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# Rates of Reaction: The effect of concentration on the rate of reaction

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Introduction

Rates of Reaction: The effect of concentration on the rate of reaction Prediction I will be investigating the effect different concentrations of sodium thiosulphate has on the rate of the reaction when combined with hydrochloric acid. The given reaction for this experiment is: Sodium Thiosulphate + Hydrochloric Acid => Sulphur + Water I presume that when the concentration of the sodium thiosulphate is increased, the rate of reaction will be higher. This is because if there are more molecules, they are more likely to collide and react within a given volume. However, the collision theory says that only a very small percentage of these collisions results in a reaction. This is because of an energy barrier - only those particles with enough energy to overcome the barrier will react when they collide. Thus, if the frequency of collisions is increased, the rate of reaction will increase. However, the percentage of successful collisions remains the same. As the experiment develops, the rate of reaction is expected to slow. This is because sodium thiosulphate and acid particles will be 'used up' after successful collisions (changed into different products). According to collision theory, and inversely to the original prediction, fewer particles in the same volume of a container will cause a decrease in the rate of reaction. ...read more.

Middle

within a container of a given volume. The first graph is consistent with this original prediction. As the concentration of sodium thiosulphate was increased, the time it took for the cross to cloud over became incrementally lower, explained above and shown on the graph as a gentle curve. On the second graph I have plotted 1000/time (secs-1 to 1dp) against the volume of sodium thiosulphate (cm3). This graph shows me a relative rate of reaction for the different concentrations of sodium thiosulphate as a reciprocal of the time it took for the cross to cloud over. The graph exhibits a linear positive correlation between the increasing volume of sodium thiosulphate and the reciprocated time. According with my initial predictions, I would expect this graph to take the form of a straight line because the success rate of collisions (the 'clouding' effect) is based on a constant percentage. This would denote the rate of reaction in the tests of different concentrations of sodium thiosulphate remaining constant relative to the volume used. This is due to the concept of collision theory - if there are a greater number of molecules of acid and thiosulphate within a container of a fixed volume, the rate of reaction will increase. ...read more.

Conclusion

monitor could be set up above the flask holding the reactants to provide a more consistent measure of the point at which the solution clouded over. * more accurate apparatus could be used in a repeat experiment, for example, a measuring cylinder marked to the nearest 1/10 of a cm3. This would decrease the grounds for measurement discrepancy to one-hundredth of a cm3. * if the experiment was still to be performed using a cross, next time I would make sure it was covered in a waterproof material. This would make sure that no smudging would occur and the cross would maintain a consistent boldness throughout the experiment. * more repetitions and a greater amount of values could be tested to provide more data for analysis Each of these possible improvements to the experiment would increase its level of accuracy and therefore the overall reliability of the tests. With a greater timescale, I would also be interested to find out how temperature effects the rate of reaction in combination with the concentration of sodium thiosulphate. All in all, I think this was a well performed and interesting experiment performed to the highest level possible considering the time and resources available. The results supported my predictions with a fair degree of reliability, and I am happy with the conclusions made. ...read more.

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