Answering Techniques For Speed of Reaction

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  • Answering techniques for examination questions concerning speed of reaction.

This article is suitable for Singapore Secondary school students who are taking the Cambridge O Level Chemistry syllabus.

Answering techniques are very important for your examination as they ensure full credit for your answer. To get full credit for your answer consistently, you need to know what keywords and template the examiners is looking for.

I am going to share with you the answering technique for the most frequently asked question concerning speed of reaction. If you remember the keywords and answering techniques that are presented below, I am pretty sure that you will always get your full credit!

All About Exam Questions

The types of questions on speed of reaction are rather limited. Questions may test on module 6.1 to 6.4, and the frequently tested questions are the factors affecting the rate of reaction.

Example Question

Let’s look at an example question (VICTORIA/SEC 4/2014/PRELIM/9A(II))

Explain how the concentration of thiosulfate ions affects the rate of reaction? [2m]

Usually, question testing the factors rate of reaction are worth two marks.

Fortunately, there is a certain template and model answer that you can remember when you answer such question in an exam.

But, before you start answering such questions, it is very important to identity the factor tested. In the sample question above, the factor that is tested is concentration.

Once you identified the factor, you can follow the template below to answer such question.

Template of Answer

As such questions are usually worth two marks in the structured open ended section, your answer must include the following two points:

  • Explain the factor that is asked. [1m]
  • Link back to the question by explaining how the factor changes the frequency of effective question and therefore, affect the rate of reaction. [1m]

In the first part of your answer, you have to explain logically how does the factor asked affect the reactants in the reaction system, since a reaction is concerned with the reactants.

For the second part of your answer, you need to link the factor asked to the frequency of collision and effective collision, and then you conclude what happens to the rate of reaction.

This is easy. You just have to remember the same concluding statement that can be used across all factors affecting the speed of reaction.

Sample Answers for Speed of Reaction Questions

Factor 1: Temperature

Sample question: How does an increase in temperature affect the rate of reaction? [1m]

Explain the factor: As temperature increases → the kinetic energy of the reactant particles increases → the particles move faster → also, more particles posses energy equal to or higher than their activation energy. [1m]

Link: The frequency of collisions increases → the frequency of effective collisions increases → rate of reaction increases [1m]

Important: Your answer must fit the context of the question as much as possible. If you know what are the reactant molecules from the question, you need to write the exact identity if the reactants.

For example, if a question is asking how does an increase in temperature of the thiosulfate solution affects the rate of reaction, you need to write the following:

… … kinetic energy of the reactant particles thiosulfate ions increases… …“.

Factor 2: Surface Area of Particles

Sample question: How does the surface area of reactants affect the rate of reaction? [2m]

Explain the factor: As the particle size decreases → the exposed surface are of the reacting particles increases. [1m]

Link: The frequency of collisions increases → the frequency of effective collisions increases → rate of reaction increases. [1m]

Important: This only applies to reaction where one or more reactant is a solid.

Factor 3: Pressure of Gas

Sample question: How does pressure affect the rate of reaction? [2m]

Explain the factor: As the pressure increases → the number of particles per unit volume increases → particles move close together. [1m]

Link: The frequency of collisions increases → the frequency of effective collisions increases → rate of reaction increases. [1m]

Important: This only applies to reaction where one or more reactant is a gas.

Factor 4: Concentration of Reactants

Sample question: How does the concentration of reactants affect the rate of reaction? [2m]

Explain the factor: As the concentration of the reactants increases → the number of reacting particles per unit volume increases. [1m]

Link: The frequency of collisions increases → the frequency of effective collisions increases → rate of reaction increases. [1m]

Important: This only applies to reaction where one or more reactant is an aqueous state.

Factor 5: Presence of Catalyst

Sample question: How does the concentration of reactants affect the rate of reaction? [2m]

Explain the factor: The catalyst provides an alternative and more efficient reaction pathway with lower activation energy → more particles posses energy equal to or higher than their activation energy. [1m]

Link: The frequency of collisions increases → the frequency of effective collisions increases → rate of reaction increases. [1m]

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