Chemistry Rates of Reaction Coursework: Sodium Thiosulphate and Hydrochloric Acid

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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.

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

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) 1.045

These results show that, when the acid reaches 0.6M, the rate of reaction does not change, no matter how much more concentrated you make the acid. This means we must modify the experiment. We will now keep the concentration of hydrochloric acid the same at 1.0M and change the concentration of the sodium thiosulphate. We will dilute it with water, and use the following concentrations: •0.20M


By doing this, we should be able to get a more accurate set of results. This will be repeated three times at each concentration.

•Place a conical flask onto a white piece of paper with a black cross drawn on it. •Pour 20ml of hydrochloric acid into the flask.
•Measure out 10ml of the selected concentration of sodium thiosulphate; prepare the stopwatch and start it as the sodium thiosulphate is added to the hydrochloric acid. •Stop the stopwatch when the cross is no longer visible from the top of the flask. •Repeat this 3 times for each chosen concentration of sodium thiosulphate.

My prediction remains the same, but the weaker the concentration of sodium thiosulphate, the longer the cross will take to disappear.

From the new experiment, we got the following results:
Concentration of sodium thiosulphate (molarity)Time taken (seconds)Rate of reaction (1/time taken) (to 2 significant figures) 0.20M280.036

Concentration of sodium thiosulphate (molarity)Average time taken (seconds)Average Rate of reaction (1/time taken) (to 2 significant figures) 0.20290.035

The results show that the rate of reaction decreases when the sodium thiosulphate is less concentrated. From the graph we can see that as the concentration decreases, the time taken increases. The gradient...
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