Investigating the Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid

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Rebecca Scott

Investigating the Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid

Aim: To find out how the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid is effected by the change of concentration.

I am investigating the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid. There are five factors which effect the rate of reaction: temperature and concentration of the reactants, pressure (in gases) and the surface area of any solid reactants. The rate of reaction is the rate of loss or the development of a product during a chemical reaction.The input variable will be the concentration, the constant variables will be the Hydrochloric Acid and the solution and the dependent variable will be the temperature. After doing some preliminary experiments, I have decided to investigate the effect of concentration on the rate of reaction. The reasons for this is that it is more practical and will be the best way to get the most accurate results. Temperature is extremely difficult to get exact results as they can always change depending on room temperate.


  • Beaker
  • 2 Measuring Cylinders (large and small)
  • Stopwatch
  • Pipette
  • A black cross
  • Sodium Thiosulphate (in different concentrations)
  • Dilute Hydrochloric Acid (has the same concentration every time)
  • Water (different concentration)


I predict that if we were to change the temperature of the Sodium Thiosulphate, the higher the temperature is and the faster the rate of reaction will be. If we change the concentration of the Sodium Thiosulphate, the higher the concentration of sodium thiosulphate, the faster the rate of reaction will be. Therefore, the black cross will fade away from sight more quickly due to the solution in the beaker becoming a cloudy pale yellow. In whatever constant variable is used (temperature or concentration), I will keep the amount of Hydrochloric Acid, the amount of solution depending on which constant variable the same. Another prediction is as the concentration is doubled, the amount of time taken to form a chemical reaction will be halved or approximately halved. If this happens, my graph would have a positive correlation throughout, and could possibly be slightly curved as the increase in the reaction will not be the same as the concentration increased.

This prediction can be answerable for by the collision theory:

If the solutions of reacting particles are made more concentrated there are more particles per unit volume. Because of this, a collision between reacting particles is more likely to occur. For a reaction to happen the particles have to collide with each other and only a small percentage result in a reaction. The reason for this is due to the energy barrier the particles have to overcome.

Only the particles with enough energy to overcome this barrier will react. The minimum energy that a particle must have to overcome the barrier is the ‘activation energy’ also known as Ea and the size of this activation energy is never the same, it is always different for different reactions.

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‘The rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules, the more the frequency of collisions is better and the reactions happen faster.’

Fair Test

Every experiment has to be fair. In this experiment the aim is to find the rate of reaction using the concentration as a factor and there is a number of things needed to be done to ensure the whole of the experiment is a fair test.


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