IGCSE Chemistry: rates of reaction and equilibrium

What changes the rate, and the collision theory reason

Cambridge Core describes these effects; explaining them with collision theory is Cambridge Supplement. Edexcel statement 3.11 asks all students to explain them with particle collision theory.

Key ideas board by board

Cambridge 0620 Core: describe the effects of concentration, pressure, surface area, temperature and catalysts (including enzymes) on rate, describe practical methods (change in mass, or volume of gas produced), and interpret rate data and graphs. Supplement: collision theory in terms of particles per unit volume, collision frequency, kinetic energy and activation energy; a catalyst lowers the activation energy; and evaluating practical methods. For reversible reactions, Core students describe heating hydrated copper(II) sulfate (blue to white) and cobalt(II) chloride (pink to blue) and reversing it with water. Supplement adds the definition of equilibrium, predicting how temperature, pressure, concentration and a catalyst affect its position, and the Haber and Contact processes with their conditions and reasons.

Edexcel 4CH1: describe rate experiments, describe and explain the effects of surface area, concentration, pressure and temperature using collision theory, catalysts as providing an alternative pathway with lower activation energy, and two required practicals: marble chips with hydrochloric acid (surface area and concentration) and the catalytic decomposition of hydrogen peroxide by different solids. Reversible reactions (dehydrating hydrated copper(II) sulfate, heating ammonium chloride) are for everyone. Reaction profile diagrams (3.14C) and dynamic equilibrium, the effect of a catalyst, and the effect of temperature and pressure on its position (3.19C to 3.22C) are Paper 2 only. Edexcel says references to Le Chatelier's principle are not required and does not include the Haber or Contact process.

Worked example 1: reading a rate from a graph

Marble chips react with hydrochloric acid and the carbon dioxide is collected in a gas syringe. After 30 s the volume is 60 cm^3; after 120 s the reaction has stopped at 90 cm^3.

Mean rate over the first 30 s = 60 ÷ 30 = 2.0 cm^3/s. Mean rate over the whole reaction = 90 ÷ 120 = 0.75 cm^3/s. The rate is highest at the start because the acid concentration is highest; it falls as acid is used up, and the curve becomes flat when one reactant runs out. To find the rate at a single moment, draw a tangent to the curve at that time and calculate its gradient (change in volume ÷ change in time).

If the experiment is repeated with powdered marble of the same mass, the curve is steeper at the start but ends at the same final volume, because the amounts of reactants are unchanged.

Worked example 2: predicting the equilibrium position

N2(g) + 3H2(g) ⇌ 2NH3(g). The forward reaction is exothermic. Pressure: there are 4 moles of gas on the left and 2 on the right, so increasing pressure moves the position of equilibrium to the right, giving more ammonia. Temperature: increasing temperature moves the position in the endothermic direction, which is to the left, so the yield of ammonia falls.

So why use 450 °C? A lower temperature would give a higher yield but the rate would be too slow; 450 °C is a compromise between rate and yield, and the iron catalyst increases the rate. 200 atm is used because higher pressures would give more ammonia but cost more and increase safety risks. A catalyst does not change the position of equilibrium; it speeds up forward and reverse reactions equally, so equilibrium is reached sooner.

Common mistakes

• Saying particles collide "more" without "more frequently" or "more often per second".
• Saying a higher temperature only gives more collisions. The key extra point is that more collisions have enough energy to react.
• Saying a catalyst "gives particles more energy". It lowers the activation energy.
• Saying larger surface area gives a larger final volume of gas. It changes the rate, not the amount of product.
• Defining equilibrium as "the reaction stops" or "equal concentrations". The rates are equal and the concentrations are constant.
• Saying a catalyst increases the yield at equilibrium.

Exam technique

For "explain" rate questions, a full answer has three links: the change, what happens to the particles, and what that does to the frequency of successful collisions. For graph questions, describe the shape with numbers, then explain each part: steep at first, gradient decreasing, then flat when a reactant is used up.

Equilibrium predictions should always state the direction and a reason from the equation: "fewer moles of gas on the right" or "the forward reaction is exothermic". Evaluation questions on the practical commonly reward using a gas syringe instead of collecting over water if the gas is soluble, measuring at regular intervals, and repeating to identify anomalous results.

How one-to-one lessons help

Rates and equilibrium questions are mostly explanation and graph work, where students lose marks by leaving out one link in the chain. A tutor has the student sketch and explain rate curves live on the shared whiteboard, practises tangents and gradients, and for Cambridge Extended or Edexcel Paper 2 students works through equilibrium predictions until the reasoning from the equation is automatic.

Self-check

1. Explain the effect of concentration, pressure, surface area, temperature and a catalyst using collision theory.
2. Calculate a mean rate and describe how to find a rate from a tangent.
3. Sketch how the gas-volume curve changes with powder or higher temperature.
4. Describe the reversible reactions of hydrated copper(II) sulfate and (Cambridge) cobalt(II) chloride.
5. Define dynamic equilibrium.
6. Predict the effect of temperature and pressure on an equilibrium from its equation.
7. (Cambridge Supplement) State and justify the Haber and Contact process conditions.