We will carry out an experiment to see how concentration affects rate of reaction between Sodium Thiosulphate and Hydrochloric Acid.

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Rates of reaction coursework

Introduction

We will carry out an experiment to see how concentration affects rate of reaction between Sodium Thiosulphate and Hydrochloric Acid. The equation for the reaction is=

S2O3(aq) + 2H(aq)                            S(s) + H2O(l) + SO2(g)

In order for a reactant to be broken down, the chemical bonds that hold the atoms together have to be broken by other particles bumping into them. The energy needed for this to happen alters from chemical to chemical, and this is called the ACTIVATION ENERGY. Increasing the temperature gives the particles more energy, and this makes them more capable of travelling faster and bumping into the bonds with more energy and breaking more, therefore breaking down the substance. Increasing the concentration of the substance also makes the reaction happen quicker, as this means that there are more particles, and in turn this means that more particles can react and more of the substances is broken down. Surface area also affects the rate of reaction. If the surface area is larger, then there are more particles exposed for reaction.  

The rate of reaction is the amount of time it takes for the reactants to disappear, and for a product to appear. There are five factors, which affect the rate of reaction; temperature – which we may encounter as the chemicals cannot be kept at a steady temperature. Pressure, but this is only in gases and gases are only being produced, not reacted; Surface area, but this is only with solids, and there are no solids involved in this reaction. Also there is concentration, which is the one we are investigating; and catalysts, which are substances that are used to speed up a reaction time. Catalysts weaken the chemical bonds in the substances, which makes it easier for them to be broken, therefore it is easier for them to be broken down. This means that the activation energy is lower.

I will be using a cross drawn on a piece of paper to determine the rate of reaction. This is because the cross will become less visible as the reaction happens, because a cloudy gas is given off. So when I can’t see cross anymore, I will measure the time of the reaction.

The concentration of the Sodium Thiosulphate will be altered throughout this experiment. We will start with 50cm3 of Sodium Thiosulphate and 5cm3 of Hydrochloric Acid. The amount of Hydrochloric Acid will remain the same throughout the experiment, but the Sodium Thiosulphate will go down in concentration by 5cm3 every time. It will be replaced with water to make sure that the results are accurate.

Fair Test

To make this series of tests fair, I will use exactly the same cross each time, so it is neither more nor less to have to see, each time. We will keep exactly the same Sodium Thiosulphate and Hydrochloric Acid, so the concentration does not differ more than we control it to do. We will do this because, for example, even if we find another bottle of Sodium Thiosulphate with 2 molar concentration, it will be different from another bottle of 2 molar concentration Thiosulphate, because when it is measured originally, the concentration is rounded up or down, so they are all different concentrations.

We replace the Thiosulphate we take out with water, as it would not be the same amount of reactants if we didn’t. For example; if we had 50cm3 of Thiosulphate and no water, then in the next experiment we changed it to 45cm3 of Thiosulphate, then there would be less reactants, and this would make the test unfair, because this isn’t altering the concentration, it is altering the amount, and altering the amount is no part of what I am investigating. This is why we replace the Thiosulphate taken out with water. We also have the same person judging whether the cross has disappeared each time, because when one person thinks the cross has disappeared to their eyes, another might think it is still visible, purely because they have better eyesight. It would be unfair if the person judging changed, because the times would alter, and this would not display the results accurately. We will also keep it in the same area, where there is little change in the light throughout the experiment, as light change would alter when we judged the cross to disappear. We will measure the temperature at the start and end of the experiment to see if the reaction was exothermic or endothermic, or kept the same temperature throughout. We decided to use the following equipment:

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A thermometer: to measure the temperature at the start and end of the reaction.

A conical flask: for the reaction to actually take place in.

2 measuring cylinders: One of them will measure the 5cm  of hydrochloric acid each time, so will be smaller, and one needs to go up to at least 50cm  to measure out the Sodium Thiosulphate and water each time.

A stopwatch: to measure the reaction time.

2 measuring pipettes: To alter the amounts when measuring them out. Using them makes it more accurate. Two are used so that the chemicals do not mix and start reacting in one pipette ...

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