OxfordAQA International GCSE Chemistry 3.8 The Rate and Extent of Chemical Change Exam Questions

About OxfordAQA International GCSE Chemistry 3.8 The Rate and Extent of Chemical Change

This section of OxfordAQA International GCSE Chemistry covers how fast reactions happen and, in the separate Chemistry course, how far they go. Everyone studies rate calculations, graphs of product formed against time, collision theory and the effect of temperature, concentration, pressure, surface area and catalysts. Separate Chemistry adds dynamic equilibrium and how temperature and pressure shift it, the Haber process for ammonia, the Contact process for sulfuric acid, and redox in terms of oxygen and electrons. Examiners ask you to calculate mean rates, explain changes using collision frequency and energy, and justify industrial conditions as a compromise between rate, yield and cost.

What you need to know

• Measuring rate Mean rate = amount of reactant used / time, or amount of product formed / time. Rate can be followed by measuring gas volume, mass loss or a colour change. The gradient of a product against time graph shows the rate.
• Collision theory Particles must collide with at least the activation energy to react. Higher temperature makes particles move faster, so collisions are more frequent and more of them have enough energy. Higher concentration, gas pressure or surface area increases collision frequency.
• Catalysts A catalyst speeds up a reaction without being used up and is specific to a reaction. It provides a pathway with lower activation energy, reducing industrial costs.
• Required practical Investigate factors affecting the rate of a reaction, for example the effect of concentration on the reaction of a metal or carbonate with acid by measuring gas volume, or a precipitate forming over a cross.
• Equilibrium (separate Chemistry) In a closed system, dynamic equilibrium is reached when forward and reverse rates are equal. Raising temperature favours the endothermic direction; raising pressure favours the side with fewer gas molecules.
• Haber process Nitrogen (from air) + hydrogen (from natural gas) ⇌ ammonia, with an iron catalyst, about 450 °C and about 200 atmospheres. Ammonia is removed by cooling and liquefying; unreacted gases are recycled. Ammonia is used to make fertilisers.
• Contact process S + O2 → SO2; 2SO2 + O2 ⇌ 2SO3 (exothermic, vanadium(V) oxide catalyst, about 450 °C, atmospheric pressure); SO3 + H2O → H2SO4.
• Redox Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons. In Fe2O3 + 3CO → 2Fe + 3CO2, iron oxide is reduced and carbon monoxide is oxidised.

Common mistakes in 3.8 The Rate and Extent of Chemical Change

• Saying particles collide more at higher temperature without mentioning energy. Also state that more collisions have energy equal to or greater than the activation energy.
• Writing just 'more collisions'. Say more frequent collisions, or more collisions per second, because rate depends on frequency.
• Saying a catalyst increases the yield at equilibrium. It speeds up both directions equally, so equilibrium is reached faster but the position does not change.
• Saying a low temperature is not used in the Haber process because it gives a low yield. A lower temperature gives a higher yield but a rate that is too slow, so 450 °C is a compromise.
• Mixing up OIL RIG. Oxidation is loss of electrons; reduction is gain.

Try three 3.8 The Rate and Extent of Chemical Change exam-style questions