Edexcel A-Level Chemistry Paper 3, June 2017: Question 9

16 marks · Hard difficulty · Calculations

Calculate the equilibrium constant Kc for the gas-phase reaction between hydrogen and iodine, process experimental data, plot graphs, determine gradients, and analyze the effects of temperature and volume changes on equilibrium.

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Question

Question text

9 The gas phase reaction between hydrogen and iodine is reversible.

H2(g) + I2(g) U 2HI(g)

(a) (i) Write the expression for the equilibrium constant, Kc, for this reaction.

(1)

(ii) If the starting concentration of both hydrogen and iodine was a mol dm−3

and it was found that 2y mol dm−3 of hydrogen iodide had formed once

equilibrium had been established, write the Kc expression in terms of a and y.

(2)

(b) The expression for the equilibrium constant in (a)(ii) can be rearranged as shown.

aKc

y =

2 + Kc

In an experiment, air was removed from a 1 dm3 flask and amounts of hydrogen

and iodine gases were mixed together such that their initial concentrations

were both a mol dm−3. This mixture was allowed to reach equilibrium at 760 K.

The equilibrium concentration of iodine was then measured.

The experiment was repeated for various initial concentrations, a mol dm−3, and

the results recorded in the table.

(i) Complete the table to give the two remaining values of y mol dm−3, to two

decimal places.

(1)

a / mol dm−3 [I ] / mol dm−3 y / mol dm−3

2 eq

0.20 0.02 0.18

0.80 0.25 0.55

1.50 0.37

2.10 0.57 1.53

2.80 0.65 2.15

3.80 0.87

4.90 1.15 3.75

(ii) Plot a graph to show how*P48954A02936*ymol dm−3varies with the initial concentrations of

hydrogen and iodine, a mol dm−3.

(2)

4.0

3.0

y / mol dm–3

2.0

1.0

01.0 2.0 3.0 4.0 5.0

a / mol dm–3

(iii) Determine the gradient of your graph.

Show your working on the graph.

(2)

*P48954A03036*

aKc

(iv) Use your answer to (b)(iii) and the expression y = to calculate the

value of K . 2 + Kc

(2)

(1)

(d) One of the experiments in part (b) was repeated using the same molar quantities

of hydrogen and iodine but in a 500 cm3 flask instead of the 1 dm3 flask.

Deduce the effect, if any, that this would have on the rate of reaction and on the

value of Kc calculated.

(2)

(e) The equation for the reaction between hydrogen and iodine is

H (g) + I (g) U 2HI(g) ǻH = –9.6 kJ mol−1

22 31

(i) Explain the effect, if any, on the value of*P48954A03136*Kcwhen the temperature is increased.

(2)

(ii) On your graph in (b)(ii), draw and label the line you would expect if the experiment

was carried out at 1000 K instead of 760 K.

(1)

(Total for Question 9 = 16 marks)

Mark scheme

Show the mark scheme

9(a)(i) 2 Ignore missing state symbols or units (1)

(Kc =) [HI(g)]

[H2(g)][I2(g)] Do not award round brackets

Question

Acceptable Answers Additional Guidance Mark

Number

9(a)(ii) (K =) 4y2 Allow square brackets (2)

(a – y)2

Numerator term correct Allow (2y)2

(1)

Denominator term correct

(1) Allow (a2 – 2ay+y2) or (a-y)(a-y)

Question

Acceptable Answers Additional Guidance Mark

Number

9(b)(i) both values correct to 2 DP 1.13 (1)

2.93

Question

Acceptable Answers Additional Guidance Mark

Number

9(b)(ii) All 7 points plotted correctly Allow TE for incorrect values from 9(b)(i) (2)

(1)

Appropriate straight line of best fit, drawn through the Do not allow all points above or below the

origin line of best fit

(1) Allow line of best fit to intersect one square

either side of the origin

Question

Acceptable Answers Additional Guidance Mark

Number

9(b)(iii) co-ordinates correctly read off the line on graph At least 1 line must be shown on the graph (2)

(1) to indicate selection of co-ordinates

gradient correctly calculated

Example of calculation

(1)

3.40 – 0.00 = gradient of graph

4.50 – 0.00

Gradient = 0.76

Ignore SF except 1SF

Do not allow units for the gradient

Allow a value from 0.71 to 0.81 inclusive

Question

Acceptable Answers Additional Guidance Mark

Number

9(b)(iv) √Kc = gradient / y Example of calculation (2)

2 + √Kc a

(1) √Kc = 0.76

re-arrangement of expression and calculation of Kc 2 + √Kc

(1) Kc = 40.1 /40 (no units)

Allow TE on gradient from part (b)(iii)

K = [(2 x grad)/(1-grad)]2

Correct answer with no working scores (2)

Question

Acceptable Answers Additional Guidance Mark

Number

9(c) hydrogen is flammable / explosive Allow iodine vapour damages eyes /toxic (1)

