Experiment to investigate the factors that affect the rate of reaction between sodium thiosulphate and hydrochloric acid

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When sodium thiosulphate and hydrochloric acid react they produce a cloudy precipitate. The two chemicals are both clear solutions and will react together to form a yellow precipitate of sulphur, the equation for which is as follows:


Na2S2O3  + 2HCl  2NaCl + SO2 + S + H2O


  As the solution will turn cloudy, we can observe the rate of reaction by placing a black cross underneath the beaker and seeing how long it takes for it to disappear.

    There are factors that affect this experiment such as temperature, concentration and time. I do not think that surface area will affect the experiment, as both chemicals are liquids.

For my experiment I will study concentration as this is easily observed and can be easily varied.

    I think that as the concentration of sodium thiosulphate increases, the amount of time taken for a reaction decreases. I know this because before two particles can react they must meet. In a low concentration, the particles will be few and widely spread. This means that the number of reactions will be limited because less particles will meet. At higher concentrations there are more particles and so they probability of them coming into contact with other particles is increased.


 However, not all collisions bring about a reaction. A reaction will only occur if the colliding particles possess more than a certain minimum amount of energy. This is known as the activation energy. (Ea) This activation energy will enable the chemical bonds that already exist between the particles to stretch and eventually break, and atoms, ions and electrons will be rearranged.

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In my experiment I will use sodium thiosulphate and hydrochloric acid. When they react, they will create a cloudy liquid. I can use this fact to test the effect of concentration on the rate of reaction.

    In the experiment, I have a range of molars of acid available to me. I will test 0.5, 1 and 2 molars and observe which will give me the best result.


  I must also decide on an amount of thiosulphate to use. I could either ...

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