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2.3 Extent of chemical change

Syllabus
First assessment 2025
Topic
2.3
Level
SL

Dynamic Equilibrium

Dynamic equilibrium occurs in a closed system when the forward and reverse processes continue at equal rates. Macroscopic amounts remain constant, but reactants and products need not be equal in amount.

The same rate-balance idea applies to physical equilibria such as vaporization and condensation as well as to reversible chemical reactions.

In a sealed liquid–vapour system at equilibrium, molecules continue evaporating and condensing at equal rates, so pressure and amounts are constant on average. Equal rates do not mean equal concentrations, and opening the system can prevent equilibrium by allowing matter to escape.

Recognizing Dynamic Equilibrium

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

Ammonia is manufactured by the Haber process.

N2( g)+3H2( g)⇌2NH3( g)ΔHr⊖=−92.0 kJ mol−1\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H_{\mathrm{r}}^{\ominus}=-92.0 \mathrm{~kJ} \mathrm{~mol}^{-1}

Outline what is meant by dynamic equilibrium.

The Equilibrium Law

Kc=[C]r[D]s/([A]p[B]q)forpA+qB⇌rC+sDKc = [C]^r[D]^s / ([A]^p[B]^q) for pA + qB ⇌ rC + sD

For a homogeneous reaction, place product concentrations over reactant concentrations and use each balanced-equation coefficient as the exponent.

Build the expression only after balancing the equation, and use equilibrium rather than initial concentrations. The numerical value of K changes with temperature; changing starting amounts can move the equilibrium composition without changing K.

Writing Kc Expressions

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

Deduce the KcK_{\mathrm{c}} expression for the reaction in part (d)(i).

Interpreting K

K range Equilibrium tendency
K << 1 reactants strongly favoured
K < 1 reactants favoured
K = 1 comparable amounts
K > 1 products favoured
K >> 1 products strongly favoured

Kreverse=1/KforwardKreverse = 1 / Kforward

K describes a ratio, not reaction speed: a very large K can still belong to a slow reaction. Reversing the equation gives 1/K, while multiplying every coefficient by a factor raises K to that factor. Interpret 'favoured' as equilibrium composition, not complete conversion.

Worked reading: if K = 0.0665 at 100 C, K < 1, so reactants are favoured and the forward reaction has a small extent. This describes equilibrium composition, not reaction speed. At the same temperature, reversing the equation gives K = 1/0.0665.

Using K to Describe Extent

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

At 100∘CKc100^{\circ} \mathrm{C} \mathrm{K}_{\mathrm{c}} for this reaction is 0.0665 . Outline what this indicates about the extent of this reaction.

Le Châtelier's Principle

An equilibrium shifts to partially counteract an imposed change. Pressure favours the side with fewer gaseous molecules; temperature favours the endothermic direction; concentration changes alter composition.

At fixed temperature, concentration and pressure changes do not change K. Temperature changes K. A catalyst changes rates in both directions and does not change equilibrium position.

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), compression favours the two-mole gas side, but K is unchanged if temperature is fixed. Heating favours the endothermic direction and changes K; a catalyst reaches the same equilibrium faster by accelerating both directions.

Disturbance Immediate evidence K at fixed/new T Direction check
concentration or pressure change Q changes before composition readjusts unchanged if T is fixed compare the new Q with K
raise temperature heat favours the endothermic direction K increases if the forward reaction is endothermic; decreases if it is exothermic use the stated forward ΔH
catalyst both forward and reverse rates increase unchanged equilibrium composition is unchanged; it is reached sooner

For gas pressure, count gaseous coefficients only. Use Q/K or opposing-rate evidence to justify the shift rather than the phrase “counteracts the change” alone.

Predicting Equilibrium Shifts

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

Explain why an increase in pressure shifts the position of equilibrium towards the products and how this affects the value of the equilibrium constant, KcK_{\mathrm{c}}.

Extent of Chemical Change Summary

Retrieve the route: define dynamic equilibrium, write K, interpret its magnitude, predict Le Châtelier shifts, compare Q with K, solve a RICE table, and connect K with ΔG.

Check closed-system and equal-rate language, exponents and direction, whether a change affects K, current versus equilibrium concentrations, stoichiometric x changes, and kelvin/unit consistency in ΔG calculations.

Objective notes

4 learning objectives
2.3.1Dynamic equilibrium• Forward rate = backward rate (closed system)• Describe physical and chemical systems at equilibriumView2.3.2Equilibrium law• K determined from stoichiometry• Deduce K expressions for homogeneous reactionsView2.3.3Equilibrium constant (K)• Magnitude indicates extent of reaction• Temperature dependent• Kreverse = 1/Kforward• Interpret K << 1, K < 1, K = 1, K > 1, and K >> 1View2.3.4Le Châtelier's principle• Predict effects of changes in concentration, temperature, pressure• Include effects on K and equilibrium compositionView