Many reactions are subject to kinetic control. To follow the rate of a reaction, one must either measure the decrease in concentration of a reactant or the increase in concentration of a product with time.
Some techniques for doing this are:
- Measure the volumes of gases evolved (gas syringe).
- Volumetric analysis – samples are removed at regular intervals, the reaction stopped by cooling, and mixture analysed by titration.
- Measuring changes in pressure (for gas reactions)
- Colorimetry may be used if one of the constituents is coloured. The colorimeter follows the change in intensity of colour.
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A conductivity meter may be used if there is a change in conductivity during the reaction i.e. if the number of ions present is changing. A pH meter is a special type of meter which will follow changes in H+.
For a chemical reaction, we often determine the order with respect to a reagent by determining the initial rate. When more than one reactant is involved, we vary the concentrations in a systematic way so that the effect of concentration of one of the reactants can be measured.
Safety Considerations:
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Take care when using nitric acid, as it is corrosive and an irritant. Wash any splashes or affected areas of skin with copious amounts of water.
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Sulphurous fumes produced in this reaction may be toxic by inhalation. Students who suffer from asthma need to take care.
- At the end of the experiment, small quantities of the chemicals can be diluted with running water and run to waste
Apparatus and Chemicals
We will need the following materials for the experiment:
- safety goggles
- three burettes for 1M sodium thiosulphate
- funnels
- 0.1M nitric acid
- Water
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100 cm3 conical flask or beaker
- a black cross drawn on a white background
- test tubes
- stirring rod
- stopwatch
Procedure
The equation for the reaction whose rate we will be measuring is as follows:
S2O32- (aq) + 2H+ (aq) S (s) + H2SO3 (aq)
The reaction is easy to follow since one sulphur atom is formed for each thiosulphate ion reacting, and the sulphur makes the reacting solution more cloudy as its concentration increases. We will place the reaction vessel over a black cross, drawn on a white background. The black cross will disappear from view, owing to the increased sulphur concentration in the solution, when a fixed amount of reaction has taken place. In this way, the rates of reaction of solutions of differing concentrations can be compared, and the effect of changes in concentration found. This is because the time taken for this fixed amount of reaction is directly proportional to the rate of reaction; that is, if the reaction is fast, the cross will disappear quickly and vice versa.
We will perform an initial trial run using 6 cm3 of sodium thiosulphate,
4 cm3 of water and 10 cm3 of nitric acid. On the basis of the time that this takes we will then plan a further three runs of the experiment in which we will vary the concentration of thiosulphate , whilst maintaining the acid concentration constant. Subsequently, we will conduct another three experiments where the thiosulphate concentration will be kept constant and the acid concentration will be varied.
We must remember to adjust the volume of water that we add, so that the total volume of the reagents is the same in each case. In addition, the concentration changes we should use should vary by a factor of around five overall, so that the times for the experiments are neither too long nor too short.
After this we will carry out the calculations needed to complete the table:
- Calculate correct molarity, as solution dilutes the other
- 1/time versus concentration of thiosulphate at constant acid concentration
- 1/time versus acid concentration at constant thiosulphate concentration
- Using the figures obtained from our calculations as described above, we will plot two graphs, drawing the best line through the points on each graph.
