A2 LEVEL CHEMISTRY

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Max 53 marks

	
Name		…………………………………………………………..

Mark		……../53	……....%	Grade ………

1.	1, 2-Dibromoethane, C2H4Br2, reacts with potassium iodide as shown in the equation below.
C2H4Br2 + 3KI → C2H4 + 2KBr + Kl3
	A series of experiments was carried out to investigate the kinetics of this reaction.
(a)	In a first experiment the concentration of C2H4Br2 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 C2H4Br2.
(i)	On the axes below, sketch a graph to show how [C2H4Br2] changed during the course of the reaction.

[2]
(ii)	Show on the graph how you would measure the initial rate of the reaction.
[1]
(iii)	How would you use the graph to show that the reaction is first order with respect to C2H4Br2?
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[1]

(iv)	The experiment was repeated using different initial concentrations of C2H4Br2.
	Using the axes below, sketch a graph to show how the initial rate of the reaction changes with different concentrations of C2H4Br2.

[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	[C2H4Br2]
/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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[2]
(c)	(i)	Construct the rate equation for the reaction.
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[1]
(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) + 2H2(g) → N2(g) + 2H2O(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	[H2]
/ 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, (CH3)2C=CH2, with dilute aqueous acid to form 2-methylpropan-2-ol, (CH3)3COH.
	The following mechanism has been proposed for this hydration.
step 1	(CH3)2C=CH2 + H+(aq) → (CH3)3C+	rate determining step
step 2	(CH3)3C+ + H2O → (CH3)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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[1]
 (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, O3, with ethene. The smog contains methanal, CH2O(g), and the equation for its production is shown below.
		O3(g) + C2H4(g) ® 2CH2O(g) + O2(g)	equation 1
(a)	The rate of the reaction doubles when the initial concentration of either O3(g) or C2H4(g) is doubled.
(i)	What is the order of reaction with respect to
O3 .............................
C2H4? .......................
[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.
		H2(g) + I2(g)  2HI(g)
(a)	Write the expression for Kc for this equilibrium.

[1]
 (b)	A student mixed together 0.30 mol H2(g) with 0.20 mol I2(g) and the mixture was allowed to reach equilibrium. At equilibrium, 0.14 mol H2(g) was present.
(i)	Complete the table below to show the amount of each component in the equilibrium mixture.
component	H2(g)	I2(g)	HI(g)	
initial amount / mol	0.30	0.20	0	
equilibrium amount / mol				
[2]
 (ii)	Calculate Kc to an appropriate number of significant figures. State the units, if any.

Kc = ...........................................
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 Kc and the composition of the equilibrium mixture under these new conditions.
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[2]
[Total 8 marks]
6.	When heated, phosphorus pentachloride, PCl5, dissociates.
		PCl5(g)  PCl3(g) + Cl2(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, Kc = 0.245 mol dm–3.
(a)	Write an expression for Kc for this equilibrium.

[1]
(b)	Use the value of Kc 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	
PCl5	0.30				
PCl3	0.30				
Cl2	0.30				
[1]
 (ii)	Explain your deduction.
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[1]

(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 Kc.
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[1]
(ii)	Explain what happened to the composition of the equilibrium mixture.
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[2]
 (d)	The chemist heated the equilibrium mixture and the equilibrium moved to the left.
(i)	Explain what happens to the value of Kc.
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[1]
(ii)	Explain what additional information this observation reveals about the reaction.
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[2]
[Total 9 marks]