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In this experiment I intend to investigate the effects of the concentration of reactants on rate of reaction.

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Introduction

Planning In this experiment I intend to investigate the effects of the concentration of reactants on rate of reaction. Specifically I will investigate what effect varying the concentration of hydrochloric acid has when it is reacted with sodium thiosulphate. The details of how a reaction works on a molecular level are described by the collision theory. [image001.gif] The collision theory explains the circumstances under which collisions between the particles of the reactants involved in a reaction must collide, in order for a reaction to occur. In order to react, the individual particles of a reactant must not only collide in the correct orientation (as explained in the diagram below), but must also collide with sufficient energy. When both of these are achieved, the collision is strong enough to break any existing bonds and form new ones. The minimum amount of energy required in a collision for two particles to react is called the activation energy, and varies according to the reactants. Thus, not all collisions are successful in inducing a reaction; those that do succeed are called fruitful collisions. The more fruitful collisions there are in a reaction, the higher the rate of reaction will be. Rate of reaction may be measured by observing the speed with which a product of the reaction is produced. Where a gas is produced it can be measured in one of three ways. By forcing it to pass through a cylinder of water, we can measure the amount of water it displaces, and therefore its volume. Similarly, we can measure its volume using a gas syringe fitted to the top of the container in which the reaction is taking place. Alternatively, we can measure the weight that the container and its contents lose as the reaction's product is given off, this being proportional to the volume of the product itself. In each of these examples, the reaction is timed until it has completely finished. ...read more.

Middle

This can be achieved by keeping certain variables constant. The amounts of each of the chemicals used must be consistent in all the experiments. As we are specifically testing the effects of concentration on rate of reaction, the other factors that may have effect must also be the same. We must therefore ensure that the temperature of the reactions is constant by conducting the whole experiment on the same day, assuming that there will be very little temperature change. Also, we must make certain that no catalysts are introduced to the solution. As all the reactants involved are liquid in form, their surface area - the final factor - will be constant throughout without intervention. It is important to avoid any physical interference with the solution as this may increase the rate of reaction by giving the particles kinetic energy. Some variables however, do need to be changed for an experiment to be of any use. In this experiment I will change only one variable. This will be the concentration of hydrochloric acid used in each experiment. Each concentration will need to be measured accurately, in order that my results and conclusion will be as close as possible to the truth. The volume of solution I add to the solutions though, must remain the same. Finally, repeating the experiment may increase the accuracy of the results. The larger the set of results you have to work with, when averaged the more precise they are likely to be. Also, with a number of results that can be compared anomalous results can be spotted more easily. I will therefore repeat the experiment for each concentration of hydrochloric acid three times. The results for each concentration can then be averaged, giving me a final set of precise results with which to work. Predictions The collision theory tells us that the higher the concentration of a solution is, the more particles it has per unit volume. ...read more.

Conclusion

The reaction of hydrochloric acid and sodium thiosulphate varied so little across the different concentrations that the equipment used in the experiment was far too inaccurate. Even the tiniest of mistakes could make a big impact on the rate of reaction. The experiment simply was not a fair enough test to allow it to provide any more detailed information than to display a general pattern of the relationship between concentration and rate of reaction, though this was sufficient to conclude the investigation. More accuracy would have been useful in the measurement of the variables in the experiment, such as a more precise way in which to measure the amounts of solution used. This could be found in more precise measuring cylinders, some more precise way in which to pinpoint the end of a reaction, and particularly in a more accurate method of timekeeping which isn't vulnerable to human inaccuracies. The exact measures that could be introduced to these effects however, are less clear without more specialist scientific knowledge. There is plenty of scope for further investigation of this topic. The same investigation could be repeated with far more accurate methods of measurement, or it could be slightly altered to make the whole process more suited to the use of simple apparatus. This could be done by reacting the varying concentrations of hydrochloric acid with an alternative reactant that would produce a gas. It is far easier to measure a substance that leaves the solution than one that remains mixed-up within it. Alternatively, the investigation could be extended to investigate the effects of concentration of different acids, such as sulphuric or citric, on the rate of reaction, and determine whether these differ to the results given by hydrochloric acid. Finally, the other factors that affect rate of reaction - temperature, surface area and the use of a catalyst - would also provide interesting topics of study with which to compare the results of this investigation. ...read more.

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