AS LEVEL CHEMISTRY

2.3.2 RATES AND EQUILIBRIUM

ASSESSED HOMEWORK

Answer all questions
Max 84 marks

	
Name		……………………………………………………………..

Mark		……../84	……....%	Grade ………

1.	The Boltzmann distribution can be used to show the effect of a change in temperature on the rate of a reaction.
·	Draw a labelled Boltzmann distribution diagram.
·	Explain the essential features of your diagram.
·	Using your diagram, explain how an increase in temperature affects the rate of a reaction.

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[Total 9 marks]

 2.	(a)	A group of students were considering the factors that affect the rate of a chemical reaction. One of the students wrote the following.
	The rate of a chemical reaction increases as the temperature is increased because there are more collisions.
	Discuss to what extent this statement is true and describe any other factors that should be considered when looking at the effect of temperature on rate of reaction.
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(b)	The students drew a Boltzmann distribution for a gas at temperature T1.
(i)	Label the axes on the students’ diagram below.

[2]
(ii)	Sketch, on the diagram, the distribution that would be obtained at a lower temperature, T2. Label this line T2.
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[Total 9 marks]

3.	(i)	On the axes below, draw and label the Boltzmann distribution to show the energies of molecules in a gas at a fixed temperature.

[2]
(ii)	Use the graph to explain the effect of a catalyst on the rate of a reaction.
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[Total 4 marks]

4.	A mixture of N2 and H2 was made and left to reach equilibrium.
	Explain how the following changes would affect the time taken to reach equilibrium.
(i)	use of a catalyst
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(ii)	a higher temperature
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(iii)	a lower pressure
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[Total 6 marks]

5.	Part of the manufacture of ammonia involves the equilibrium below.
	N2(g) + 3H2(g)  2NH3(g)	DH = − 92 kJ mol–1
State le Chatelier’s principle.
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[Total 2 marks]

6.	The following equation represents another equilibrium reaction.
	2NO2(g)  N2O4(g)	 = -58 kJ mol–1
	brown	colourless
Use le Chatelier’s principle to describe and explain the colour change (if any) that might take place when
(i)	a mixture of NO2(g) and N2O4(g) is compressed at constant temperature,
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(ii)	a mixture of NO2(g) and N2O4(g) is heated at constant pressure.
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[Total 4 marks]

7.	Esters are used as flavourings. They are made by a reversible reaction between a carboxylic acid and an alcohol.
carboxylic acid + alcohol  ester + water
(a)	Give two features of a reversible reaction, when a dynamic equilibrium has been set up.
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(b)	The production of esters is catalysed by the presence of acids.
(i)	What is meant by a catalyst?
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(ii)	Using the fact that acids are needed to catalyse this reaction, deduce the formula of the ion that acts as the catalyst.
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(iii)	Catalysts do not affect the position of an equilibrium. Explain why not.
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[1]
[Total 5 marks]

8.	The following equation represents an equilibrium reaction.
	Cr2O7 2–(aq) + H2O(l)  2CrO4 2–(aq) + 2H+(aq)
	orange	yellow
Use le Chatelier’s principle to describe and explain the colour change (if any) that might take place when dilute HCl(aq) is added to a solution containing K2CrO4(aq).
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[Total 2 marks]
 
9.	Limestone contains calcium carbonate, CaCO3.
	Limestone is an important source of commercially important chemicals such as lime, calcium oxide, CaO.
(a)	If calcium carbonate is heated strongly in an open container, it decomposes according to the equation below.
	CaCO3(s) ® CaO(s) + CO2(g)	DH = +180 kJ mol–1
	Suggest two reasons why it is necessary to heat the calcium carbonate strongly to achieve decomposition.
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(b)	Another sample of calcium carbonate was placed in a closed container before being heated strongly. This allows an equilibrium to be set up. This equilibrium is shown below.
		CaCO3(s)  CaO(s) + CO2(g)
 (i)	What can you say about the rates of the forward and reverse reactions when the calcium carbonate starts to decompose?
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 (ii)	What can you say about the rates of the forward and reverse reactions when the equilibrium has been established?
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(iii)	A valve allowed some of the carbon dioxide to escape.
	State and explain what happens to the composition of the mixture in the container.
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[3]
[Total 7 marks]

