Exit to H2 Chemistry (9476): Chemical Equilibria

Structured Practice: Chemical Equilibria (A Level)

Short, exam-style questions to practise Le Chatelier reasoning, Kc/Kp expressions, and ICE-style calculations.

GCE A-Level H2 Chemistry 9476-2027

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Dynamic equilibrium language.

Part a: State what is equal at dynamic equilibrium. (1 mark)
Part b: State one condition needed for equilibrium to be established. (1 mark)

Le Chatelier on the Haber equilibrium. $\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$, forward reaction exothermic.

Part a: Predict the equilibrium shift when the mixture is compressed to increase pressure at constant temperature. (1 mark)
Part b: Predict the equilibrium shift when temperature is increased. (1 mark)
Part c: State whether the value of $K_c$ changes when pressure changes at constant temperature. (1 mark)

$K_c$ expression and value. For $\ce{2SO2(g) + O2(g) <=> 2SO3(g)}$, an equilibrium mixture has $[\ce{SO3}]=0.60$, $[\ce{SO2}]=0.40$, $[\ce{O2}]=0.20$ mol dm$^{-3}$.

Part a: Write the $K_c$ expression. (1 mark)
Part b: Calculate $K_c$. (1 mark) Enter the final answer here and show your method in the following field.: Include the unit dm3/mol.
Working for part b (1 mark)

ICE-style equilibrium composition. For $\ce{A(g) <=> B(g)}$, $K_c=4.0$ at this temperature. Initially, $[\ce{A}]=1.00$, $[\ce{B}]=0$ mol dm$^{-3}$. The vessel volume is constant.

Part a: Calculate $[\ce{B}]$ at equilibrium. (1 mark) Enter the final answer here and show your method in the following field.: Include the unit mol/dm3.
Working for part a (2 marks)
Part b: State what the magnitude of this $K_c$ value implies about equilibrium position. (1 mark)

$K_p$ and omitted species. Consider $\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$.

Part a: Write the $K_p$ expression. (1 mark)
Part b: State why solids are omitted from $K$ expressions. (1 mark)

Compressing the dinitrogen tetroxide equilibrium. $\ce{N2O4(g) <=> 2NO2(g)}$ is at equilibrium in a sealed gas syringe with total pressure $p_0$. The volume is suddenly halved, and the temperature is then kept constant while a new equilibrium is established.

Part a: What happens to the partial pressures immediately after the volume is halved, before any reaction occurs? (1 mark)
Part b: By what factor does the ratio $p_{\ce{NO2}}^2/p_{\ce{N2O4}}$ change immediately after compression? (1 mark)
Part c: In which direction does the position of equilibrium then shift? (1 mark)
Part d: How does the total pressure at the new equilibrium compare with $p_0$? (1 mark)
Part e: What happens to the value of $K_p$? (1 mark)
Part f: Explain, in terms of the rates of the forward and reverse reactions, how the new equilibrium is reached. (2 marks)

$K_p$ for the dissociation of phosphorus(V) chloride. $\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$ At a fixed temperature, an equilibrium mixture has partial pressures $p_{\ce{PCl5}}=40.0$ kPa, $p_{\ce{PCl3}}=60.0$ kPa and $p_{\ce{Cl2}}=60.0$ kPa.

Part a: Which is the $K_p$ expression for this equation? (1 mark)
Part b: Calculate $K_p$ for this equation, in kPa. (1 mark)
Part c: Calculate the equilibrium constant, in terms of partial pressures, for $\ce{2PCl5(g) <=> 2PCl3(g) + 2Cl2(g)}$ at the same temperature, in kPa$^2$. (1 mark)
Part d: Calculate the equilibrium constant, in terms of partial pressures, for $\ce{PCl3(g) + Cl2(g) <=> PCl5(g)}$ at the same temperature, in kPa$^{-1}$. (1 mark)
Part e: The equilibrium mixture is the same in parts b and c. Why do the two constants differ? (1 mark)
Part f: More chlorine is added at constant temperature and a new equilibrium is established. What happens to $K_p$ for the original equation? (1 mark)

Equilibrium concentrations from one measured value. $\ce{A(g) + B(g) <=> C(g)}$ In a sealed vessel of constant volume at a fixed temperature, the initial concentrations are $[\ce{A}]=0.600$, $[\ce{B}]=0.400$ and $[\ce{C}]=0$ mol dm$^{-3}$. At equilibrium, $[\ce{C}]=0.250$ mol dm$^{-3}$.

Part a: Calculate $[\ce{A}]$ at equilibrium, in mol dm$^{-3}$. (1 mark)
Part b: Calculate $[\ce{B}]$ at equilibrium, in mol dm$^{-3}$. (1 mark)
Working for parts a and b: show how the change in each concentration follows from the equation. (1 mark)
Part c: Calculate $K_c$ at this temperature, in dm$^3$ mol$^{-1}$. (1 mark)
Part d: A second mixture in the same vessel at the same temperature has $[\ce{A}]=0.200$, $[\ce{B}]=0.100$ and $[\ce{C}]=0.0500$ mol dm$^{-3}$. What happens as it reaches equilibrium? (1 mark)

Evaluating proposed Haber process conditions. $\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$ $\quad \Delta H=-92\ \text{kJ mol}^{-1}$ A plant operator proposes running the iron-catalysed converter at about 50 °C and 1000 atm, because both conditions give a higher equilibrium yield of ammonia than the conditions currently used.

Part a: What is the main drawback of operating at about 50 °C? (1 mark)
Part b: Why is a pressure of about 200 atm used rather than 1000 atm? (1 mark)
Part c: What does the iron catalyst do? (1 mark)
Part d: Only about 15% of the nitrogen and hydrogen is converted in each pass through the converter. How does the plant still convert most of its feed into ammonia? (1 mark)
Part e: Evaluate the operator's proposal, and justify the temperature and pressure actually used. (3 marks)

Structured Practice: Chemical Equilibria (A Level)

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