Ch 3 · Reversible Reactions and Equilibrium · O Level Chemistry 5070
Topic 3 of 4 in Chemical Reactions — O Level Chemistry 5070 — 48 lessons.
Introduction
Not every reaction runs to completion. Some go backwards as easily as forwards, and in a sealed container they settle into a balance called equilibrium. Making ammonia and sulfuric acid depends on choosing conditions that push this balance towards the product without making the reaction too slow. Questions here are usually six-mark explanations, so the reasoning must be complete.
Key ideas — Reversible reactions
Some chemical reactions are reversible: the products can react to re-form the reactants. This is shown with the leftharpoons symbol, and changing the conditions can change the direction of the reaction.
🧑🏫 Teacher's explanation
Imagine a busy revolving door at a hotel. People are walking in (reactants becoming products) and people are walking out (products becoming reactants) at the same time. If the door is spinning very fast in one direction, more people enter than leave. But if the door spins at the same speed in both directions, the number of people inside the hotel stays constant, even though individuals are constantly moving through the door. This is the essence of a reversible reaction.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass (M_r) of hydrated copper(II) sulfate, CuSO_4 · 5H_2O, given A_r values: Cu=64, S=32, O=16, H=1.
🔍 Worked example — Standard
A reaction is endothermic in the forward direction: A(s) + heat leftharpoons B(g). If the temperature is increased, which way does the equilibrium shift?
🔍 Worked example — Exam-style
In the reaction mathrmN_2(g) + 3H_2(g) leftharpoons 2NH_3(g) + heat, how would increasing the pressure affect the yield of NH_3?
⚠️ Common errors
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🎯 Exam question
Key ideas — Dynamic equilibrium
the definition. A reversible reaction in a closed system (nothing can enter or leave) is at equilibrium when the rates of the forward and reverse reactions are equal and the concentrations of reactants and products are no longer changing. Both reactions continue, so the equilibrium is dynamic, but the amounts present stay constant. The position of equilibrium describes whether it lies to the left (mostly reactants) or the right (mostly products).
🧑🏫 Teacher's explanation
Imagine you are walking on an escalator that is moving downwards. If you walk upwards at the exact same speed the escalator moves down, you stay in the same spot. To an observer, you aren't moving, even though you are working hard and the escalator is moving. This is 'dynamic equilibrium'.
📖 Definitions
🔍 Worked example — Easy
In a reversible reaction, the rate of the forward reaction is 0.05 mol/dm^3s and the rate of the reverse reaction is 0.05 mol/dm^3s. Is the system at equilibrium? Explain.
🔍 Worked example — Standard
Consider the reaction ceN2(g) + 3H2(g) leftharpoons 2NH3(g). If the concentration of ceNH3 is increasing, what is happening to the rate of the reverse reaction?
🔍 Worked example — Exam-style
A reaction ceA(g) + B(g) leftharpoons C(g) is at equilibrium. If the volume of the container is decreased, what happens to the position of equilibrium? (Assume the forward reaction has fewer moles of gas).
⚠️ Common errors
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🎯 Exam question
Key ideas — Changing the position of equilibrium
predicting the shift. When a condition changes, the position of equilibrium moves in the direction that opposes the change.
🧑🏫 Teacher's explanation
Imagine a busy shop where people are constantly entering and leaving through the front and back doors. Equilibrium is like the moment when the number of people entering equals the number of people leaving. Even though people are moving, the total number of people inside stays constant.
📖 Definitions
🔍 Worked example — Easy
In the reaction ceN2(g) + 3H2(g) leftharpoons 2NH3(g) + heat, what is the effect of increasing the temperature on the yield of ammonia?
🔍 Worked example — Standard
Consider the reaction cePCl5(g) leftharpoons PCl3(g) + Cl2(g). If the pressure of the system is increased, in which direction will the equilibrium shift?
🔍 Worked example — Exam-style
A reaction is at equilibrium in a closed container. If the concentration of a reactant is doubled, how does this affect the rate of the forward reaction and the position of equilibrium?
⚠️ Common errors
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🎯 Exam question
(a) Predict and explain the effect of increasing the pressure on the yield of SO_3. [2] (b) Predict and explain the effect of increasing the temperature on the yield of SO_3. [2] [4 marks]
Key ideas — The Haber process and the Contact process
N_2(g) + 3H_2(g) leftharpoons 2NH_3(g) forward reaction exothermic
🧑🏫 Teacher's explanation
Imagine a busy shop where people are constantly entering and leaving. If the number of people entering equals the number of people leaving, the total number of people inside stays the same, even though individuals are moving. This is 'equilibrium'. In chemistry, we call this a 'closed system'—a container where nothing can escape, so the reactants and products just keep swapping back and forth.
📖 Definitions
🔍 Worked example — Easy
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g) is exothermic. If the temperature is increased, what happens to the yield of NH_3?
🔍 Worked example — Standard
Calculate the change in moles of gas for the reaction N_2(g) + 3H_2(g) leftharpoons 2NH_3(g) when moving from reactants to products.
🔍 Worked example — Exam-style
In the Contact process, 2SO_2(g) + O_2(g) leftharpoons 2SO_3(g) is exothermic. If the pressure is increased, how does the position of equilibrium shift and why?
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Ammonia is manufactured by the Haber process at 450 °C and 200 atm using an iron catalyst. (a) Explain why a higher temperature would give a lower yield of ammonia. [2] (b) Explain why the process is not run at a much lower temperature, even though the yield would be higher. [1] (c) Explain why a high pressure is used. [2] (d) State the effect of the iron catalyst on the position of equilibrium. [1]
🔍 Worked example — Easy
In the reaction N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), the forward reaction is exothermic. If the temperature is increased, what happens to the yield of NH_3?
🔍 Worked example — Standard
A reaction A_(g) + B_(g) leftharpoons C_(g) has a Δ H = -50 kJ/mol. If the pressure is increased, which way does the equilibrium shift?
🔍 Worked example — Exam-style
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), the equilibrium constant K_c is calculated. If the concentration of N_2 is doubled while keeping temperature and volume constant, how does the concentration of NH_3 at equilibrium change?
⚠️ Common errors
Common mistakes
Practice
Answers Pink solid turns blue on heating [B1]; turns pink again when water is added [B1]; CoCl_2 · 6H_2O leftharpoons CoCl_2 + 6H_2O [B1]. Rate of forward reaction equals rate of reverse reaction [B1]; concentrations of reactants and products no longer change [B1]. Equilibrium shifts to the right, increasing the yield of SO_3 [B1]; because there are fewer moles of gas on the right (2 compared with 3) [B1]. Sulfur dioxide from burning sulfur or from sulfide ores [B1]; oxygen from the air [B1]; 450 °C and 200 kPa (2 atm) [B1]; vanadium(V) oxide catalyst [B1].