2. (a) (i) Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction - CIE - A-Level Chemistry - Question 2 - 2015 - Paper 1
Question 2
2. (a) (i) Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction.
H2(g) + I2(g) ⇌ 2HI(g)
(ii) If the starting concentrati... show full transcript
Worked Solution & Example Answer:2. (a) (i) Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction - CIE - A-Level Chemistry - Question 2 - 2015 - Paper 1
Step 1
Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction.
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Answer
For the reaction:
[ H_2(g) + I_2(g) \rightleftharpoons 2HI(g) ]
The equilibrium constant, Kc, is expressed as:
[ K_c = \frac{[HI]^2}{[H_2][I_2]} ]
Step 2
If the starting concentration of both iodine and hydrogen was y mol dm⁻³ and it was found that 2y mol dm⁻³ of hydrogen iodide had formed once equilibrium had been established, write Kc in terms of a and y.
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Answer
Let the initial concentrations of hydrogen and iodine be ( y ) mol dm⁻³. After reaching equilibrium:
Change in concentration of hydrogen and iodine will be ( y - y ) (decreasing by y)
The concentration of HI formed is ( 2y ).
Thus:
At equilibrium, [ [H_2] = y - y = 0 \
[ I_2] = y - y = 0 \
[HI] = 2y. ]
This gives us:
[ K_c = \frac{(2y)^2}{(y-y)(y-y)} ]
Since the concentrations of ( H_2 ) and ( I_2 ) effectively become 0, their concentrations need to be considered as hardly affecting the equilibrium constant at high concentrations.
Step 3
Complete the table of the values of y mol dm⁻³ to three decimal places.
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Fill in the missing value for y based on the provided data in the table. Ensure that all results are rounded to three decimal places as necessary.
Step 4
Plot a graph to show how y mol dm⁻³ varies with initial concentrations of hydrogen and iodine, a mol dm⁻³.
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Using the values from the completed table, plot the graph with 'a mol dm⁻³' on the x-axis and 'y mol dm⁻³' on the y-axis. Mark each point based on the corresponding values.
Step 5
Determine the slope of your graph. State the co-ordinates of both points used for your calculation.
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To determine the slope,
Select two clear points on the best fit line of your graph.
Let these points be ( (x_1, y_1) ) and ( (x_2, y_2) ).
The slope is calculated as:
[ \text{slope} = \frac{y_2 - y_1}{x_2 - x_1} ].
Record the coordinates and the slope to three significant figures.
Step 6
Use the value of your slope and the equation in (b) to calculate the value of Kc.
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Substitute the calculated slope into the equilibrium constant equation derived earlier. Ensure to show all working steps clearly for full credit.
Step 7
Explain why, for safety reasons, it is necessary to remove air from the 1 dm³ flask.
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Removing air prevents the introduction of oxygen, which can be explosive in conjunction with hydrogen. This ensures safety during the reaction, avoiding any potential hazards.
Step 8
What effect, if any, would this have on the rate of reaction and the value of Kc measured?
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Using a smaller flask may increase the rate of reaction due to increased concentration of reactants; however, Kc remains constant as it is a ratio at equilibrium and is not influenced by changes in total volume, provided the temperature remains consistent.
Step 9
On your graph, draw and label the line you would expect if the experiment were performed at 1000 K instead of 760 K.
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At a higher temperature, the equilibrium will shift according to Le Châtelier's principle. Draw a new line that is steeper than the original, indicating decreased y values for the same a values, since the equilibrium shifts to favor the endothermic reaction at elevated temperatures.
Step 10
What effect, if any, would the higher temperature have on the value of Kc?
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At higher temperatures, for an exothermic reaction, the value of Kc decreases as the equilibrium shifts to favor the reactants according to Le Châtelier's principle.
