A2 LEVEL CHEMISTRY

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ASSESSED HOMEWORK

Answer all questions
Max 72 marks

	
Name		…………………………………………………………..

Mark		……../72	……....%	Grade ………

1.	One cause of low-level smog is the reaction of ozone, O3, with ethene, C2H4. The smog contains methanal, HCHO(g).
	The equation for methanal production is shown below.
		O3(g) + C2H4(g) ® 2HCHO(g) + ½ O2(g)
	The rate of the reaction was investigated, using a series of different concentrations of either C2H4(g) or O3(g), by measuring the initial rate of formation of HCHO(g).
	The results are shown below.
experiment	[O3(g)]
/ 10–7mol dm–3	[C2H4(g)]
/ 10–8 mol dm–3	initial rate
/ 10–12 mol dm–3 s–1	
1	0.5	1.0	1.0 	
2	2.0	1.0	4.0	
3	4.0	2.0	16.0	
 (i)	Analyse and interpret the results to deduce the order of reaction of each reactant and the rate equation.
	Explain your reasoning.
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[5]
(ii)	Calculate the value of the rate constant and state the units.

rate constant =.............................. units…….
[3]
 
(iii)	Using the equation above, deduce the initial rate of formation of O2(g) in experiment 1.
	Explain your reasoning.

answer = ................................. mol dm–3 s–1
[1]
 (iv)	The experiment was repeated at a higher temperature.
	How would the new conditions affect the rate of the reaction and the value of the rate constant?
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[1]
[Total 10 marks]

2.	Nitrogen dioxide is one of the major pollutants in air, formed by reaction of nitrogen monoxide with oxygen.
		2NO(g) + O2(g) ® 2NO2(g)
(a)	What is meant by the rate of reaction?
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[1]
 (b)	A series of experiments was carried out to investigate the kinetics of this reaction. The results are shown in the table below.
Experiment	[O2]
/ mol dm–3	[NO]
/ mol dm–3	initial rate
/ mol dm–3 s–1	
1	0.00100	0.00100	7.10	
2	0.00400	0.00100	28.4	
3	0.00400	0.00300	256	
(i)	For each reactant, deduce the order of reaction. Show your reasoning.
O2(g) .....................................................................................................
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NO(g) ....................................................................................................
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[4]
 (ii)	Deduce the rate equation for this reaction.
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[1]

(iii)	Calculate the rate constant, k, for this reaction. State the units for k.

k = ................................... units ...................................
[2]
[Total 8 marks]

3.	In this question, one mark is available for the quality of use and organisation of scientific terms.
	Propanone reacts with iodine in the presence of dilute hydrochloric acid.
	A student carried out an investigation into the kinetics of this reaction.
	He measured how the concentration of propanone changes with time. He also investigated how different concentrations of iodine and hydrochloric acid affect the initial rate of the reaction.
 	The graph and results are shown below.

[CH3COCH3]
/ mol dm–3	[I2]
/ mol dm–3	[H+]
/ mol dm–3	initial rate
/ mol dm–3 s–1	
1.5 ×10–3	0.0300	0.0200	2.1 ×10–9	
1.5 ×10–3	0.0300	0.0400	4.2 ×10–9	
1.5 ×10–3	0.0600	0.0400	4.2 ×10–9	
	The overall equation for the reaction is given below.
		CH3COCH3 + I2 → CH3COCH2I + HI
	This is a multi-step reaction.
•	What conclusions can be drawn about the kinetics of this reaction from the student’s investigation? Justify your reasoning.
•	Calculate the rate constant for this reaction, including units.
•	Suggest the equations for a possible two-step mechanism for this reaction. Label the rate-determining step and explain your reasoning.
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Quality of Written Communication [1]
[Total 14 marks]
4.	The decomposition of dinitrogen pentoxide, N2O5, at 45 °C was investigated. The reaction that takes place is shown below.
		2N2O5 → 4NO2 + O2
	In an experiment, N2O5 with a concentration of 0.60 mol dm–3 was decomposed at
45 °C.
	At this temperature, the reaction has a constant half-life of 1200 s.
 
(i)	How can you tell that this reaction is first order with respect to N2O5?
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[1]
(ii)	Write down an expression for the rate equation of this decomposition.
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[1]

(iii)	Complete the graph below to show how the [N2O5] changes over the first 3600 s of the reaction.

