How does the Temperature affect the rate of the reaction between Sodium Thiosulphate and Acid.

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For our investigation we will be looking at how the temperature affects the rate of reaction between sodium thiosulphate and an acid. To make sure the experiment is a fair test we must first see which other factors may effect the investigation and how we can control them. The temperature, concentration of the two solutions, the light intensity, the depth of the solution and the person judging whether they can still see the cross are all factors which may affect the rate of the reaction.

Description of reaction:

When sodium thiusulphate are added together they react as shown below:

HCL + sodium thiosulphate        sodium chloride + sulphur dioxide + sulphur + water.
HCL(aq)   +  Na2S2O3(aq)                 NaCl(aq)            +        SO2(g)         +    S(s)     +  H2O(l)

The sulphur produced is held in suspension turning what was a colourless solution into a clouded one. This is what causes the cross to disappear. When we are measuring the time for the cross to disappear we are also measuring the time for a set amount of sulphur to be produced.

The rate:

The rate of reaction is the speed at which the reaction takes place. Speed are measured as distance over time such as metres a second (m/s) and miles per hour (mph). A speed could also be the speed at which someone works out at, say a factory worker makes 5 footballs per hour so there work speed would be number of footballs over hours so in this case 5 footballs/hour. The speed (rate) at which a reaction happens at is written in the same way but in stead of a distance over time or footballs over time it is the point of the reaction you are measuring upto over time. The point you measure upto in this experiment is

Theoretical background and prediction:

Since starting chemistry in year 7 we have performed and been shown experiment and demonstrations which involve reactions. The speed and how vigorously these reactions take place have been shown to vary greatly. The reaction between a strong acid and a strong alkali can happen in a few seconds and they react together extremely vigorously while the rusting (oxidising) of a metal statue can take years. Reactions occur as described in the collision theory when two molecules collide (intermolecular collision); this is shown in the diagram below:

In our experiment we will be measuring the rate of the reaction by measuring the time taken for the reaction between the hydrochloric acid and the sodium thiosulphate to reach a set point (when we could no longer see the cross). The time taken to reach this point could be changed by increasing or decreasing the number of reactions, which took place every second. This could be done in two ways:

  1. Increasing the number of   molecules of the two reactants (increasing concentration) in a given volume of the solution
  2. Increasing the speed of the molecules by increasing the temperature of the reactant.

Both of these increase the chance of collisions occurring. Kinetic theory tells us that the higher the temperature then the more energy the molecules have and the faster they move around. A basic rule followed by most chemists is that for every 10 Kelvin’s the temperature is raised through the rate of reaction is doubled therefore for each 1 Kelvin raised the rate of reaction increases by about 10 percent. However experiments using the kinetic theory show however that the increase in total number of intermolecular collisions is only about 2 per cent for each 1 Kelvin rise in temperature. Only a certain proportion of collisions actually produce a reaction. This proportion rises more rapidly with increase in temperature than the total number of collisions. It was first suggested in 1889 by a man named Arrhenius that a molecule would only react on collision if it had higher than the average energy i.e. a necessary amount of energy is required for the reaction to occur. If they do not have this minimum amount of energy they will just bounce off each other and no reaction will occur. The minimum amount of energy required for the reaction to take place is called the activation energy (Eact).

Light intensity will affect how long it takes before the person judging cannot see the cross anymore. As if there is a higher light intensity it will be a lot easier to see the cross. Also the person who is judging whether or not they can see the cross makes a difference as we all have different eyesight’s and judgement. If two different beakers are used and both filled with the same volume of water then if one has a small surface area then there will be a greater depth of solution. The greater the depth then the quicker the time taken before the cross can no longer be seen as the light would have to pass through a greater amount of sulphur.

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I predict that the higher the temperature is the quicker the time will be for the reaction to take place and that the higher the temperature the higher the rate of reaction. I predict this because as I have stated above that the more energy the molecules are given by heating them the faster they move thus more collisions occur with a greater force. This results in more reactions taking place in a shorter space of time. If more reactions are taking place in a shorter space of time as the temperature increases then more sulphur will be produced per ...

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