6.4 Required practical: rates of reaction

The aim

To investigate how changing the concentration affects the rate of reaction. This is AQA required practical 5, Edexcel core practical 7.1 and OCR PAG C8. AQA uses two methods: a colour change (turbidity) and volume of gas. Edexcel's core practical uses marble chips and hydrochloric acid (gas) and sodium thiosulfate (colour change).

Method 1: disappearing cross

1. Draw a black cross on a piece of paper and put a conical flask on it.
2. Add 50 cm³ of sodium thiosulfate solution of a known concentration.
3. Add 10 cm³ of dilute hydrochloric acid, swirl and start a stopwatch.
4. A pale yellow sulfur precipitate forms, making the mixture cloudy. Stop the clock when you can no longer see the cross from above.
5. Repeat with different concentrations of sodium thiosulfate (dilute it with water, keeping the total volume the same).

Method 2: volume of gas

1. Put a measured volume of hydrochloric acid of known concentration in a conical flask.
2. Add a set mass of magnesium ribbon (or marble chips of the same size), and quickly fit a bung connected to a gas syringe (or an upturned measuring cylinder in a trough of water).
3. Record the volume of gas every 10 seconds.
4. Repeat with different concentrations of acid. Plot volume against time for each; the steeper the initial slope, the faster the rate.

Making it reliable

• Keep the temperature the same: it has a big effect on rate.
• The same person should judge when the cross disappears, looking from the same height. A light sensor and data logger removes the judgement.
• Repeat each concentration at least three times and calculate a mean.

Safety

Wear eye protection. Sulfur dioxide gas is produced in the thiosulfate reaction, so keep the room well ventilated, and people with asthma should take care.