Chemical Reactions: IGCSE Chemistry Exam Guide

Chemical reactions is a broad topic on the 0620 syllabus, covering rates of reaction, reversible reactions, equilibrium (Extended), redox, and the energy changes that accompany reactions. Questions from this topic appear on every paper, and the highest-mark questions often require collision theory explanations or equilibrium predictions. The full content is under chemical reactions.

Rates of reaction: the collision theory framework

Every rates question can be answered using the same framework. The rate of a reaction depends on the frequency and energy of collisions between reacting particles. Anything that increases either of these increases the rate.

The five factors and their explanations

1. Temperature Increasing temperature increases the rate because particles have more kinetic energy, they move faster, they collide more frequently, AND a greater proportion of collisions have energy equal to or greater than the activation energy.

Both ideas are needed for full marks. Writing only “particles collide more often” typically earns 1 of 2 marks. The second mark requires the idea that collisions are more energetic.

2. Concentration (solutions) Increasing concentration means more particles per unit volume. More particles in the same space means more frequent collisions, increasing the rate.

3. Pressure (gases) Increasing pressure forces gas particles closer together, equivalent to increasing concentration. More particles in a smaller volume means more frequent collisions.

4. Surface area (solids) Breaking a solid into smaller pieces increases the surface area exposed to the other reactant. More surface area means more particles available for collision at any moment, increasing the frequency of collisions.

5. Catalyst A catalyst increases the rate by providing an alternative reaction pathway with a lower activation energy. This means a greater proportion of particles have enough energy to react on collision. The catalyst is not used up and can be recovered unchanged at the end.

Interpreting rate graphs

The exam frequently gives you a graph of volume of gas produced against time, or mass lost against time, and asks you to compare experiments.

• Steeper initial gradient = faster rate.
• Curve levels off = reaction is complete (limiting reagent used up).
• Same final volume/mass but steeper curve = same amount of product formed faster (e.g., higher temperature or catalyst).
• Higher final volume/mass = more product formed (e.g., more reactant used).

When asked to sketch a second curve on the same axes (for example, same reaction at higher temperature), ensure the curve is steeper initially but levels off at the same final value if the amounts of reactants are the same. Guidance on interpreting graphs is in interpreting graphs in chemistry.

Reversible reactions and equilibrium (Extended)

A reversible reaction proceeds in both directions simultaneously. The symbol is a double arrow.

At equilibrium:

• The rate of the forward reaction equals the rate of the backward reaction.
• The concentrations of reactants and products remain constant (not equal, constant).
• The system is closed (nothing added or removed).

Predicting equilibrium shifts

Use Le Chatelier’s principle: if a change is made to a system at equilibrium, the position of equilibrium shifts to oppose the change.

Temperature:

• Increase temperature: equilibrium shifts in the endothermic direction (to absorb the extra heat).
• Decrease temperature: equilibrium shifts in the exothermic direction.

Pressure (gaseous equilibria):

• Increase pressure: equilibrium shifts to the side with fewer gas molecules (fewer moles of gas).
• Decrease pressure: equilibrium shifts to the side with more gas molecules.

Concentration:

• Increase concentration of a reactant: equilibrium shifts to the right (towards products).
• Increase concentration of a product: equilibrium shifts to the left (towards reactants).

Catalyst: No effect on the position of equilibrium. A catalyst increases the rate of both the forward and backward reactions equally, so equilibrium is reached faster but the proportions of products and reactants are unchanged.

Redox: the definitions

The exam tests redox using three definitions, and you need all three:

1. In terms of oxygen: Oxidation is gain of oxygen; reduction is loss of oxygen.
2. In terms of hydrogen: Oxidation is loss of hydrogen; reduction is gain of hydrogen.
3. In terms of electrons: Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).

The electron definition is used most frequently on Paper 4. Given a reaction, you need to identify which substance is oxidised (loses electrons) and which is reduced (gains electrons).

Example: Zn + CuSO4 -> ZnSO4 + Cu

• Zinc is oxidised: Zn -> Zn2+ + 2e- (loses electrons).
• Copper ions are reduced: Cu2+ + 2e- -> Cu (gains electrons).

Worked exam question

Q (Paper 4): The graph shows the volume of carbon dioxide gas produced when excess calcium carbonate reacts with 50 cm3 of 1.0 mol/dm3 hydrochloric acid at 25 degrees C.

On the same axes, sketch the curve you would expect if the experiment were repeated using 50 cm3 of 1.0 mol/dm3 hydrochloric acid at 40 degrees C with the same mass of excess calcium carbonate.

Explain the shape of the curve you have drawn. (4 marks)

Model answer: The curve should be steeper (faster initial rate) (1) but reach the same final volume of gas (1). At the higher temperature, the particles have more kinetic energy and move faster, so collisions are more frequent (1) and a greater proportion of collisions exceed the activation energy, increasing the rate (1). The same final volume is produced because the same amount of acid is used (acid is the limiting reagent, and calcium carbonate is in excess).

The sketch mark requires the curve to be visibly steeper but levelling off at the same height. A curve that is both steeper AND reaches a higher final volume is wrong — the amount of product depends on the amount of reactant, not the rate.

Chemical reactions questions reward understanding over memorisation. If collision theory explanations or equilibrium predictions are costing marks, a trial lesson can build the systematic reasoning these questions demand.