10.	Many industrial processes, used to manufacture important chemicals, involve equilibrium reactions. Chemists use their understanding of rates of reaction and of yields at equilibrium to find the most economic conditions for the reactions.
Chemists were investigating the production of a chemical, X2Y, that could be formed from X2 and Y2 as shown in equilibrium 3.1 below.
	2X2(g) + Y2(g)  2X2Y(g)	equilibrium 3.1
(a)	State le Chatelier’s principle.
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(b)	State and explain the effect on equilibrium 3.1 of a decrease in pressure on:
(i)	the equilibrium position of the reaction,
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(ii)	the rate of the reaction.
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(c)	The chemists measured the percentage conversion of X2 at various temperatures. The results are shown in the graph below.

(i)	Use the graph to predict the percentage conversion at 350 °C.
answer = .....................................................%
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(ii)	The forward reaction in equilibrium 3.1 is exothermic. Explain how the graph supports this statement.
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(d)	The chemists decided to use a catalyst in the process.
State, and explain, the effect of using a catalyst on:
(i)	the rate of conversion of X2 and Y2 into X2Y,
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(ii)	the percentage conversion at equilibrium of X2 and Y2 into X2Y.
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[Total 13 marks]

11.	Diamond and graphite are two allotropes of carbon. Allotropes are different forms of the same element. Diamonds are made industrially from the much cheaper graphite.
Under high pressure the following equilibrium exists.
C(graphite)  C(diamond)	DHө = +1.8kJmol-1
substance	Density / gem-3	
graphite	2.25	
diamond	3.51	
(a)	Complete the following enthalpy profile diagram for the conversion of graphite into diamond, labelling DHө.

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(b)	Suggest which allotrope of carbon is thermodynamically more stable. Give a reason for your choice.
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(c)	In which allotrope do the carbon atoms take up less space? Give a reason for your choice.
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(d)	State Le Chatelier's principle.
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(e)	Use Le Chatelier's principle and the information given at the start of the question to deduce the likely conditions of pressure and temperature needed to turn graphite into diamond. Explain your answer.
pressure ..........................................................................................................
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temperature ....................................................................................................
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[Total 10 marks]

12.	A chemical C is made by reacting chemical A with chemical B in a reversible reaction. A, B and C are all gases under the reaction conditions.
Research chemists wanted to know the optimum conditions to use in the manufacture of C. They carried out a series of reactions under different conditions of temperature and pressure. The percentage conversion of A at equilibrium is shown in the table below.
pressure / MPa	temperature / °C	% A converted	
	350	8	
10	450	12	
	550	16	
	350	11	
20	450	21	
	550	29	
	350	18	
40	450		
	550	49	
(a)	Suggest the percentage of A that is converted at 450 °C and 40MPa.
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[1]

(b)	(i)	Use the data in the table above to state the effect of increasing pressure on the percentage of A converted.
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(ii)	What can be deduced, from this change, about the total number of moles of reactants A and B compared with the number of moles of product C in the equation for the reaction? Explain how you reached your conclusion.
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(c)	Use the data in the table to deduce whether the reaction between A and B is exothermic or endothermic. Explain how you reached your conclusion.
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(d)	It was found necessary to use a catalyst in the production of C.
(i)	What is meant by a catalyst?
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(ii)	Suggest and explain two reasons why catalysts are used in industrial processes.
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(e)	Conditions were used that should have given a conversion of A of 39%. In the manufacture, using these conditions, it was found that only 20% conversion was achieved.
Suggest why the conversion was much less than theory suggested.
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(f)	Give two economically important processes that use catalysts.
process 1 ........................................................................................................
catalyst used in process 1 ..............................................................................
process 2 ........................................................................................................
catalyst used in process 2 ..............................................................................
[2]
[Total 13 marks]