Meet Pip, your intelligent revision partner

Learning Outcomes

• Explain what reaction rate means and how it is measured in the lab.
• Describe how concentration, temperature, surface area, and pressure change the rate of a reaction.
• Use collision theory to explain why each factor affects rate.
• Interpret rate graphs and describe what the gradient represents.

Syllabus coverage

Topic C6 covers rates of reaction and reversible reactions. You must be able to interpret graphs of mass or volume against time.

• Required practical: investigate how changes affect the rate of a reaction.
• Explain the effect of catalysts on reaction rate without being consumed.
• Compare mean rate at an instant with rate over a whole interval.
• Describe reversible reactions and dynamic equilibrium in closed systems.

Collision theory

• State that particles must collide with sufficient energy for a reaction to occur.
• Explain why increasing temperature raises the proportion of successful collisions.
• Describe how a greater collision frequency increases rate when concentration rises.
• Link surface area to the number of collisions per second at a solid interface.

Concentration, temperature, and pressure

• Higher concentration packs more particles into the same volume, so collisions happen more often.
• A temperature rise gives particles more kinetic energy and more collisions exceed activation energy.
• Increasing pressure in a gaseous reaction raises concentration when volume is fixed.
• Crushing a solid increases surface area so more particles are exposed to react.

Catalysts

A catalyst speeds up a reaction by providing an alternative pathway with lower activation energy.

• Catalysts are not consumed in the overall reaction.
• Enzymes are biological catalysts with high specificity for one substrate.
• Catalysts do not change the energy transferred in a reaction.

Reading rate graphs

A rate graph plots a measurable quantity against time. The shape tells you how fast the reaction is changing.

• A steep gradient means a fast reaction at that moment.
• Curves flatten as reactants are used up and the rate falls.
• Draw tangents carefully when finding the rate at a single instant.
• Mass-loss graphs slope downward when a gas leaves the flask.

Required practical: marble chips and acid

• Measure volume of gas collected at regular time intervals.
• Repeat with smaller chips, warmer acid, or stronger acid to compare rates.
• Plot results and describe which change gave the steepest initial gradient.
• Control variables such as acid volume and chip mass between runs.

Reversible reactions

In a closed system, products can reform reactants. The forward and reverse reactions can reach dynamic equilibrium.

• At equilibrium, the concentrations of reactants and products stay constant.
• Changing conditions can shift the position of equilibrium.
• The Haber process uses an iron catalyst to make ammonia at high pressure.

Exam technique

• Quote collision theory whenever you explain why a factor changes rate.
• State what you measure and the units when describing an experiment.
• Sketch a tangent on a graph before you calculate a numerical rate.
• Check whether the question asks for mean rate or rate at an instant.

Topic C7 preview: Chemical analysis

Next you will interpret chromatograms, calculate Rf values, and compare purity of samples using melting-point data.

• Rf values depend on solvent choice and must be measured from the same baseline.
• Pure compounds melt sharply at one temperature; mixtures melt across a range.