Investigating Rates of Reaction

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Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid


Investigation, to find out how the rate of reaction between Sodium Thiosulphate and Hydrochloric acid is affected by changing the concentration.


I must produce a piece of coursework investigating the rate of reaction, and the effect different changes have on them. The rate of reaction is the rate of loss of a reactant or the rate of development of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place. There is five factors which affect the rate of a reaction, according to the collision theory of reacting particles: temperature, concentration (of solution), pressure (in gases), surface area (of solid reactants), and catalysts. I have chosen to investigate the effect of concentration on the rate reaction. This is because it is the most practical to investigate. Dealing with temperatures is a difficult task especially when we have to keep constant high temperatures. Secondly the rate equation and the constant k changes when the temperature of the reaction changes. We have no gases and solids involved therefore it is easy to deal with solutions. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

The theory behind this experiment is that 'Increasing the
concentration can increase the rate of the reaction by increasing the rate of molecular collisions.'


I will place the reaction mixture on a paper with a black cross-drawn on it. When the cross is completely obscured, the reaction will have finished. The time taken for this to happen is the measure of the rate of reaction. We must do this several times, and change the

concentration of Sodium Thiosulphate.

The rate of reaction is a measure of the change, which happens during a reaction in a single unit of time. The things that affect the rate of reaction are as follows:

• Surface area of the reactants
• Concentration of the reactants
• The temperature at which the reaction is carried out
• Use of a catalyst

Reaction equation is mentioned above but rate equation could only be decided by doing experiments. So the following procedure can be used to carry out the experiment.



• 2 Measuring cylinders
• Beaker
• Stopwatch
• Paper with black cross on it
• Sodium Thiosulphate (different concentrations)
• Hydrochloric acid (same concentration each time)
• Water (different concentrations)
• Pipette


I predict that the greater the concentration of Sodium Thiosulphate solution the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution. I predict that as concentration is doubled the amount of time taken for the reaction is halved. This means that both graphs drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by relating to the collision theory.

If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself: For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions.

I think that the concentration of a solution effects the rate of reaction because 'the rate of reaction depends on how frequently the molecules of...
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