Eduqas GCSE Chemistry 9 Rate of Chemical Change and Dynamic Equilibrium Exam Questions

About Eduqas GCSE Chemistry 9 Rate of Chemical Change and Dynamic Equilibrium

This Eduqas GCSE Chemistry topic covers how fast reactions happen and what happens in reversible reactions. You learn to measure rate by gas collection, mass loss or a colour change, plot and interpret rate graphs, and explain the effects of temperature, concentration, pressure, surface area and catalysts using collision theory. The second part covers reversible reactions, dynamic equilibrium and how changing conditions shifts the position of equilibrium, which leads into the Haber process in topic 11. There are three specified practicals, SP9A, SP9B and SP9C. Examiners often ask you to calculate a mean rate or a gradient and explain changes using collision frequency and activation energy.

What you need to know

• Measuring rate Mean rate = quantity of reactant used or product formed / time, for example cm3/s or g/s. The rate at a point is the gradient of a tangent on the graph. SP9A: effect of one factor on rate using gas collection.
• Collision theory Particles must collide with at least the activation energy. Higher concentration or pressure gives more frequent collisions. Higher temperature makes particles move faster, giving more frequent collisions and a greater proportion with energy above the activation energy.
• Surface area Smaller pieces or powders have a larger surface area to volume ratio, so more particles are exposed and collisions are more frequent. SP9B: effect of one factor on the reaction of dilute hydrochloric acid with sodium thiosulfate, timing until a cross disappears.
• Catalysts and enzymes A catalyst speeds up a reaction without being used up by providing a pathway with a lower activation energy. Enzymes are biological catalysts. SP9C: effect of catalysts such as manganese(IV) oxide on the decomposition of hydrogen peroxide.
• Reversible reactions Some reactions go both ways, shown by a double arrow. If a reversible reaction is exothermic in one direction it is endothermic by the same amount in the other, for example heating hydrated copper(II) sulfate.
• Dynamic equilibrium In a closed system, equilibrium is reached when the forward and reverse rates are equal, so concentrations stay constant. Increasing temperature favours the endothermic direction; increasing pressure favours the side with fewer gas molecules; adding reactant shifts equilibrium towards products.

Common mistakes in 9 Rate of Chemical Change and Dynamic Equilibrium

• Saying particles collide more at higher temperature without mentioning energy. The main reason is that more collisions have energy greater than the activation energy.
• Writing that a catalyst gives particles more energy. It provides an alternative pathway with lower activation energy.
• Saying there are more collisions. Examiners want more frequent collisions, or more collisions per second.
• Saying the reaction stops at equilibrium. Both reactions continue at the same rate; it is a dynamic equilibrium.
• Thinking a catalyst changes the position of equilibrium. A catalyst speeds up both forward and reverse reactions equally, so equilibrium is reached faster but its position is unchanged.

Try three 9 Rate of Chemical Change and Dynamic Equilibrium exam-style questions