OCR GCSE Chemistry A C5.2 Controlling Reactions Exam Questions

About OCR GCSE Chemistry A C5.2 Controlling Reactions

C5.2 explains how chemists measure and control the rate of reactions. On the OCR Gateway Chemistry A course it is part of Topic C5 Monitoring and controlling chemical reactions, assessed in Paper 2 (Foundation) or Paper 4 (Higher). You study the factors affecting rate, explain them using collision theory and activation energy, and learn how catalysts work. It builds on the particle model from C1.1 and reaction profiles from C3.2, and links to equilibria in C5.3 and industrial processes in C6.1. Examiners commonly test interpreting rate graphs, calculating mean rates and gradients, describing the PAG rate practical and explaining factors with collision theory.

What you need to know

• Measuring rate Rate can be measured by the volume of gas produced (gas syringe), the loss of mass as a gas escapes, or the time for a solution to become cloudy (disappearing cross). Mean rate = amount of product formed / time, or amount of reactant used / time.
• Rate graphs On a graph of product against time, the gradient shows the rate: steeper means faster, and the line levels off when a reactant is used up. The rate at a particular time is the gradient of a tangent to the curve.
• Collision theory Particles react only when they collide with energy equal to or greater than the activation energy. Increasing the frequency of successful collisions increases the rate.
• Factors affecting rate Higher concentration of solutions or higher gas pressure puts more particles in the same volume, so collisions are more frequent. Smaller pieces have a larger surface area to volume ratio, exposing more particles. Higher temperature makes particles move faster, so collisions are more frequent and more have energy above the activation energy.
• Catalysts A catalyst increases the rate of reaction without being used up, by providing an alternative pathway with a lower activation energy. Catalysts are not included in the overall equation.
• Enzymes Enzymes are biological catalysts, used industrially for example in fermentation. They work best at an optimum temperature and pH and are denatured outside these conditions.

Common mistakes in C5.2 Controlling Reactions

• Saying particles collide more 'so there are more reactions'. Say there are more frequent successful collisions per second.
• Explaining temperature only in terms of more collisions. The main reason is that more particles have energy greater than the activation energy.
• Writing that a catalyst gives particles more energy. It lowers the activation energy by providing an alternative reaction pathway.
• Stating that larger surface area produces more product. It gives a faster rate, but the final amount of product depends on the amount of limiting reactant.
• Calculating rate from a single point on a curved graph instead of drawing a tangent when asked for the rate at a specific time.

Try three C5.2 Controlling Reactions exam-style questions