The rate and extent of chemical change — AQA GCSE Chemistry

This topic explores how fast chemical reactions occur (kinetics) and how far they go (equilibrium). You'll learn to calculate reaction rates using graphs and tangents, and understand factors like temperature, concentration, pressure, surface area, and catalysts that speed up reactions. The concept of activation energy and collision theory is central to explaining why these factors matter.

The second part introduces reversible reactions and dynamic equilibrium. You'll study Le Chatelier's principle to predict how changing conditions (temperature, pressure, concentration) shifts the position of equilibrium. This is crucial for industrial processes like the Haber process, where optimising yield and rate is economically important. Understanding equilibrium helps explain why some reactions don't go to completion.

Mastering this topic connects to energy changes (exothermic/endothermic reactions) and prepares you for more advanced chemistry. It's a high-weight topic in exams, often tested with graph analysis, calculation questions, and explanations of industrial applications.

• →Collision theory: For a reaction to occur, particles must collide with sufficient energy (≥ activation energy) and correct orientation.
• →Rate of reaction = change in concentration (or mass/volume) per unit time. Measured by monitoring product formation or reactant loss.
• →Factors affecting rate: temperature (more energetic collisions), concentration/pressure (more frequent collisions), surface area (more exposed particles), catalysts (lower activation energy).
• →Reversible reactions: can go in both directions; at equilibrium, forward and reverse rates are equal, concentrations constant.
• →Le Chatelier's principle: if a system at equilibrium is disturbed, it shifts to counteract the change. Used to predict effects of temperature, pressure, concentration changes.

• Calculation of mean rate of reaction using quantity of reactant used or product formed over time.
• Units for rate of reaction (g/s, cm3/s, or mol/s for HT).
• Interpretation of graphs showing quantity against time.
• Use of tangents to determine rate at a specific time (HT).
• Explanation of factors affecting rate using collision theory.
• Role of catalysts in providing alternative pathways with lower activation energy.
• Definition of reversible reactions and dynamic equilibrium.
• Application of Le Chatelier’s Principle to predict shifts in equilibrium position (HT).
• Effect of changing temperature, pressure, and concentration on equilibrium position (HT).

• 💡Always include units in your final answers for rate calculations.
• 💡When drawing tangents, ensure you use a large triangle to calculate the gradient for better accuracy.
• 💡Remember that catalysts do not change the overall energy change of a reaction.
• 💡For equilibrium questions, clearly state whether the equilibrium shifts to the left or right.
• 💡Use the term 'frequency of collisions' rather than just 'more collisions' when explaining rate factors.
• 💡When drawing tangents on rate graphs, use a ruler and ensure the line touches the curve at exactly one point. Calculate the gradient correctly (change in y / change in x) and include units.
• 💡For equilibrium questions, always state both the change (e.g., increase temperature) and the effect on the position (shifts to endothermic direction) and why (to oppose the increase).
• 💡Remember that catalysts do not affect the position of equilibrium; they only speed up the rate at which equilibrium is reached. This is a common exam point.

• Confusing the effect of catalysts with changing reaction conditions.
• Incorrectly interpreting the slope of a tangent as the average rate rather than the instantaneous rate.
• Failing to mention 'frequency' of collisions when explaining rate increases.
• Misapplying Le Chatelier’s Principle regarding pressure changes in reactions with equal numbers of gas molecules on both sides.
• Assuming that a catalyst is consumed in a reaction.
• Misconception: A catalyst is used up in the reaction. Correction: Catalysts are not consumed; they provide an alternative pathway with lower activation energy and are chemically unchanged at the end.
• Misconception: At equilibrium, the amounts of reactants and products are equal. Correction: Equilibrium means the rates are equal, not the concentrations. The position can be anywhere; it depends on conditions.
• Misconception: Increasing temperature always increases rate. Correction: For exothermic reactions, increasing temperature increases rate but shifts equilibrium to the left (favours reactants), so yield may decrease.