7.2 - Choosing methods to measure reaction rates

Choosing a measurable change

The rate of reaction is how fast reactants are used up or products are formed. A practical method must therefore measure something that changes because the reaction is happening.

The measured "amount" does not always have to be moles. At GCSE it is often a practical quantity such as volume of gas, mass, concentration, colour intensity, or the time taken for a fixed visible change.

The first question to ask is not "what apparatus do I remember?" It is "what change can this reaction produce that I can measure reliably?"

[DIAGRAM: asset_name: rate_method_selection_apparatus: Choosing a practical method for measuring reaction rate - diagram 1; asset_slug: 164_7_2_practical_methods_for_determining_the_rate_of_a_diagram1; file: 164_7_2_practical_methods_for_determining_the_rate_of_a_diagram1_imagegen_v1.png; recommended_method: deterministic_drawn; description: Exact monochrome apparatus comparison showing three suitable rate-method routes: mass-loss on a balance for gas escaping, gas volume collection with a gas syringe and the alternative of an upturned measuring cylinder over water, and a precipitate/colour endpoint using a conical flask on a cross with a stop clock. Labels identify the measured quantity, time reading, and when each method is suitable. Assessed practical visual; deterministic drawing is used because apparatus names and method-choice labels must be exact.]

Gas and mass methods

If a reaction produces a gas, two common methods are gas collection and mass loss. The apparatus sketches summarise the choices. For collection over water, the delivery tube must lead under the submerged opening of a water-filled, inverted measuring cylinder.

For gas collection, place the reactants in a conical flask and fit a bung with a delivery tube. The gas can be collected in a gas syringe, or over water in an upturned measuring cylinder. Start the stop clock as soon as the reactants are mixed, then record the gas volume at regular time intervals.

A gas syringe gives direct volume readings in cm3 and is usually more precise than collecting gas over water. However, the bung must be fitted quickly and tightly, because gas escaping before the apparatus is sealed will make the measured volume too low.

For mass loss, place the reaction vessel on a balance and let the gas escape. A cotton wool plug can reduce spray without sealing the flask. Start the stop clock when the reactants are mixed, then record the balance reading at regular time intervals. The mass decreases because gas leaves the flask.

Mass loss is only suitable if a gas leaves the apparatus. It is not suitable for a reaction in a sealed flask, and it is a poor choice if the gas is produced very slowly or the mass change is too small for the balance to measure reliably.

Colour and precipitate methods

Some reactions do not produce a gas that is easy to collect. Instead, the reaction may cause a colour change or form a precipitate. A precipitate is a solid formed in a solution, and it can make the mixture cloudy.

In the Edexcel rates context, sodium thiosulfate solution reacts with dilute hydrochloric acid. Solid sulfur forms, so the mixture becomes cloudy and a cross under the flask gradually disappears from view. The method measures the time taken to reach the same visible endpoint each time.

A suitable method is:

1. Place a white paper cross under a conical flask.
2. Measure a fixed volume of sodium thiosulfate solution into the flask.
3. Measure a fixed volume of dilute hydrochloric acid separately.
4. Mix the acid into the flask and start the stop clock immediately.
5. Look down through the solution from the same position.
6. Stop the clock when the cross can no longer be seen.
7. Repeat under controlled conditions.

This gives a time for a fixed amount of cloudiness to form. If the endpoint is the same each time, a shorter time means a faster reaction. A relative rate can be calculated using:

Colour-change methods can also use a fixed endpoint, for example the time taken for a colour to disappear. If a colorimeter is supplied in a question, it can make the colour measurement more objective by measuring intensity rather than relying only on eyesight. Do not assume a colorimeter is required for the Edexcel core practical unless the question gives it.

Sodium thiosulfate and hydrochloric acid can produce sulfur dioxide. The room should be well ventilated, small volumes should be used, and eye protection should be worn.

Controls and timing

A rate method is valid only if the measured change is caused by the variable being investigated, not by an uncontrolled difference in the setup.

The variable you deliberately change is the independent variable. The variable you measure is the dependent variable. Control variables are kept the same so the comparison is fair.

Keep the variables below constant unless one is the independent variable being investigated: