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O-Level Rate of Reactions
the factors that change reaction speed and how collisions explain them
What the syllabus expects
- Describe how concentration, pressure, particle size and temperature affect reaction rates, and explain each effect in terms of collisions between reacting particles.
- Define a catalyst and describe how catalysts, enzymes included, affect reaction rates.
- Explain how routes with lower activation energies account for a faster reaction rate.
- State that some compounds serve as catalysts in various industrial processes, while enzymes act as biological catalysts.
- Propose a workable method for studying how a chosen variable influences the rate of a reaction.
- Interpret data gathered from rate-of-reaction experiments.
How it's examined
Questions on this topic most often ask you to name, suggest. About 4% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Apart from the acid concentration and the temperature, name one factor that would speed up the reaction of silver carbonate with nitric acid, and account for your answer using the idea of collisions between particles.
Show the worked answer
One suitable factor is increasing the surface area of the silver carbonate (using it in smaller pieces / more finely powdered form). Breaking the solid into smaller particles exposes a larger total surface area to the acid. This means there are more silver carbonate particles available at the surface for the acid particles to collide with, so the frequency of collisions between the reacting particles increases. A greater frequency of (effective) collisions per unit time increases the rate of reaction.
Example 2 (2 marks)
Suggest one way of following how fast hydrogen peroxide reacts with iodide ions, and give a reason why that method is suitable.
Show the worked answer
The reaction is H2O2 + 2I^- + 2H^+ -> I2 + 2H2O, which produces iodine. Iodine is coloured (yellow-brown) whereas the reactant solution is colourless. Method: follow the reaction by colorimetry / by measuring how the intensity of the brown colour builds up with time (or time how long the colour takes to appear). Suitable because iodine is coloured so the change in colour intensity can be measured and used to follow the rate at which product forms.
More O-Level Pure Chemistry topics
Experimental Chemistry · The Particulate Nature of Matter · Chemical Bonding and Structure · Chemical Calculations · Acid-Base Chemistry · Qualitative Analysis · all of O-Level Pure Chemistry