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.

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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 5cmi?? and 45cmi?? (10% water, 90% sodium thiosulphate) as opposed to the 50cmi?? of sodium thiosulphate in the first reading, then the reaction will take longer as there are fewer sodium thiosulphate particles to collide with the 5cmi?? 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 50cmi?? every time). I will keep the other variables constant. 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 5cmi?? each time, and the volume of sodium thiosulphate will begin with 50cmi??. Before each further reading, I will reduce the amount of sodium thiosulphate by 5cmi?? , and replace it with the same amount of water. For example, the second reading will involve the standard 5cmi?? of HCl, plus a solution containing 45cmi?? of sodium thiosulphate and 5cmi?? of water.

The third reading will involve 40cmi??of sodium thiosulphate and 10cmi?? of water, and so on, until there is 30cmi?? of sodium thiosulphate solution and 20cmi??. The ratios between HCl and Na2S3O2 vary at each reading, beginning with 1:10, then 1:9, then 1:8, then 1:7 and finally 1:6. These variables are incorporated into the varying of concentration of sodium thiosulphate. At each reading I will take 3 measurements, to work out fair average results at the end. I will take a range of 5 readings, as explained before, beginning with 50cmi?? of sodium thiosulphate solution and ending with a solution of 30cmi??and 20cmi?? water.

I estimate that any change in temperature will have a very small effect on the results. Therefore I will disregard the matter of temperature control in this experiment as it is of little importance to my investigation. Apparatus:  Glass flask  Safety goggles  100cmi?? measuring cylinder 10cmi?? measuring cylinder  Paper with cross X marked on it  Sodium thiosulphate solution  Dilute hydrochloric acid solution  Distilled water Stopwatch  Beakers Experimental Procedure I will put on my safety goggles before beginning measurements.

I will place the flask over the paper with a black cross X marked on it in the middle. I will take the solution of sodium thiosulphate and measure out 50cmi??. I will then take the solution of hydrochloric acid and measure out 5cmi??. I will pour the sodium thiosulphate into the flask, and pick up the stopwatch. Upon pouring in the 5cmi?? of hydrochloric acid I will begin timing the reaction between the sodium thiosulphate and the hydrochloric acid solutions. Looking down upon the flask and the solution in it, I will stop the watch when the solution turns opaque to the point when the cross X is no longer visible.

I will record the time taken for the solution to turn murky, and repeat the reading twice, being sure to clean out the flask container before the next measurement. I will then repeat the experiment, being sure to rid the flask of any trace of solution before pouring any sodium thiosulphate or hydrochloric acid into the flask, only this time reducing the amount of sodium thiosulphate by 5cmi?? each time. Again I will take 3 measurements at each different reading of concentration of sodium thiosulphate.