Finding catalysts for industrial reactions is a vitally important field of research. For example, without a catalyst the conversion of nitrogen and hydrogen into the valuable compound ammonia in the Haber process would be too slow to be useful. An iron catalyst speeds up the reaction and makes the process fast enough to be economically worthwhile.
If reacting substances are heated, the rate of the reaction usually rises; equally, if they are cooled, the reaction slows down. In order to react, the particles in the substances must collide with each other. Heat gives them more energy to move around and so increases the chances of a collision. When particles do collide, they are more likely to react, rather than just bounce off each other, if they are moving faster. Cooling has the opposite effects. For example, when sodium thiosulphate is mixed with dilute hydrochloric acid the mixture becomes cloudy as solid sulphur comes out of the solution (precipitates). If the mixture is heated, it becomes cloudier more quickly. If it is cooled, it takes longer to become cloudy. Measuring gas volumes could be appropriate in a reaction such as the evolution of carbon dioxide gas when dilute hydrochloric acid is added to calcium carbonate,
CaCO3 + 2HCl → CaCl2 + H2O + CO2.
The rate at which carbon dioxide is evolved can be followed by attaching a gas syringe to a test tube in which the reaction is occurring and noting the volumes at fixed time intervals.
By sampling the reaction mixture and the concentration of one of the components of the reaction mixture (either reactants or products) at regular intervals, the concentration of the reaction mixture can be estimated by titration. An example is the acid-catalysed hydrolysis of an ester such as ethyl ethanoate,
CH3COOC2H5 + H2O → CH3COOH + C2H5OH.
The reaction mixture is titrated at intervals against a standard solution of sodium hydroxide; there is a constant concentration of acid present as catalyst, so as the reaction progresses, more alkali is required due to the formation of ethanoic acid.
In some reactions, electrical conductivity can be measured, as this changes as the reaction proceeds. For example, during the alkaline hydrolysis of bromoethane,
C2H5Br + OH- → C2H5OH + Br-,
the conductivity decreases because the fast-moving hydroxyl (OH¯) ions are replaced by the slower-moving bromide ions.
Other properties which can be followed in order to determine reaction rates include changes of pressure, for gaseous reactions; changes in optical rotation, where optically active materials are involved; and absorption of electromagnetic radiation, such as light, using a spectrophotometer.
Increasing the concentration of reactants can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate. Particle size can also affect reaction rate. Marble chips will dissolve in hydrochloric acid more slowly than an equal amount of ground marble, because less surface area is exposed for the acid to attack.
Aim: How much concentration of sodium thiosulphate (Na2S2O3) affects the rate of reaction when tested with Hydrochloric acid (HCL).
The chemical reaction is Na2S2O3 + 2HCL → 2NaCL + H2O + SO2 + S
Safety: You should always wear goggles through all of the experiment as your eyes may come into contact with the solution.
Fair Test: You should always use the same type of apparatus through out the experiment. To initiate a fair experiment you should try to keep all of the solutions at the same temperature for all experiments.
Prediction: I predict that the higher concentration of the catalyst the faster the reaction will take. The less the concentration the slower it will take.
Apparatus:
Conical Flask Stop Clock
Measuring Cylinder Goggles
Method:
- Gather all of the equipment all together
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(Every time I do the experiment I change the concentration of the Na2S2O3 but the concentration of the HCL will be the same. I will change the Na2S2O3 solution from 0.2ml to 0.4ml, 0.6ml, 0.8ml, and finally 1ml in separate experiments). Using two measuring cylinders pour 0.5ml of HCL into one of the cylinders and the first amount of Na2S2O3 Into the other cylinder.
- Draw an X symbol onto a clear sheet of paper.
- Put the flask onto the paper and empty both of the cylinders into the flask and start the stop clock.
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Repeat this experiment with all of the other concentrations of Na2S2O3 three times.
Results:
Conclusion: My prediction was correct. As I added more Na2S2O3
the reaction speeded up and was shorter with the more Na2S2O3
I used. In my first experiment my average time of the X disappearing was 31 seconds and in my second concentration the time was only 23.5 seconds (average) and in my last experiment it the average was only 12 seconds.
