R2.3Reactivity 2 — How much, how fast, how far?

How far? The extent of chemical change — IB Chemistry HL Revision Notes & Topical Questions

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📚 IB Chemistry HL Hub / Topical Revision

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Lesson summary

R2.3 How far? The extent of chemical change — IB CHEMISTRY HL notes: Describe dynamic equilibrium, write and use equilibrium constants, apply Le Chatelier's principle and relate K to Q and Gibbs energy (HL).Reversible reaction — products can react to re-form the reactants, shown …

Constant concentrations
Macroscopic properties
Either direction
Physical systems
Closed system
Open system

Part 1 of 10Reversible reactions and dynamic equilibrium

Reversible reactions and dynamic equilibrium

Reversible reaction

Reversible reaction products can react to re-form the reactants, shown by two opposing half arrows.

Dynamic equilibrium

Dynamic equilibrium the rate of the forward reaction equals the rate of the backward reaction in a closed system.

Constant concentrations

Constant concentrations reactant and product concentrations stay constant, but they are not necessarily equal.

Macroscopic properties

Macroscopic properties colour and density do not change.

Either direction

Either direction the same equilibrium can be reached starting from reactants or from products.

Physical systems

Physical systems equilibrium also occurs, for example liquid ethanol and its vapour in a bottle.

Part 2 of 10Closed and open systems

Closed and open systems

Closed system

Closed system none of the reactants or products escape.

Open system

Open system some matter is lost to the surroundings.

In solution

In solution equilibrium can be reached in open flasks.

With gases

With gases equilibrium can be reached only in a closed system.

Example

CaCO₃ decomposing in an open system goes to completion as CO₂ escapes.

Part 3 of 10The equilibrium constant expression

Expression

Expression for aA + bB ⇌ cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b).

Concentrations

Concentrations equilibrium concentrations in mol dm⁻³.

Solids

Solids ignored in the expression.

Specific

K belongs to a given equation and temperature.

Brackets

Brackets square brackets must be used because they mean concentration.

Part 4 of 10Magnitude of K

K	Equilibrium position	Mixture
K ≪ 1	Far to the left	Mostly reactants; hardly proceeds
K < 1	Left	Favours reactants
K = 1	Balanced	Significant amounts of both
K > 1	Right	Favours products
K ≫ 1	Far to the right	Almost complete reaction

Temperature

K is constant at a given temperature but depends on temperature.

Reverse reaction

Reverse reaction the K for the reverse equation is the reciprocal, K′ = 1/K.

Acids

Acids stronger acids have larger K values.

Part 5 of 10Le Chatelier's principle

Statement

Statement if a change is made to a system at equilibrium, the position moves to minimise the change.

Position of equilibrium

Position of equilibrium the relative amounts of reactants and products;
- a shift to the right increases products.

Concentration

Concentration adding a reactant or removing a product shifts the equilibrium right;
- removing a reactant or adding a product shifts it left.

K it is not changed by changes in concentration or pressure.

Dilution

Dilution adding water to an aqueous equilibrium with equal numbers of ions on each side has no effect.

Part 6 of 10Pressure, temperature and catalysts

Pressure

Pressure affects only equilibria involving gases;
- an increase shifts toward fewer gas molecules, and a decrease toward more.

Equal gas moles

Equal gas moles if both sides have the same number of gas molecules, pressure has no effect.

Temperature increase

Temperature increase shifts the equilibrium in the endothermic direction.

Temperature decrease

Temperature decrease shifts it in the exothermic direction.

K and temperature

K increases with temperature for endothermic reactions and decreases for exothermic reactions.

Catalyst

Catalyst speeds up forward and reverse reactions equally, so equilibrium is reached faster but its position and K are unchanged.

Compromise

Compromise for an exothermic reaction a low temperature gives a higher yield but a high temperature gives a faster rate,
- so a compromise temperature is used.

Part 7 of 10Reaction quotient (HL)

Reaction quotient

Q has the same expression as K but uses concentrations that may not be at equilibrium.

Q = K

Q = K the reaction is at equilibrium, with no net change.

Q < K

Q < K the reaction proceeds to the right, favouring products.

Q > K

Q > K the reaction proceeds to the left, favouring reactants.

Constancy

Q can be measured at any time, but K is constant at a given temperature.

Part 8 of 10Calculations with K (HL)

Concentrations

Concentrations from moles and the volume of the reaction mixture, concentration = moles ÷ volume.

ICE table

Initial, Change, Equilibrium, using the molar ratio to find unknown equilibrium amounts.

Equilibrium amount

Equilibrium amount initial amount + change in amount.

Volume cancels

Volume cancels when the numerator and denominator have the same number of concentration terms, moles can be used directly.

Small K

Small K when K < 10⁻³ the change is negligible, so [reactant]_initial ≈ [reactant]_equilibrium;
- state this approximation and its reason.

Algebra

Algebra assign the changes by stoichiometry and solve for x.
- (for example −2x, +3x, +x)

Part 9 of 10K and Gibbs energy (HL)

1Equation

ΔG° = −RT ln K, in section 1 of the data booklet.

2Independence

K gives no information about the rate.

3K > 1

K > 1 products favoured, ΔG° < 0.

4K = 1

K = 1 neither side favoured, ΔG° = 0.

5K < 1

K < 1 reactants favoured, ΔG° > 0.

6Units

ΔG° in kJ mol⁻¹ and R in J K⁻¹ mol⁻¹, so one value is adjusted by 1000.

7More negative ΔG°

More negative ΔG° the forward reaction is more favoured and K is larger.

8Equilibrium

Equilibrium the system is at equilibrium when Q = K.

Part 10 of 10More from the specification

1Equilibrium constant (Kc) expression

The ratio of product concentrations to reactant concentrations, each raised to their stoichiometric powers at dynamic equilibrium.

2Esterification equilibrium

Reversible reaction between ethanoic acid and ethanol yielding ethyl ethanoate and water, analyzed using ICE tables.

3Gaseous equilibrium (H₂ + I₂ 2HI)

Calculating equilibrium concentrations of hydrogen and hydrogen iodide from initial amounts and given Kc.

4Magnitude of Kc interpretation

Kc >> 1 indicates the equilibrium mixture contains mostly products, whereas Kc << 1 indicates reactants predominate.

5Hydrogen

A gas that gives a squeaky pop with a lighted splint.

6Ethyl ethanoate

The ester CH₃COOC₂H₅.

Key terms

Term	Meaning
Dynamic equilibrium	Forward and backward rates are equal and concentrations are constant.
Closed system	A system in which no matter is lost.
Equilibrium constant	The ratio of product to reactant concentrations at equilibrium.
Le Chatelier's principle	An equilibrium shifts to oppose an imposed change.
Reaction quotient	The K expression evaluated for any concentrations.
Position of equilibrium	The relative amounts of reactants and products.
ICE table	Initial, change and equilibrium table used in K calculations.

Avoid these mistakes

Saying the concentrations at equilibrium are equal — they are constant, not equal.

Including solids or pure liquids in the K expression.

Saying a catalyst changes K or the position of equilibrium.

Using round brackets instead of square brackets in the K expression.

Forgetting that K changes with temperature but not with concentration or pressure.

Last updated: 18 October 2026