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A solution forms when a solute dissolves in a solvent.
In chemistry the word concentration refers to the mass (or number of moles) of a solute dissolved in a certain volume of solvent.
Concentration is measured in moles per litre (mol l-1 or mol/l) and we can change the concentration of a solution by diluting it with water.
For example if we have 500 cm3 of a solution with a concentration 1 moles per litre and we add another 500 cm3 of water to this solution, we have diluted the solution and its concentration is half of what it was, in other worlds 0.5 moles per litre.
We will now consider how changing the concentration of a reactant solution can change the rate of a chemical reaction.
There are many ways of measuring the rate of a reaction for example, as has been shown in the previous section, measuring how quickly a gas is produced. Another method which can be used to measure the rate of some reactions is shown below. This method can be used when a precipitate (an insoluble solid) is produced in a reaction.
The method of the experiment used to obtain the results further down thew page is shown below:
Method:
An X was drawn on a piece of white paper and a 100ml (small) beaker was placed on top.
Exactly 10ml of hydrochloric acid was added to the beaker.
The measuring cylinder was washed thoroughly and 10ml of sodium thiosulfate of the required concentration was measured and added to the beaker containing the acid.
Timing using a stopwatch was started immediately when the sodium thiosulfate was added.
When the cross on the paper was no longer visible the timing was stopped.
The experiment was repeated using different concentrations of the thiosulfate solution.
The video below goes into much more detail than you need to know at this level but it allows you to see what the reaction looks like and gives an indication what what happens when the concentration of the sodium thiosulfate is changed.
A set of example results from the experiment described above are shown in the table below.
As before this information can also be presented on a graph which is included below the table.
From the results of this experiment you can see that as the concentration of thiosulfate increases the time taken for the X to disappear decreases. This means the reaction is finished sooner and therefore is proceeding at a faster rate. Increasing the concentration increases the rate of the reaction.
In a reaction as the concentration increases, there are more particles in the same volume of solvent. which means that there is an increase in the number of collisions between them. This causes an increase in the rate of the reaction.
Increasing the concentration of reactants increases the rate of reaction.
Increasing the concentration of reactants increases the rate of reaction.
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