Required Practical 3

Question 1

Describe an experiment between sodium thiosulfate and hydrochloric acid to investigate how rate of reaction changes with temperature.

Answer 1

METHOD

1. Measure 10 cm³ of 0.2 mol dm⁻³ hydrochloric acid and 10cm³ of sodium thiosulfate in separate clean measuring cylinders. Put the solutions in separate boiling tubes.
2. Choose a temperature to investigate and use a water bath to get the two solutions to that temperature by placing the boiling tubes in the water bath.
3. Place a flask on the centre of a large cross and add sodium thiosulfate to it.
4. Then add the hydrochloric acid and start the stopwatch and swirl to mix the solutions.
5. Stop the clock when the cross disappears and note the time.
6. Repeat the experiment for four more different temperatures.

Question 2

In the experiment between sodium thiosulfate and hydrochloric acid, why do we usually measure the reaction rate as 1/time?

(Where time is the time taken for a cross placed underneath the reaction mixture to disappear due to cloudiness of sulfur.)

Answer 2

• This is an approximation for initial rate of reaction as it does not include the change in concentration term.
• We can use this because we can assume the amount of sulfur produced to block out the cross in each experiment is fixed and constant so each time the experiment is repeated, the same amount of sulfur is produced.
• The change in concentration of sulfur is therefore the same for each experiment so only the time taken to reach this concentration is relevant.