Measuring Rate of Reaction

Methods to Measure Rate

The rate of a chemical reaction tells us how fast reactants are used up or products are formed. There are several ways to measure this rate depending on the reaction type and what changes during the reaction:

Volume of Gas Produced

If a reaction produces a gas, you can measure how much gas is made over time. This is often done using a gas syringe or an inverted measuring cylinder in a water trough. The faster the gas is produced, the faster the reaction.

Mass Loss

Some reactions release a gas that escapes from the reaction mixture. Measuring the loss of mass on a balance over time shows how quickly the gas is produced and lost. The quicker the mass decreases, the faster the reaction.

Change in Colour

If the reaction mixture changes colour as the reaction proceeds, the rate can be measured by monitoring this colour change. This can be done visually or more accurately with a colourimeter.

Change in Concentration

The concentration of reactants or products can be measured over time using titration or instrumental methods. A faster change in concentration means a faster reaction.

Practical Techniques

Each method above has practical ways to measure the rate accurately:

Using a Gas Syringe

A gas syringe collects gas produced in a reaction. The volume of gas is recorded at regular time intervals. This method is accurate and suitable for reactions producing gases like hydrogen or carbon dioxide.

Measuring Mass on a Balance

Place the reaction vessel on a digital balance and record the mass at regular intervals. As gas escapes, the mass decreases. This method is simple and useful for reactions producing gases like carbon dioxide.

Colourimeter for Colour Change

A colourimeter measures how much light passes through a coloured solution. As the reaction proceeds and the colour changes, the absorbance changes. This gives a quantitative measure of the rate of colour change.

Titration for Concentration

Samples are taken from the reaction mixture at timed intervals and titrated to find the concentration of a reactant or product. This method is useful for reactions where colour change is not visible.

Calculating Rate

The rate of reaction is calculated by measuring how much a reactant is used or a product is formed per unit time.

Rate = amount of reactant used or product formed / time taken

Units of rate depend on what is measured:

• Volume of gas: cm³/s or dm³/s
• Mass loss: g/s
• Change in concentration: mol/dm³/s
• Colour change (absorbance): arbitrary units per second

The average rate over a time period can be found by dividing the total change by the total time.

For instance, if 20 cm³ of gas is produced in 50 seconds, the average rate is:

$\text{Rate} = \frac{20 \text{ cm}^3}{50 \text{ s}} = 0.4 \text{ cm}^3/\text{s}$

Required Practicals

AQA GCSE Chemistry requires students to investigate how rate changes with concentration and temperature. These practicals involve:

Effect of Concentration on Rate

By varying the concentration of a reactant (e.g., hydrochloric acid) and measuring the rate (e.g., volume of gas produced), students observe how increasing concentration increases the rate.

Effect of Temperature on Rate

By changing the temperature of the reactants and measuring the rate (e.g., mass loss or volume of gas), students see how higher temperatures increase the rate.

Recording and Interpreting Data

Students must record measurements at regular intervals, calculate average rates, and interpret how changes in conditions affect rate.