The rate of a chemical reaction is a measure of how fast of how slow the reaction takes place. It is important to remember that a rapid reaction is completed in a short time. Some reactions are very fast, e. g. the formation of silver chloride precipitate when silver nitrate and hydrochloric acid solutions are mixed. Other reactions are very slow, e. g. the rusting of iron. For practical reasons, reactions used in the laboratory for studying rates of reactions must not be too fast or too slow.
Having selected a suitable reaction it is necessary to find a change that can observed during the reaction. An estimate of the rate of reaction can be obtained from the time taken for a measurable change to take place. Suitable changes include: 1. Colour 2. Formation of precipitate 3. Change in mass (e. g. A gas evolved causing a loss of mass) 4. Volume of gas evolved 5. Time taken for a given mass of reagent to disappear 6. pH 7. Temperature In following experiment I will be investigating the time of the reaction between Potassium Persulphate and Potassium Iodide.
By adding starch indicator to the reaction, at the start, there will be a distinctive colour change when the two substances react. This is because of the reaction with iodide turns the solution blue/black when reacted with the starch indicator. Therefore the solution will change colour and it can be timed how long it takes to change colour and go cloudy or how long a cross drawn underneath takes to disappear when viewed through the solution, using different concentrations. Equation of reaction: Potassium + Potassium Potassium + Iodine.
Persulphate Iodide Sulphate K2 S2 O8 (aq) + 2KI (aq) 2K2 SO4 (aq) + I2 (s) Prediction I predict that when the concentration of the reactant, Potassium Persulphate, is increased while all other factors are kept constant then the rate of reaction will increase because if the solution is more concentrated it means there are more particles of reactant knocking about between the Potassium Iodide molecules which makes collisions between the particles more likely. Therefore the concentration is directly proportional to the rate of reaction. Preliminary Experiment.
The preliminary experiment was to test the quantities of amounts, range of solutions and the volume of equipment needed. Starch was used as an indicator because iodine was produced in the experiment. When iodine is mixed with starch it turns a blue/black colour. For this experiment the Potassium Persulphate and Potassium Iodide were kept constant and the amount of starch was varied Concentration of Potassium Persulphate Concentration of Potassium Iodide Amount of starch (cm3) Time (secs).
From looking at the results from the preliminary experiment the amount of starch to use was decided. The amount of starch that I will use is 0. 1cm3. I chose this amount because it was the amount when the time of the reaction didn’t change. I have chosen to start the Potassium Persulphate and Potassium Iodide at volumes of 10cm3 and vary the concentration of the Potassium Persulphate by adding 5cm3 of water to it each time. I have decided to use a 100ml beaker for the reaction with a cross drawn on a piece of paper with a black marker pen underneath.