O bond (bond polarised)
This causes the C to have a higher partial positive charge, causing it to be attacked by the halide ion
This causes a water molecule to become the leaving group
When water is used as a nucleophile to react with a haloalkane, what is the/are the:
General equation
Product(s)
Reaction conditions
Equation:
R-Hal + H2O → R-OH + H+ + Hal-
Product:
Alcohol + halogen ion + hydrogen ion
Reaction Condition:
Heat under reflux - this is sometimes called hydrolysis
When OH- is used as a nucleophile to react with a haloalkane, what is the/are the:
General equation
Product(s)
Reaction conditions
General Equation:
R-Hal + OH- → R-OH + Hal-
Product:
Alcohol + halogen ion
Reaction Conditions:
Heated under reflux with NaOH(aq) with ethanol as a solvent
When ammonia is used as a nucleophile to react with a haloalkane, what is the/are the:
General equation
Product(s)
Reaction conditions
General Equation:
R-Hal + NH3 → R-NH2 + Hal- + H+
Product:
Amine + halogen ion + hydroxide ion
Reaction Conditions:
Haloalkane heated with conc. ammonia solution in a sealed tube
What are the general conditions for heterolytic fission?
Dissolved in a polar solvent such as an ehtanol/water mixture
What must nucleophilic substitution reactions to make haloalkanes (from alcohols) be done in the presence of?
A strong acid
To provide H+
What does the polarity of a molecule depend on?
It’s shape- if symmetrical it is non polar
Name the 3 intermolecular bonds in order of most to least strength
Hydrogen bonding
Permanent dipole-permanent dipole bonds
Instantaneous dipole- indices dipole bonds
Which atoms have stronger IDIDBs?
The heavier- bigger atoms as they have more electrons
What two factors influence the boiling point of organic molecules? And why?
Length of the chain
- longer means stronger IDIDBs as more molecular surface contact
Branching
- less branching means more surface contacts so more IDIDBs form
What is a homogeneous catalyst and how does it work?
Catalyst at same state as reactants
They work by forming intermediate compounds with the reactants. Breaks down to give the product and reform the catalyst.
Which factors affect electronegativity?
Atomic charge, distance from the nucleus, shielding
Describe trends in haloalkanes.
Carbon-halogen bonds are polar. Immiscible with water. Boiling and melting points increase down the group.
