Rate of Reaction

Rate of reaction is the amount of reactant used up or amount of product formed per unit time.

Rate of reaction = (amount of reactant used up) ÷ (time taken)

or

Rate of reaction = (amount of product formed) ÷ (time taken)

Catalyst

• A catalyst is a substance that speeds up the chemical reaction without itself being changed or used up.
• A catalyst speeds up the chemical reaction by providing an alternative pathway with a lower activation energy. 
• There is no change in the enthalpy of a reaction with or without catalyst.

For IGCSE Chemistry, it's important to understand how these factors 👇 affect rate of reaction

Factors that Affect Rate of Reaction

I. Surface Area of a Solid

• Increasing the surface area of a solid increases the rate of reaction.
• Increase in surface area means there are more particles on the solid that are exposed to other reactants.
• There are more frequent and successful collisions between the reactants, so the rate of reaction increases.

II. Concentration of a Solution

• Increasing the concentration of a solution increases the rate of reaction.
• A more concentration solution contains more particles of the reactants per volume.
• There are more frequent and successful collisions between the reactants, so the rate of reaction increases.

III. Temperature

• Increasing the temperature increases the rate of reaction.
• With an increase in temperature, more reactant particles have kinetic energy larger than the activation energy.
• There are more frequent and successful collisions between the reactants, so the rate of reaction increases.

IV. Catalyst

• Using a catalyst increases the rate of reaction.
• Catalyst provides an alternative pathway with lower activation energy, so more particles have the necessary activation energy to react.
• There are more frequent and successful collisions between the reactants, so the rate of reaction increases.

Let's take a look at some IGCSE Chemistry questions on rate of reactions 😄

Example 1

A student investigates the rate of decomposition of hydrogen peroxide solution. The diagram shows the apparatus he uses in his experiments. The equation for the decomposition is

2H₂O₂ → 2H₂O + O₂

(a) The student keeps the amount, in moles, of H₂O₂ in the solution constant at the start of each experiment. State two properties of the solution that he should keep the same to ensure that the amount of H₂O₂ is the same in each experiment.

• volume of hydrogen peroxide solution
• concentration of hydrogen peroxide solution

(b) The student carries out the experiment five times. He uses a different solid in each experiment to see how effective each solid is as a catalyst in the decomposition. He removes the bung, adds a small amount of one of the solids and quickly replaces the bung. He records the time taken to collect 100 cm³ of oxygen in the syringe.

(i) Which solid does not seem to act as a catalyst?

• B does not act as a catalyst.
• No oxygen being collected means there was no reaction. Since a catalyst increases the rate of reaction, B cannot be a catalyst.

• D is the most effective catalyst.
• It took the least time to collect the same amount of oxygen with D, which means the rate of reaction is highest with D.

(c) In the first experiment the student added 1g of solid A. Describe what he could do with the contents of the conical flask at the end of the experiment to show that A was a catalyst, and not a reactant.

• Remove solid A from the solution and weigh it.
• Since catalysts remain unchanged after the reaction, A should still weigh 1g.
• If A was a reactant, the mass would have decreased after the reaction.

Example 2

The student investigated the effect that changing the concentration of the hydrogen peroxide solution has on the rate of the decomposition of hydrogen peroxide. He used solid manganese(IV) oxide as the catalyst in each experiment. This is the method he used. 

1. Pour some hydrogen peroxide solution into a conical flask on a top-pan balance. 
2. Add the catalyst and place some cotton wool loosely in the neck of the flask. 
3. Record the balance reading and start a timer. 
4. Record the balance reading every minute until the mass no longer changes. 
5. Repeat the experiment several times using different concentrations of hydrogen peroxide solution.

(a) (i) State one property of each substance that the student should keep the same in each experiment.

• total volume of hydrogen peroxide solution used
• surface area of solid manganese(IV) oxide

(ii) What is the purpose of the cotton wool?

• To prevent hydrogen peroxide solution from escaping the flask

(b) The graph shows the results of one of the student’s experiments.

(i) Why does the balance reading decrease during the experiment?

• The decomposition of hydrogen peroxide produces oxygen gas.
• Oxygen gas escapes from the flask, so the balance reading decreases as the reaction proceeds.

(ii) What does the slope of the curve indicate about the reaction?

• Slope of the curve = loss of mass per unit = rate of reaction

(d) The results of some of the student’s other experiments are shown on this graph.

(i) Which one of the experiments was the fastest?

• T was the fastest experiment.
• It took the least time to lose the most mass in T.

• Rate of reaction: R < S < T
• Increase in concentration of solution increases the rate of reaction. Therefore,
• Concentration of hydrogen peroxide solution: R < S < T

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