module 3.5

Question 1

What is meant by the term average bond enthalpy?

Answer 1

(Average enthalpy change) when one mole of bonds

of (gaseous covalent) bonds is broken

Question 2

Describe and explain the effect of increasing the pressure on the reaction rate.

Answer 2

Increased rate
AND
greater concentration of molecules / more molecules per
(unit) volume 
More collisions per second / more frequent collisions 

Question 3

State le Chatelier’s principle.

Answer 3

The (position of a dynamic) equilibrium shifts to minimise

(the effect of) any change 

Question 4

Explain why the actual conditions used by the chemical industry might be different.

Answer 4

Low temperature gives a slow rate
OR high temperatures needed to increase rate 
High pressure is expensive (to generate)
OR high pressure provides a safety risk 

Question 5

Explain why the use of a catalyst can reduce the demand for energy.

Answer 5

Allows reactions to take place at lower temperatures

Question 6

Apart from heat loss, suggest two reasons for the difference.
Assume that the calculation has been carried out correctly.

Answer 6

incomplete combustion
non-standard conditions
evaporation of alcohol/water
specific heat capacity of beaker/apparatus

Question 7

A closed system is required for dynamic equilibrium to be established.
State one other feature of this dynamic equilibrium.

Answer 7

Rate of the forward reaction is equal to the rate of the reverse
reaction 
OR
concentrations do not change

Question 8

The student heats the equilibrium mixture keeping the volume constant.
Predict how the composition of the equilibrium mixture changes on heating.
Explain your answer.

Answer 8

More H2 and I2 OR less HI 
(equilibrium position shifts) to the left
AND
(Forward) reaction is exothermic
OR reverse reaction is endothermic
OR in the endothermic direction

Question 9

Predict and explain what effect, if any, an increase in the pressure would have on the position of the equilibrium.

Answer 9

No effect
AND
Same number of (gaseous) moles on both sides

Question 10

What is meant by the term standard enthalpy change of formation?
Give the standard conditions.

Answer 10

(Enthalpy change) when one mole of a compound 
is formed from its elements 
298 K / 25 °C AND 1 atm / 100 kPa / 101 kPa / 1 bar

Question 11

Explain how increasing the temperature increases the rate of reaction.Include a labelled sketch of the Boltzmann distribution, on the grid below.Label the axes.

Answer 11

axes labelled (number of) molecules and (kinetic) energy 
Correct drawing of a two Boltzmann distributions
i.e. both curves must start within the first small square nearest to the
origin AND must not touch the x axis at high energy 
Drawing of Boltzmann distribution at two different
temperatures with higher and lower temperature clearly
identified ( ie T2>T1) 
QWC - (At a higher temperature) more molecules have energy
above activation energy
OR greater area under the curve above the activation energy

Question 12

Describe and explain the effect of decreasing the pressure on the rate of a reaction.

Answer 12

(Decreasing the pressure) decreases the rate of reaction
AND
Decreased concentration of molecules
OR
Number of molecules remains the same but the volume increases
OR
Less molecules per (unit) volume 
Less frequent collisions 

Question 13

What is meant by the term enthalpy change of combustion, ΔHc?

Answer 13

(Enthalpy change that occurs) when one mole of a
substance 
completely combusts OR reacts fully with oxygen 

Question 14

The student noticed that compound I burnt with a yellow flame and produced black smoke.
Suggest an explanation for these observations.

Answer 14

incomplete combustion OR not sufficient oxygen available

AND carbon is formed 

Question 15

reaction in catalytic converters?

Answer 15

2NO + 2CO 2CO2 + N2 

Question 16

Propenenitrile is manufactured from propene as shown in the equation.
C3H6(g) + NH3(g) + 1½O2(g) CH2CHCN(g) + 3H2O(g) ΔH = –540 kJ mol–1
The conditions used are 450 °C and 2.5 atmospheres in the presence of a catalyst.Describe and explain, using le Chatelier’s principle, the effect on the position of equilibrium of the following changes:
• a temperature above 450 °C
• a pressure above 2.5 atmospheres
• the absence of a catalyst.

Answer 16

(as temperature rises) position of equilibrium changes to
minimise effect of temperature rise by absorbing energy
OR
(as pressure rises) position of equilibrium changes to
minimise the pressure increase by reducing the
pressure and making fewer gas molecules 
as temperature rises the position of equilibrium shifts to
the left AND increase in pressure shifts the equilibrium to
the left 
relates change with temperature to the
(forward) reaction being exothermic OR reaction releases
energy or heat
OR
reverse reaction is endothermic OR reverse reaction
takes in heat or energy 
change with pressure because there are fewer moles of
reactants OR more moles of products 
removing the catalyst does not change the position of
equilibrium 

Question 17

The chemical industry uses temperature and catalysts to control the rate of reactions.Using Boltzmann distribution diagrams, explain the effect on the rate of a reaction of:
• increasing the temperature
• adding a catalyst.

