Two Rates Experiments

It is important in this core practical to use appropriate apparatus to make and record a range of measurements accurately, including time, temperature and volume.
- This includes the safe use of apparatus, and monitoring chemical changes.
- This outlines one way to carry out the practical. Eye protection must be worn.

To investigate the effect of changing the temperature on the rate of a reaction.

Sodium thiosulfate solution reacts with dilute hydrochloric acid:
- sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur
- Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The sulfur forms a cloudy yellow-white precipitate during the reaction.
The time taken for this to achieve a given cloudiness provides a way to measure the reaction time.

Using a measuring cylinder, add 50 cm3 of dilute sodium thiosulfate solution to a conical flask.
Place the conical flask on a piece of paper with a black cross drawn on it.
Using a different measuring cylinder, add 10 cm3 of dilute hydrochloric acid to the conical flask. Immediately swirl the flask to mix its contents, and start a stop clock.
Look down through the reaction mixture. When the cross can no longer be seen, record the time on the stop clock.
Measure and record the temperature of the reaction mixture, and clean the apparatus as directed by a teacher.
Repeat steps 1 to 5 with different starting temperatures of sodium thiosulfate solution.