6. The Rate and Extent of Chemical Change

5.6.1 Rate of reaction

The rate of a chemical reaction can be found by measuring the amount of reactant used or the amount of product formed over time.

Calculating rate of reaction:
Rate = amount of reactant used ÷ time
Rate = amount of product formed ÷ time
Units depend on the quantities measured (e.g. g/s, cm³/s, mol/s).

Measuring quantities:

  • Mass: Use a balance to record change in mass as a reaction proceeds.

  • Volume: Use a gas syringe or an inverted measuring cylinder in water to measure gas produced.

Factors affecting rate of reaction:

  1. Concentration of solutions – Higher concentration = more particles in the same volume = more frequent collisions.

  2. Pressure of gases – Higher pressure = particles closer together = more frequent collisions.

  3. Surface area of solids – Greater surface area = more exposed particles = faster rate.

  4. Temperature – Higher temperature = particles have more kinetic energy = more frequent and more energetic collisions.

  5. Catalysts – Increase rate by lowering activation energy; not used up in the reaction.

Collision theory:
For a reaction to occur, particles must collide with enough energy (activation energy). Increasing frequency and energy of collisions increases rate.

Required practical:
Investigate how changes in concentration affect the rate of a reaction by measuring:

  • Production of gas over time.

  • Change in colour or turbidity (e.g. disappearance of cross in sodium thiosulfate and hydrochloric acid reaction).

5.6.2 Reversible reactions

Some reactions are reversible — the products can react to form the original reactants.

Example:
Ammonium chloride ⇌ ammonia + hydrogen chloride

The direction taken depends on the conditions.

Energy in reversible reactions:

  • If forward reaction is exothermic, the reverse is endothermic (and vice versa).

  • The same amount of energy is transferred in each direction.

5.6.3 Equilibrium

In a closed system, a reversible reaction will reach equilibrium — the rate of the forward and reverse reactions become equal and the concentrations of reactants and products remain constant.

Le Chatelier’s Principle:
If conditions are changed, the system shifts to counteract the change.

Changes affecting equilibrium:

  1. Concentration: Increasing concentration of a reactant shifts equilibrium to the products; increasing product concentration shifts to reactants.

  2. Temperature: Increasing temperature favours the endothermic direction; decreasing temperature favours the exothermic direction.

  3. Pressure (for gases): Increasing pressure favours the side with fewer gas molecules; decreasing favours the side with more.

Applications:

  • Understanding how industrial processes (e.g. Haber process) are optimised for yield and rate.