Answer 17

Correct drawing of one Boltzmann distribution (could be
temperature or catalyst) 
axes labelled (number of) molecules and energy 
Increasing the temperature
Correct drawing of Boltzmann distribution at two different
temperatures with higher and lower temperature clearly
identified 
idea that activation energy is lowered with a catalyst
Collision theory
reaction is faster with catalyst AND when temperature is
increased 
Greater proportion of molecules with energy above
activation energy (with increased temperature or when
catalyst is used ) 
more effective collisions OR more successful collisions
(with increased temperature or when catalyst is used) 

Question 18

The pressure of the equilibrium mixture is increased.Explain what happens to the position of the equilibrium.

Answer 18

(equilibrium position shifts) to the left 
(because there are) fewer moles (of gas) on the reactant
side
OR
(there are) more moles (of gas) on product side 
This explanation mark is dependent on the correct shift of
the equilibrium

Question 19

The temperature of the equilibrium mixture is increased.

Explain what happens to the position of the equilibrium.

Answer 19

(equilibrium position shifts) to the right 
(because forward) reaction is endothermic
OR
reverse reaction is exothermic 
This explanation mark is dependent on the correct shift of
the equilibrium

Question 20

The reaction is actually carried out in the presence of a nickel catalyst at a pressure of 30 atmospheres. Suggest why the manufacturer uses a pressure of 30 atmospheres.

Answer 20

Gives a high rate of reaction OR reaction is fast OR
reasonable rate of reaction without shifting equilibrium too
much to the left 

Question 21

The nickel catalyst increases the rate.

Use a labelled diagram of the Boltzmann distribution of molecular energies to explain why.

Answer 21

y-axis label is ‘(fraction of or number of) molecules’ AND xaxis
label is ‘energy’ AND correct curve 
Lowers activation energy 
More molecules with energy above activation energy with
a catalyst OR more effective collisions OR more
successful collisions 

Question 22

Outline the stages that allow nitrogen monoxide and carbon monoxide to react in a catalytic converter.

Answer 22

CO and NO are adsorbed (onto surface) OR reactants are
adsorbed (onto surface) 
weakening of bonds OR chemical reaction OR new bonds
are made OR carbon dioxide and nitrogen are made 
CO2 and N2 desorbs (from the surface) OR products
desorbs (from the surface) 

Question 23

It is not possible to measure the enthalpy change of formation of N2O(g) directly.
Suggest why it is not possible.

Answer 23

Reaction gives NO OR reaction gives NO2 OR reaction
gives a mixture of oxides OR activation energy too high
OR rate of reaction is too slow 

Question 24

The reaction is repeated at a higher pressure.

Describe and explain what happens to the rate of the reaction between H2(g) and Cl2(g).

Answer 24

more concentrated (particles) OR more particles per (unit)
volume 
more collisions per second
OR more frequent collisions 

Question 25

The reaction is repeated again. This time the temperature is decreased.
Describe and explain, by drawing appropriately labelled Boltzmann distributions, what happens to the rate of reaction between H2(g) and Cl2(g).

Answer 25

y-axis label is ‘(number of) molecules’ AND x-axis label is
‘energy’ AND one correct curve 
Correct curve for lower temperature (labelled) 
Activation energy does not change
OR clearly labelled on diagram, e.g. Ea OR E 
Fewer molecules have energy above activation energy
OR fewer molecules have enough energy to react 
So fewer successful collisions 

Question 26

Write the equation for the aerobic respiration of C6H12O6.

Answer 26

C6H12O6 + 6O2  6CO2 + 6H2O 

Question 27

Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.

Answer 27

Bond breaking absorbs energy AND bond forming releases
energy 
More energy released than absorbed 

Question 28

Suggest why the enthalpy change of formation of C6H12O6cannot be determined directly.

Answer 28

Would make carbon dioxide and water instead
OR activation energy (too) high
OR rate is (too) slow
OR do not react together 

Question 29

Explain the meaning of the term endothermic.

Answer 29

Reaction in which energy enters the system (from the

surroundings) 

Question 30

Explain the meaning of the term activation energy.

Answer 30

Activation energy is the minimum amount of energy needed

for the reactants to react 

Question 31

Explain, in terms of the rate of the forward reaction and the rate of the backward reaction, how the mixture of N2(g) and O2(g) reaches a dynamic equilibrium containing N2(g), O2(g) and NO(g).

Answer 31

Rate of forward reaction slows down and rate of backward
reaction speeds up 
(Until) rate of forward reaction is the same as the rate of the
backward reaction 

Question 32

The research chemist repeats the experiment at the same temperature using the same initial amounts of N2(g) and O2(g). This time she carries out the experiment at a much higherpressure.
Suggest why
• much less time is needed to reach dynamic equilibrium
• the composition of the equilibrium mixture is the same as in the first experiment.

