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A2 LEVEL CHEMISTRY

5.1.1 HOW FAST? 
5.1.2 HOW FAR?

TEST

Answer all questions
Max 53 marks

| Name		………………………………………………………….. |
| Mark		……../53	……....%	Grade ……… |

1.	1, 2-Dibromoethane, C_2H_4Br_2, reacts with potassium iodide as shown in the equation below.
C_2H_4Br_2 + 3KI → C_2H_4 + 2KBr + Kl_3
	A series of experiments was carried out to investigate the kinetics of this reaction.
(a)	In a first experiment the concentration of C_2H_4Br_2 was measured during the course of the reaction and a concentration-time graph was plotted. The reaction was shown to be first order with respect to C_2H_4Br_2.
(i)	On the axes below, sketch a graph to show how [C_2H_4Br_2] changed during the course of the reaction.

[2]
(ii)	Show on the graph how you would measure the initial rate of the reaction.
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(iii)	How would you use the graph to show that the reaction is first order with respect to C_2H_4Br_2?
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(iv)	The experiment was repeated using different initial concentrations of C_2H_4Br_2.
	Using the axes below, sketch a graph to show how the initial rate of the reaction changes with different concentrations of C_2H_4Br_2.

[1]
 (b)	In a second experiment, the initial concentration of KI was varied and the initial rate was measured. The results are shown in the table below.

experiment

[C_2H_4Br_2]
/mol dm^–3

[KI]
/mol dm^–3

initial rate
/mol dm^–3s^–1

1 | 0.50 | 0.18 | 0.027
2 | 0.50 | 0.72 | 0.108
	Deduce the order of reaction with respect to KI. Show your reasoning.
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(c)	(i)	Construct the rate equation for the reaction.
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(ii)	Calculate the rate constant, k, for this reaction. State the units for k.

k = ............................ units ..................................
[3]
[Total 11 marks]
2.	Nitrogen monoxide reacts with hydrogen at 500 °C as in the equation below.
		2NO(g) + 2H_2(g) → N_2(g) + 2H_2O(g)
	A series of experiments was carried out to investigate the kinetics of this reaction. The results are shown in the table below.

experiment

[NO]
/ mol dm^–3

[H_2]
/ mol dm^–3

initial rate
/ mol dm^–3 s^–1

1 | 0.10 | 0.20 | 2.6
2 | 0.10 | 0.50 | 6.5
3 | 0.30 | 0.50 | 58.5
	In this question, one mark is available for the quality of spelling, punctuation and grammar.
(i)	For each reactant, deduce the order of reaction. Show your reasoning.
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 [4]
Quality of Written Communication [1]
 (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 ....................................................
[3]
[Total 9 marks]

3.	A student investigated the hydration of 2-methylpropene, (CH_3)_2C=CH_2, with dilute aqueous acid to form 2-methylpropan-2-ol, (CH_3)_3COH.
	The following mechanism has been proposed for this hydration.
step 1	(CH_3)_2C=CH_2 + H^+(aq) → (CH_3)_3C^+	rate determining step
step 2	(CH_3)_3C+ + H_2O → (CH_3)_3COH + H^+(aq)
(i)	Step 1 is the rate-determining step for this hydration.
	What is meant by the term rate-determining step?
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[1]
 (ii)	Write a balanced equation for the overall hydration reaction.
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 (iii)	Suggest the role of H^+(aq) in this mechanism. Explain your reason.
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[2]
(iv)	Use the mechanism above to suggest the rate equation for this hydration.
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[1]
[Total 5 marks]

4.	One cause of low-level smog is the reaction of ozone, O_3, with ethene. The smog contains methanal, CH_2O(g), and the equation for its production is shown below.
		O_3(g) + C_2H_4(g) → 2CH_2O(g) + O_2(g)	equation 1
(a)	The rate of the reaction doubles when the initial concentration of either O_3(g) or C_2H_4(g) is doubled.
(i)	What is the order of reaction with respect to
O_3 .............................
C_2H_4? .......................
[1]
(ii)	What is the overall order of the reaction?
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[1]
(iii)	Write the rate equation for this reaction.
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[1]

(b)	For an initial concentration of ozone of 0.50 × 10^–7 mol dm^–3 and one of ethene of 1.0 × 10^–8 mol dm^–3, the initial rate of methanal formation was
1.0 × 10^–12 mol dm^–3 s^–1.
(i)	How could the initial rate of methanal formation be measured from a concentration/time graph?
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[2]
(ii)	Calculate the value of the rate constant and state the units.

rate constant =.......................... units....................................
[3]
(iii)	The initial rate of methanal formation is different from that of oxygen formation in equation 1.
	Explain why.
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[1]
(iv)	The experiment was repeated but at a higher temperature. What would be the effect of this change on the rate and the rate constant of the reaction?
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[2]
[Total 11 marks]

5.	The preparation of hydrogen iodide, HI(g), from hydrogen and iodine gases is a reversible reaction which reaches equilibrium at constant temperature.
		H_2(g) + I_2(g)  2HI(g)
(a)	Write the expression for K_c for this equilibrium.

[1]
 (b)	A student mixed together 0.30 mol H_2(g) with 0.20 mol I_2(g) and the mixture was allowed to reach equilibrium. At equilibrium, 0.14 mol H_2(g) was present.
(i)	Complete the table below to show the amount of each component in the equilibrium mixture.
component | H_2(g) | I_2(g) | HI(g)
initial amount / mol | 0.30 | 0.20 | 0
equilibrium amount / mol | | |
[2]
 (ii)	Calculate K_c to an appropriate number of significant figures. State the units, if any.

K_c = ...........................................
units, if any ...........................................................................................
[3]
(c)	The student compressed the equilibrium mixture so that its volume was reduced. The temperature was kept constant.
	Comment on the value of K_c and the composition of the equilibrium mixture under these new conditions.
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[2]
[Total 8 marks]
6.	When heated, phosphorus pentachloride, PCl_5, dissociates.
		PCl_5(g)  PCl_3(g) + Cl_2(g)
	A chemist placed a mixture of the three gases into a container. The initial concentration of each gas was the same: 0.30 mol dm^–3. The container was left until equilibrium had been reached.
	Under these conditions, K_c = 0.245 mol dm^–3.
(a)	Write an expression for K_c for this equilibrium.

[1]
(b)	Use the value of K_c for this equilibrium to deduce whether the concentration of each gas increases, decreases or stays the same as the mixture approaches equilibrium.
(i)	Show your answer by placing a tick in the appropriate cells in the table below.
| initial concentration / mol dm^–3 | greater than 0.30 mol dm^–3 | less than 0.30 mol dm^–3 | equal to 0.30 mol dm^–3
PCl_5 | 0.30 | | |
PCl_3 | 0.30 | | |
Cl_2 | 0.30 | | |
[1]
 (ii)	Explain your deduction.
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(c)	The chemist compressed the equilibrium mixture at constant temperature and allowed it to reach equilibrium under these new conditions.
(i)	Explain what happens to the value of K_c.
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(ii)	Explain what happened to the composition of the equilibrium mixture.
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 (d)	The chemist heated the equilibrium mixture and the equilibrium moved to the left.
(i)	Explain what happens to the value of K_c.
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(ii)	Explain what additional information this observation reveals about the reaction.
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[Total 9 marks]

Paddington Academy