Measuring Rates of Reaction (AQA A-Level Chemistry): Revision Notes

1.7.2 Measuring Rates of Reaction

What is the Rate of Reaction?

The rate of reaction measures how quickly a reactant is consumed or a product is formed during a chemical reaction. It is defined as the change in concentration of a reactant or product over a given period. The formula for rate is:

$$\text{Rate of reaction} = \frac{\text{change in concentration}}{\text{time}}$$

The units of rate are typically:

$$\text{mol} \, \text{dm}^{-3} \, \text{s}^{-1}$$

Factors Affecting the Rate of Reaction

Several factors influence the rate of reaction, all explained by collision theory.

This theory states that reactions occur when particles collide with sufficient energy (activation energy) and the correct orientation.

1. Concentration

• Effect: Increasing the concentration of a reactant increases the number of particles in a given volume, leading to more frequent collisions.
• Outcome: As the number of successful collisions per unit time increases, the rate of reaction increases.

$$\text{Concentration} \uparrow \quad \Rightarrow \quad \text{Rate} \uparrow$$

2. Pressure (for Gaseous Reactions)

• Effect: Increasing the pressure of a gas forces the particles closer together, increasing the frequency of collisions.
• Outcome: More successful collisions per unit time mean the rate of reaction increases.

$$\text{Pressure} \uparrow \quad \Rightarrow \quad \text{Rate} \uparrow$$

3. Temperature

• Effect: Increasing the temperature provides particles with more kinetic energy, causing them to move faster.
• Outcome: More particles collide with energy greater than or equal to the activation energy, increasing the number of successful collisions and speeding up the reaction.

$$\text{Temperature} \uparrow \quad \Rightarrow \quad \text{Rate} \uparrow$$

4. Surface Area of Solid Reactants

• Effect: Increasing the surface area of a solid reactant exposes more particles to collisions.
• Outcome: This leads to more successful collisions per unit time, increasing the rate of reaction.

$$\text{Surface Area} \uparrow \quad \Rightarrow \quad \text{Rate} \uparrow$$

5. Presence of a Catalyst

• Effect: A catalyst provides an alternative reaction pathway with a lower activation energy.
• Outcome: With a lower activation energy, a greater proportion of particles will have sufficient energy to react, leading to more successful collisions and an increased rate of reaction.

$$\text{Catalyst} \quad \Rightarrow \quad \text{Rate} \uparrow$$

Catalysts are widely used in industrial processes to increase efficiency and reduce costs by speeding up reactions without being consumed in the process.

Measuring the Rate of Reaction

To measure the rate of reaction, you can monitor changes in:

• Concentration of reactants or products over time.
• Volume of gas produced in reactions that produce gases.
• Mass loss in reactions where a gas is released.
• Time taken for a visible change such as a colour change or precipitate formation.

Common Experimental Methods:

1. Volume of Gas Produced: Using a gas syringe to measure the volume of gas released at regular time intervals.
2. Change in Mass: Measuring the loss in mass using a balance for reactions that produce gas.
3. Color Change/Precipitation: Using a stopwatch to time how long it takes for a visible change to occur.