Rates of reaction

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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. They react as 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.

. 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. I will not test this variable because the school doesn't have the facilities to test it. However pressure is a continuous variable.

2. Using a catalyst is another method I could use. A catalyst is a separate substance which speeds up a reaction. After the reaction has happened it gets left behind. This makes this variable unsuitable for the type of experiment I am going to do. A catalyst is also a discontinuous variable with only one likely useful catalyst emerging.

3. Energy. By giving the particles extra energy they will move faster. This means that they cover more ground and are therefore more likely to hit each other which in turn makes the reaction faster. The best way to give energy to a particle is as heat and so I can predict that raising temperature will increase the rate of reaction. This is a continuous, independent variable. I shall test this variable - see below. I predict that temperature is proportional to rate of reaction.

4. Concentration. Just as increasing the pressure will increase the number of particles colliding, so will the concentration. By putting more particles into the reaction, the chance of them colliding increases and so the rate increases. This variable is continuous and independent. I shall test this variable. I predict that by doubling the concentration of the acid, the rate of reaction will double.

5. Surface area. Particles can only collide when the two sorts can meet. Therefore a reaction can only occur on the surface of the material. Therefore by increasing the area of the material which is available to collide the speed of the reaction will increase. I predict that doubling the surface area will double the speed of the reaction. This variable is continuous but I shall not test it because it is hard to control the exact surface area of the two reactants as they both come in an aqueous solution.
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I am going to test the two variables concentration and temperature. Both of these are independent, continuous variables. I think that concentration will have the biggest affect because the reaction is exothermic. Therefore even while I am testing concentration, heat will be given out by the reaction which will give more energy to the particles and so cause them to reach their activation energy sooner. In addition to this, looking at the original equation, it becomes clear that for every one mole of sodium thiosulphate, you need two moles of hydrochloric acid. Therefore increasing the number of hydrochloric ...

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