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Chemical Reactions - Reversible reactions and equilibrium

Grade 12IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A reversible reaction is one where the products can react together to reform the reactants, indicated by the symbol ⇌\rightleftharpoons.

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Dynamic equilibrium occurs in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction, resulting in constant concentrations of reactants and products.

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Le Chatelier's Principle states that if a change in conditions (concentration, temperature, or pressure) is applied to a system at equilibrium, the system moves to counteract that change.

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Increasing the concentration of a reactant shifts the equilibrium to the right to produce more products; removing a product has the same effect.

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In gaseous reactions, increasing the pressure shifts the equilibrium towards the side with the smaller number of moles of gas, as shown by the coefficients in the balanced equation.

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Increasing the temperature shifts the equilibrium in the direction of the endothermic reaction (ΔH>0\Delta H > 0) to absorb the added heat.

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A catalyst increases the rate of both the forward and reverse reactions equally, allowing equilibrium to be reached faster without changing the equilibrium position or the yield.

📐Formulae

aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dD

N2(g)+3H2(g)⇌2NH3(g)ΔH=−92 kJ/molN_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92\text{ kJ/mol}

2SO2(g)+O2(g)⇌2SO3(g)ΔH=−197 kJ/mol2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -197\text{ kJ/mol}

CH3COOH(l)+C2H5OH(l)⇌CH3COOC2H5(l)+H2O(l)CH_3COOH(l) + C_2H_5OH(l) \rightleftharpoons CH_3COOC_2H_5(l) + H_2O(l)

💡Examples

Problem 1:

For the Haber Process reaction N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) where ΔH=−92 kJ/mol\Delta H = -92\text{ kJ/mol}, predict the effect of decreasing the temperature on the yield of NH3NH_3.

Solution:

The yield of NH3NH_3 will increase.

Explanation:

The forward reaction is exothermic (ΔH\Delta H is negative). According to Le Chatelier's Principle, decreasing the temperature will cause the equilibrium to shift in the direction that produces heat. Therefore, the equilibrium shifts to the right, increasing the production of NH3NH_3.

Problem 2:

In the reaction 2NO2(g)⇌N2O4(g)2NO_2(g) \rightleftharpoons N_2O_4(g), what happens to the equilibrium position if the volume of the container is decreased?

Solution:

The equilibrium shifts to the right, favoring the formation of N2O4N_2O_4.

Explanation:

Decreasing the volume increases the total pressure. The system counteracts this by shifting to the side with fewer moles of gas. There are 22 moles of gas on the left (2NO22NO_2) and only 11 mole on the right (1N2O41N_2O_4), so the equilibrium moves to the right.

Reversible reactions and equilibrium Revision - Grade 12 Chemistry IGCSE