Senior Secondary (HKDSE) · Chemistry
5 multiple-choice questions marked as you go, and 3 written questions with worked solutions. All on Reversible reactions; dynamic equilibrium.
Consider the gaseous system at dynamic equilibrium in a sealed container at a constant temperature:
\( H_2(g) + I_2(g) \rightleftharpoons 2HI(g) \)
Which of the following observations provides the strongest evidence that the equilibrium is dynamic rather than static?
Consider the concentration-time graph for the gaseous equilibrium system: \( PCl_{5}(g) \rightleftharpoons PCl_{3}(g) + Cl_{2}(g) \). At time \( t_{1} \), the volume of the reaction vessel is suddenly decreased while keeping the temperature constant. Which of the following describes the behavior of the system immediately after \( t_{1} \) and the subsequent shift to a new equilibrium?
Consider the following gaseous equilibrium occurring at high temperature:
\( 4NH_{3}(g) + 5O_{2}(g) \rightleftharpoons 4NO(g) + 6H_{2}O(g) \)
What is the unit of the equilibrium constant, \( K_{c} \), for this reaction?
A mixture of \( SO_{2}(g) \), \( O_{2}(g) \), and \( SO_{3}(g) \) is at equilibrium in a container of volume \( V \) at temperature \( T \). The reaction is represented by:
\( 2SO_{2}(g) + O_{2}(g) \rightleftharpoons 2SO_{3}(g) \quad \Delta H = -198 \, kJ \, mol^{-1} \)
If the volume of the container is increased to \( 2V \) and the temperature is simultaneously increased to \( T + 50 \, K \), which of the following will occur once a new equilibrium is established?
The decomposition of solid ammonium carbamate follows the equation below:
\( NH_{2}COONH_{4}(s) \rightleftharpoons 2NH_{3}(g) + CO_{2}(g) \)
A sample of the solid is placed in an evacuated, sealed container at a constant temperature. Which of the following statements is CORRECT when the system reaches dynamic equilibrium?
Consider the reversible gaseous reaction: \(2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)\), where the forward reaction is exothermic.
Predict and explain the combined effect on the equilibrium yield of sulfur trioxide (\(SO_3\)) and the value of the equilibrium constant (\(K_c\)) if the temperature is simultaneously decreased and the total pressure of the system is increased by reducing its volume.
Write your answer out first, then check it against the worked solution.
Phosgene (\(\text{COCl}_2\)) is a vital industrial precursor for the synthesis of polyurethanes and polycarbonates. The production of phosgene involves the following reversible reaction:
\(\text{CO(g)} + \text{Cl}_2\text{(g)} \rightleftharpoons \text{COCl}_2\text{(g)}\)
In a laboratory investigation, 0.40 mol of \(\text{CO(g)}\) and 0.80 mol of \(\text{Cl}_2\text{(g)}\) were introduced into an evacuated sealed container of volume 2.0 \(\text{dm}^3\) at a constant temperature \(T_1\). When the system reached equilibrium, the concentration of \(\text{COCl}_2\text{(g)}\) was found to be 0.15 \(\text{mol dm}^{-3}\).
(a) Calculate the equilibrium constant \(K_c\) for this reaction at \(T_1\), stating its units. (3 marks)
(b) If the volume of the reaction container was suddenly reduced to 1.0 \(\text{dm}^3\) while maintaining the temperature at \(T_1\), explain the effect on the equilibrium yield of \(\text{COCl}_2\text{(g)}\) using Le Chatelier's Principle. (2 marks)
(c) The formation of phosgene is an exothermic process (\(\Delta H < 0\)). The temperature of the system was subsequently changed to a new temperature \(T_2\). At the new equilibrium, it was observed that the percentage decomposition of \(\text{COCl}_2\text{(g)}\) had increased. State and explain whether \(T_2\) is higher or lower than \(T_1\), and predict how the value of \(K_c\) would change compared to its value at \(T_1\). (3 marks)
Write your answer out first, then check it against the worked solution.
The industrial synthesis of methanol involves the following reversible reaction:
\(\text{CO(g)} + 2\text{H}_2\text{(g)} \rightleftharpoons \text{CH}_3\text{OH(g)}\) \(\Delta H = -91 \text{ kJ mol}^{-1}\)
In a laboratory investigation, 2.00 mol of \(\text{CO(g)}\) and 5.00 mol of \(\text{H}_2\text{(g)}\) were introduced into an evacuated sealed container of volume 2.00 \(\text{dm}^3\) at a constant temperature \(T_1\). When the system reached equilibrium, the amount of \(\text{CH}_3\text{OH(g)}\) was found to be 0.80 mol.
(a) Calculate the equilibrium constant \(K_c\) for the reaction at temperature \(T_1\), stating its units. (4 marks)
(b) If the volume of the reaction container was suddenly reduced to 1.00 \(\text{dm}^3\) while maintaining the temperature at \(T_1\), explain the effect on the equilibrium yield of \(\text{CH}_3\text{OH(g)}\) using Le Chatelier's Principle. (2 marks)
(c) The temperature of the system was subsequently increased from \(T_1\) to a higher temperature \(T_2\). State and explain how the value of the equilibrium constant \(K_c\) would change compared to its value at \(T_1\). (2 marks)
Write your answer out first, then check it against the worked solution.
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