Chemistry Rates of Reaction Coursework: Sodium Thiosulphate and Hydrochloric Acid

An investigation to show how the rate of reaction between hydrochloric acid and sodium thiosulphate is affected by the concentration of the acid

Simple Procedure
Place a conical flask on a piece of paper with a cross on it. Add hydrochloric acid and sodium thiosulphate, and record the amount of time taken for the cross to disappear through the solution from the top of the flask. Record this time and repeat this for different concentrations of hydrochloric acid.

Fair Test
The variables in this test are:
• The concentration of hydrochloric acid
• The concentration of sodium thiosulphate
• The amount of hydrochloric acid
• The amount of sodium thiosulphate
• The temperature
• The surface area of the reactants
• The size and colour of the cross
• The size of the conical flask

To keep this test fair the concentration of the hydrochloric acid will be the variable that will be changed and all of the others will be kept consistent. The same person must do the timing and watch the cross through the top of the flask, because different people have different reaction times. The concentration of acid used will be: 1.0M, 0,8M, 0.6M, 0.4M and 0.2M.

Apparatus
The apparatus for the test is:
• A 100ml conical flask
• A piece of paper with a black cross drawn on it
• Sodium thiosulphate
• Hydrochloric acid
• 2 measuring cylinders – one 10ml and one 25ml
• Stopwatch

Prediction
Using the particle collision theory, I predict that the less concentrated the acid the slower the cross will disappear. This is because in a more concentrated solution there will be more hydrochloric acid molecules, which means that there will be more collisions between the hydrochloric acid molecules and the sodium thiosulphate molecules.

Preliminary Results
After completing the test once, the results were as follows:
Concentration of Hydrochloric acid (molarity) Time taken for cross to disappear (seconds)
1.0 45
0.8 46
0.6 44
0.4 36
0.2 29

These results show that,