Aim: To determine how the concentration of each species in a reaction affects the rate of reaction

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Aim: To determine how the concentration of each species in a reaction affects the rate of reaction



In this coursework, the rate of reaction between two reactants will be investigated. Rate of reaction can be defined as the time during which a reactant is lost or a product forms during a chemical reaction. This is calculated by dividing the value of concentration by the time in seconds. There are factors other than concentration that affect the reaction rate, which are temperature, surface area and a catalyst. However, the effect of concentration is the only factor being investigated, meaning that the other factors need to remain constant, and in the absence of any catalyst.


Increasing the concentration of a solution increases the reaction rate. This is because there are more particles in the solution, making molecular collisions more likely. Therefore, more collisions between particles take place.  

The reaction that will be examined in order to fulfil the aim is the reaction between sodium thiosulphate solution and dilute hydrochloric acid:

2HCl(aq)  +  Na2S2O3(aq)    2NaCl(aq)  +  SO2(g)  +  S(s)  +  H2O(l) 

The experiment is carried out by constructing a large cross on a piece of paper, and placing this beneath the reaction mixture. Hydrochloric acid is then mixed together with sodium thiosulphate. The concentration of one of the reactants would change, while the other remains constant, to determine exactly how the concentration affects the rate of reaction. As soon as the two reactants are mixed together, a stopwatch would be started and be stopped as soon as the cross is no longer visible (becomes opaque). This time recorded represents the time taken for the cross to disappear. As both the concentration and reaction time are known, the rate of reaction can be calculated by dividing the concentration by the reaction time. This is achieved by constructing a concentration – time graph, where the gradient at the different concentrations represents the rate of reaction at those concentrations.

Once this has been done, the species that initially had a varying concentration becomes constant, while the other species which had constant concentration now has a varying concentration. The same procedure is then followed.

        It then becomes clear exactly how the concentration of each species affects the rate of reaction, which is the aim of this investigation.

Finding the order of reaction

By finding the order of the reaction in regards to each species, the rate equation can be produced. This can be achieved by using the following two methods:

  1. Concentration – Time graph and Initial rate of reaction – Concentration graph

Once the results are collected, a graph of concentration against time is plotted, where the time is the time taken for the cross to disappear from beneath the solution (end of reaction). Straight line tangents are then drawn corresponding to the different concentrations. By calculating the gradient of each of these tangents, the values calculated represent the rate of reaction at those concentrations. On a second graph, these rates of reaction are plotted against the concentration, in order to find the order of reaction with respect to the concentration of that particular species.

        The graphs that are produced are then compared to the standard order of reaction graphs for each order. If the graph constructed takes a similar shape to one of these graphs, then it can be assumed that the species has that particular order of reaction. Below are the graphs which differentiate between the three orders of reaction:

Zero Order

First Order

Second Order

  1. Using rate equation

For the following reaction:

Rate = k[HCl]a[Na2S2O3]b

where k = reaction rate constant

           a = order of reaction of HCl

           b = order of reaction of Na2S2O3

Once the order of reaction with respect to the concentration of each species is known, both the value and units of k can be calculated. The rate equation can be used in order to check whether calculations have been correct. When k has been calculated, and the order of reaction with respect to the concentration of each species is known, then substituting concentrations into the rate equation should produce the same rate of reaction that was earlier calculated using the Concentration – Time graph.

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Other methods

  1. Using the Half-life values

A Concentration – Time graph has to be plotted for each experiment. The successive half-lives are then calculated, where the half-life is the time taken for the concentration of the species to fall from any chosen value to half that value. The following observations determine which order of reaction is present:

  • Half-life is equal to zero – Zero Order reaction

  • Successive half-lives are the same – First Order reaction

  • Successive half-lives increase – Second Order reaction

Although the use of ...

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