Answer 32

Reaction is faster 
Increasing pressure mean more particles per unit volume
OR increasing pressure gives more crowded particles
OR increasing pressure gives more concentrated (particles) 
So more collisions per second
OR higher collision frequency
OR collisions more often 
(Changes of pressure) do not change the (position of)
equilibrium 
Both sides of equation have same number of moles (of gas) 

Question 33

The reaction between nitrogen and oxygen in a car engine does not reach a dynamic equilibrium.
Suggest why not.

Answer 33

not a closed system

Question 34

Some scientists believe that it is more important to have a high percentage yield in fermentation but others think that a high atom economy is more important.Is it more important to have high percentage yield or a high atom economy in fermentation?
Explain your answer.

Answer 34

High percentage yield with a simple reason
e.g. because the aim is to manufacture ethanol;
to reduce waste; increases sustainability 
BUT
High percentage yield because there is very efficient
conversion from reactant to product
OR to reduce the waste of starting materials 
OR
High atom economy with a simple reason
e.g. because it is cheaper or makes less harmful products;
to reduces waste; increases sustainability 
BUT
High atom economy to reduce the amount of waste products
OR less by products
OR more desired product 

Question 35

Suggest why poly(ethenol) dissolves in water.

Answer 35

Poly(ethenol) has (many) O–H group(s) 

Poly(ethenol) forms hydrogen bonds with water 

Question 36

What is meant by the term enthalpy change of reaction?

Answer 36

(enthalpy change when) the number of moles of reactants 

as specified in the (balanced) equation react together 

Question 37

The actual value for the enthalpy change of this reaction is +81 kJ mol–1.
Suggest a reason why the actual value for the enthalpy change of this reaction is different from the calculated value.

Answer 37

Bond enthalpies may not be the same as the average bond
enthalpy
OR
The idea that bonds have different strengths in different
environments 

Question 38

Explain, using an enthalpy profile diagram and a Boltzmann distribution, how the presence of a catalyst increases the rate of reaction.

Answer 38

one activation energy labelled on enthalpy profile diagram 
idea that activation energy is lowered 
catalyst has a different reaction pathway
OR different reaction mechanism
OR two curves drawn on profile 
QWC – correct diagram of reaction profile for endothermic or
exothermic reaction with products and reactants at different
heights – y axis labelled as energy or enthalpy 
Drawing of Boltzmann distribution AND axes labelled (number
of) molecules and energy 
More molecules with energy above activation energy with a
catalyst OR
More molecules that overcome the activation energy 
More effective collisions OR more successful collisions 

Question 39

Explain why many industrial manufacturing processes use catalysts.Include in your answer ideas about sustainability, economics and pollution control.

Answer 39

Enable reactions to occur with less waste
OR enable reactions to take place with higher atom economy
OR fewer undesired products 
Enable reactions to happen with less toxic solvents/reactants
OR enable reactions to produce less toxic waste/side products 
Reactions can happen at room temperature
OR reactions can happen at atmospheric pressure
OR reactions can happen at a lower pressure
OR reactions can happen at a lower temperature 
Saves energy (costs) 
Reduce carbon dioxide emissions
OR reduces amount of fuel burnt
OR reduces greenhouse gas emissions 
Enable reactions to occur with more specificity
OR enable reactions to produce correct stereoisomer 

Question 40

The relative atomic mass of the sulfur found by the space probe was different from the relative atomic mass of sulfur on Earth.
Suggest why.

Answer 40

Different isotopic abundance 

Question 41

NO and CO react very slowly without a catalyst.The catalyst in a catalytic converter increases the rate of reaction.Explain, using an enthalpy profile diagram and the Boltzmann distribution model, how the use of a catalyst increases the rate of reaction.

Answer 41

One activation energy correctly labelled on enthalpy profile
diagram 
Idea that activation energy is lowered 
Catalyst has a different reaction pathway OR different
reaction mechanism OR two curves drawn on profile 
Correct diagram of reaction profile for exothermic reaction
with product below reactants with y axis as enthalpy or
energy and ∆H label – arrow should go down. Ignore a
small gap between at either end of ∆H line 
Drawing of Boltzmann distribution – axes labelled number of
molecules and energy 
More molecules with energy above activation energy with a
catalyst 
More effective collisions OR more successful collisions 

Question 42

The actual conditions used in the catalytic oxidation of ammonia include 900 °C and an increase in pressure. Suggest why these conditions are a compromise.

Answer 42

Increased pressure speeds up reaction / ora 
900 oC increases the rate OR increased temperature
speeds up reaction / ora 
Idea that high enough temperature without compromising
yield OR idea that high enough pressure without
compromising yield 