[2]
(iv)	The rate of this reaction can be determined from this graph.
	Show on the graph how the rate can be measured after 1200 s.
[1]
 
(v)	The rate can also be calculated from the rate equation. The rate constant for this reaction is 6.2 × 10–4 s–1.
	Calculate the initial rate of this reaction. State the units.

rate =………….…..……. units………..….………..
[2]
[Total 7 marks]

 5.	Nitrogen dioxide reacts with carbon monoxide emitted from car exhausts in the following reaction.
		NO2 + CO ® NO + CO2
	The rate equation for this reaction is rate = k[NO2]2.
	This is a multi-step reaction. The first step is the rate-determining step.
 
(i)	What is meant by the rate-determining step?
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[1]
(ii)	Suggest a two-step reaction mechanism for this reaction that is consistent with the kinetic data and the overall reaction.

[2]
[Total 3 marks]

6.	Nitrogen monoxide, NO, is involved in formation of ozone at low levels and the breakdown of ozone at high levels.
(i)	In the lower atmosphere, NO is produced by combustion in car engines. Ozone is then formed following the series of reactions shown below.
		NO(g) + 1/2O2(g) → NO2(g)
		NO2(g) → NO(g) + O(g)
		O2(g) + O(g) → O3(g)
	Write the overall equation for this reaction sequence.
	Identify the catalyst and justify your answer.
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[3]
(ii)	In the upper atmosphere, NO removes O3 by the following reaction mechanism.
		NO(g) + O3(g) → NO2(g) + O2(g)	slow
		O(g) + NO2(g) → NO(g) + O2(g)	fast
	Suggest the rate equation for this process. Explain your reasoning.
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[2]
[Total 5 marks]

7.	Some ammonia plants are run at 200–300 atm and 500 °C, with an iron catalyst.
(a)	The hydrogen for the plants is obtained by reacting methane with steam.
	Construct a possible equation for this reaction.
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[1]
(b)	Nitrogen gas and hydrogen gas produce ammonia gas as shown below.
N2(g) + 3H2(g)  2NH3(g)	ΔH = –92 kJ mol–1
(i)	Write the expression for Kc for this equilibrium.

[1]
(ii)	At 500 °C, Kc = 8.00 x 10–2 dm6 mol–2.
	At equilibrium, the concentration of N2 is 1.20moldm–3 and the concentration of H2 is 2.00moldm–3.
	Calculate the equilibrium concentration of ammonia under these conditions.

equilibrium concentration of NH3 = ........................................ moldm–3
[3]

(c)	In this question one mark is available for the quality of the use and organisation of scientific terms.
	Discuss the advantages and disadvantages of running this reaction
•	at a pressure of 200–300 atm;
•	at a temperature of 500°C;
•	with an iron catalyst.
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[6]
Quality of Written Communication [1]
[Total 12 marks]

8.	Syngas is a mixture of carbon monoxide and hydrogen gases, used as a feedstock for the manufacture of methanol.
	A dynamic equilibrium was set up between carbon monoxide, CO, hydrogen, H2, and methanol, CH3OH, in a 2.0 dm3 sealed vessel.
	The equilibrium is shown below.
		CO(g) + 2H2(g)  CH3OH(g)
	The number of moles of each component at equilibrium is shown below
component	CO(g)	H2(g)	CH3OH(g)	
number of moles at equilibrium	6.20 × 10–3	4.80 × 10–2	5.20 × 10–5	
(a)	State two features of a system that is in dynamic equilibrium.
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[2]
 (b)	(i)	Write an expression for Kc for this equilibrium system.

[1]
(ii)	Calculate Kc for this equilibrium. State the units.

Kc = ……………………... units:…………..…..
[4]

(c)	The pressure was increased whilst keeping the temperature constant. The mixture was left to reach equilibrium.
	The equilibrium position above shifted to the right.
(i)	Explain why the equilibrium position shifted to the right.
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(ii)	What is the effect, if any, on the value of Kc?
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 (d)	The temperature was increased whilst keeping the pressure constant. The mixture was left to reach equilibrium.
	The value of Kc for the equilibrium above decreased.
(i)	Explain what happened to the equilibrium position in the equilibrium.
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 (ii)	Deduce the sign of the enthalpy change for the forward reaction shown in the equilibrium above.
	Explain your reasoning.
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(e)	Methanol can be used as an additive to petrol.
(i)	Write an equation for the complete combustion of methanol, CH3OH.
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(ii)	Suggest why methanol is added to petrol.
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[Total 13 marks]