Required Practical 11: Rates of Reaction

About this practical...

Here there are two experiments. Firstly, you measure how long it takes for a solution to become so cloudy that you can no longer see the cross underneath the conical flask (turbidity), secondly, you will take regular reading s of gas as it is evolved from a reaction. Both will show how variables affect the rate of a reaction, however, the end point only is given in the turbidity experiment whereas when collecting gas, we can plot a graph and look at how the gradients change over time.


Method 1: Disappearing cross (initial rate method)

1. Pour 40cm3 or room temperature Sodium thiosulphate solution into the conical flask using the 100cm3 measuring cylinder.

2. Place the conical flask onto the cross provided

3. Add 5cm3 of hydrochloric acid to the 10cm3 measuring cylinder

4. Simultaneously, pour in the acid and start the timer

5. Record the time when you can no longer see the cross when looking down through the flask

6. Repeat the experiment using sodium thiosulphate solution from the water bath to observe the effect of raising the temperature on the rate of reaction.

7. Ensure that you do not mix up the measuring cylinders or the reaction will begin early.

All waste must go into the waste bucket, not down the sinks

You can repeat this experiment with a different concentration of acid or acid at a different temperature to show what effects rates.

Method 2: Gas syringe CO2 collection (continuous monitoring method)

1. Using a weighing boat, weigh out 2g of calcium carbonate chips

2. Place these into the bottom of the conical flask (with side arm).

3. Connect the arm to the securely setup gas syringe

4. Carefully measure 50cm3 of 0.2 moldm-3 hydrochloric acid.

5. Simultaneously, pour the acid into the conical flask, insert the bung and start the timer.

6. Record the volume on the gas syringe every 5 seconds.

7. Repeat using 0.5 moldm-3 hydrochloric acid.

8. Empty waste into sieve/bucket provided

You can repeat this experiment with a different concentration of acid or calcium carbonate chips with a different surface area to show what effects rates.

Safety & Managing Risks

Usual lab rules must be followed: loose hair tied back, goggles worn throughout the lesson, bags and stools tucked away and notify the teacher of any spills or breakages immediately. Take care with the chemicals as some are corrosive. During the reaction, compounds of sulfur are produced, take care not to inhale them deeply. For more detailed information, please consult CLEAPSS.

Technician notes:

Disappearing cross method:

Sodium thiosulphate solution - 40gdm-3.

Hydrochloric acid 1.0 moldm-3

Conical flasks

10 cm-3 and 100 cm-3 measuring cylinders

Printed/laminated crosses


Water bath at 50 ℃ with sodium thiosulphate solution

Waste bucket

Collecting CO2 method:


Weighing boats


Calcium carbonate chips

Conical flasks, tubes and gas syringes

100cm-3 measuring cylinders

0.2 and 0.5 moldm-3 hydrochloric acid


Bucket and sieve for waste

This page was updated on: 8th January 2022