Allow hydrogen iodide is corrosive / acidic /

irritant (if qualified) / lachrymator

Ignore references to high pressure

Ignore references to safety precautions

Question

Acceptable Answers Additional Guidance Mark

Number

9(d) Faster rate of reaction / increased rate Ignore references to shifting position of (2)

(1) equilibrium

Kc unchanged

(1)

Question

Acceptable Answers Additional Guidance Mark

Number

9(e)(i) An explanation that makes reference to the following points: (2)

(Kc is) smaller / decreases / gets less (1)

(forward) reaction is exothermic (1) Allow reverse/backwards reaction is

endothermic

MP2 dependent on MP1

Question

Acceptable Answers Additional Guidance Mark

Number

9(e)(ii) straight line drawn on the graph with a less steep Do not allow if lines cross (1)

gradient (and goes through the origin)

(Total for Question 9 = 16 marks)

TOTAL FOR PAPER = 120 MARKS

How to answer it

Equilibrium Constants and Graphical Analysis Study Guide

What this question tests

This multi-step question assesses your mastery of chemical equilibria (Kc expressions, ICE tables, and algebraic manipulation), graphical analysis (plotting, determining gradients, and linking experimental data to theoretical equations), reaction kinetics (effects of volume changes), and thermodynamics (Le Chatelier's principle and temperature effects on Kc).

Question 9(a)

Equilibrium Expressions & ICE Derivations

✅ Correct Answers

(a)(i): Kc = [HI]² / ([H₂][I₂])
(a)(ii): Kc = 4y² / (a - y)² or (2y)² / (a - y)²

💡 Key Knowledge

Construct Kc expressions using stoichiometric coefficients as powers.
Set up an Initial, Change, Equilibrium (ICE) table using algebraic terms ( a and y ).

Mark Breakdown: (a)(i) 1 mark for correct expression. (a)(ii) 2 marks (1 for correct numerator, 1 for correct denominator). Note: Round brackets around terms are penalized.

Question 9(b)

Data Processing, Graphing, and Calculating Kc

📐 Calculations & Table Completion

Missing Table Values (9b(i)): Calculate remaining y values using the given rearranged formula to 2 decimal places:
Row 3 ( a = 1.50 ): 1.13
Row 6 ( a = 3.80 ): 2.93
Gradient Determination (9b(iii)): Construct a large triangle on your line of fit.
Example: (3.40 - 0.00) / (4.50 - 0.00) = 0.76 (Acceptable range: 0.71 to 0.81).
Calculating Kc (9b(iv)): Equate gradient to the algebraic term:
√Kc / (2 + √Kc) = 0.76
Rearranging yields Kc = 40.1 or 40 (No units required).

🧠 Exam Technique

Graphing (9b(ii)): Ensure all 7 points are plotted accurately and draw a straight line of best fit forcing or passing cleanly through the origin.
Gradient Construction (9b(iii)): Examiners explicitly look for construction lines on the graph grid to prove coordinate selection.
Significant Figures: Match data precision (2 decimal places requested in table).

❌ Common Errors

Forgetting to show triangle coordinate selection lines on the printed graph.
Algebraic rearrangement slips when solving for √Kc in part (iv).
Inconsistent rounding during table calculations.

Mark Breakdown: (b)(i) 1 mark. (b)(ii) 2 marks (plotting + line of best fit). (b)(iii) 2 marks (coordinates read + gradient calculation). (b)(iv) 2 marks (equating expression + final calculation).

Question 9(c) & (d)

Safety, Kinetics, and Volume Changes

✅ Correct Answers: Safety (c)

Hydrogen is flammable / explosive.
(Note: Iodine vapour damages eyes/is toxic is also accepted).

✅ Correct Answers: Volume Change (d)

Rate of reaction: Faster rate / increased rate (due to higher concentration/collision frequency).
Value of Kc: Unchanged (Kc only changes with temperature).

Mark Breakdown: (c) 1 mark for hazard. (d) 2 marks (1 for rate, 1 for Kc). Note: Ignore references to shifting equilibrium position for part (d).

Question 9(e)

Thermodynamics & Temperature Effects

💡 Key Knowledge (Le Chatelier's Principle)

Given ΔH = -9.6 kJ mol⁻¹ , the forward reaction is exothermic.
Increasing temperature shifts equilibrium in the endothermic direction (backwards), decreasing the concentration of products relative to reactants. Therefore, Kc decreases.

🧠 Sketching Graphs (9e(ii))

Draw a straight line passing through the origin with a less steep gradient than the 760 K line, reflecting the lower equilibrium yield at higher temperatures for an exothermic reaction.

Mark Breakdown: (e)(i) 2 marks (1 for Kc decreases, 1 for forward reaction is exothermic, dependent on point 1). (e)(ii) 1 mark for less steep straight line through origin.

Topics

Physical Chemistry · Topic 10: Equilibrium I · Topic 11: Equilibrium II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.