Experiment to find the effect of change of concentration on the rate of reaction

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George Maund

Experiment to find the effect of change of concentration on the rate of reaction

Introduction

Na2S2O3 + 2HCl ---> 2NaCl + S + H20 + S02

I am going to be studying the rate of reaction between hydrochloric acid (HCl) and sodium thiosulphate (Na2S2O3). By varying the concentration of sodium thiosulphate (using water to dilute it) I will time how long it takes for the solution to turn opaque. As I mix the hydrochloric acid and the sodium thiosulphate, the solution will turn from a transparent liquid to a murky, yellow-coloured mixture. I will judge when the solution has gone opaque by looking at a cross on a piece of paper beneath the beaker containing the solution.

Plan

I predict that the rate of the reaction will be inversely proportional to the percent concentration of sodium thiosulphate in the solution. That is to say, as I decrease the concentration the time taken for the solution to turn murky will increase.

This prediction can be explained using the particle collisions theory, which states that the more particles there are in a solution, the more likely they are to collide. If the number of sodium thiosulphate particles is increased then they are more likely to collide with the hydrochloric acid molecules, thus speeding up the reaction. If there is a more dilute solution of sodium thiosulphate, for instance the second reading I intend to take will be using 5cm³ and 45cm³ (10% water, 90% sodium thiosulphate) as opposed to the 50cm³ of sodium thiosulphate in the first reading, then the reaction will take longer as there are fewer sodium thiosulphate particles to collide with the 5cm³ of hydrochloric acid.

There are several variables that will have an affect on the rate of reaction. These include:        

  • Temperature of solution

  • Concentration of hydrochloric acid

  • Concentration of sodium thiosulphate

  • Volume of solution

  • Ratios between HCl and Na2S3O2

I intend to look at the effect of varying the concentration of sodium thiosulphate, as it is the easiest, the most cost-effective and the most energy efficient variable to study (Na2S3O2 is expensive and difficult to produce, so I don’t want to waste it by using 50cm³ every time). I will keep the other variables constant.

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The sodium thiosulphate will be made to a constant concentration before the experiment takes place, as will the hydrochloric acid. I will not be diluting the solution of HCl, as it will already be quite diluted so as not to be dangerous and also to control it as a variable.

For the hydrochloric acid I will use 5cm³ each time, and the volume of sodium thiosulphate will begin with 50cm³. Before each further reading, I will reduce the amount of sodium thiosulphate by 5cm³, and replace it with the same amount of water. For example, the second reading will involve the ...

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