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The effects of temperature and concentration on the rate of a precipitation reaction.

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Introduction

The effects of temperature and concentration on the rate of a precipitation reaction This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction. The reaction is shown in the equations below: sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l) A reaction will only occur where the particles of the reactants meet and combine. This is called the collision theory. Therefore it stands to reason that to increase the rate of reaction it is necessary to cause more particles to collide harder and make it happen more often. There are several ways to do this and these make up the variables for this experiment. They are listed below along with predictions as to their affect on the reaction. 1. Increasing the pressure. By reducing the volume in which the same amount of particles exist the pressure is increased. Once the same number of particles are in a smaller area there is less space in which to move and so the particles are more likely to hit each other. It is therefore possible to predict that increasing the pressure will result in an increase in the rate of reaction. ...read more.

Middle

This time I will make four 20ml samples and put them in individual beakers. I will heat the beakers to temps of 20, 40, 60, and 80 degrees centigrade. Here I recognise that degrees centigrade are not the most preferable units as 40 degrees doesn't give double the energy to 20 degrees. However the school cannot go low or high enough to make use of the kelvin scale. Despite this, if the prediction is correct and it is proportional, I should still be able to tell from the ensuing graph. Because there aren't enough computers for me to test my second variable using this method, I shall use an optical method. I shall set up the equipment as in the diagram below, drawing a black cross on a piece of paper and setting it on a heat proof mat. I shall take each beaker in turn, having heated the hydrochloric acid to the correct temperature on a bunsen burner, and place it on the mat over the cross. I shall then add 20ml of sodium thiosulphate and start a stopwatch simultaneously. I shall stop the watch when it is no longer possible to see the cross. I shall test each temperature three times and then take an average to ensure that the result is balanced and not just a result of a fluke. Diagram 2 - optical method Doing I conducted the experiment as per my plan, although I had to disregard the first few computer results as the system took a while to configure. ...read more.

Conclusion

By 40� many of the particles had reached activation energy. They then gave out heat themselves and so speeded up the reaction causing the large increase after 40�. If it had been possible to test 80� on the computer, I might be able to see the temperature at which all of the particles have reacted and the reaction stops. However the collision theory can still be applied to both variables because when temp/concentration increases, so does reaction rate. Although I predicted that concentration would have the greatest affect on the reaction rate, it was in fact temperature. This was mainly because the concentration was tested at room temperature which wasn't enough to get many of the particles to activation energy levels. In addition, in the solution there was plenty of particles so concentration wasn't important. The investigation could have been improved by testing the temperature variable on the computer as the stop watch I used could not cope with the speed of the reaction. It would also have helped to test each concentration more than once to ensure that the results were true. When using the light sensor I should have covered the underside of the sensor with black material rather than sticking on paper as this could have let in some light. In addition I should have used an artificial source of light as the natural light in the room was constantly changing as clouds pass in front of the sun. I could also have used a burette to measure out the reactants although the measuring cylinder was quite accurate. ...read more.

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