Potassium Iodide

I will use 100cm3 cylinders to measure out the solutions, and a pipette to measure the starch. Procedure Serially diluted concentrations of Potassium persulphate were measured out, starting with 10cm3 of Potassium Persulphate giving a concentration of 0.05M. A further 10cm3 of 0.05M Potassium Iodide was measured out. A beaker was taken and using a pipette, 0.1ml of starch was placed into the beaker. The beaker must be wrapped in paper to keep light out so that the temperature is kept constant.

Under the beaker was placed a piece of paper with a cross drawn on it with a marker pen.

A stop clock was taken and as the two solutions were poured in to the beaker the stop clock was started. A person observed above the beaker until the two solutions had reacted and the cross had disappeared and then the stop clock was stopped. This was repeated three times and an average was taken. Next the experiment was repeated as before but this time decreasing the concentration of persulphate.

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This was done adding 5ml of distilled water to dilute it; the concentration works out at 0. 0375M. This is reacted with the 10cm3 of iodide and 0.03ml starch and the reaction was timed until the cross disappeared and again was repeated three times and an average was taken. This same process was repeated, by adding to the 10cm3, 10ml, 15ml, 20ml, 25ml, 30ml, 35ml, 40ml, 45ml, 50ml and 55ml of distilled water each time. To ensure better results it is important not to mix up the solutions, so by using separate pipettes and measuring cylinders for each of the chemicals is necessary.

It may also be helpful to label each container holding each chemical before the solution is mixed.

Safety

For safety, safety goggles must be worn during the experiment with a lab coat for protection against the chemicals. Normal laboratory rules apply, for example - no running in the laboratory, care must be taken when poring chemicals, bags under bench. Collision Theory for Rates of Reaction Before looking at the factors that can alter the rate of reaction, we must consider what happens when a reaction takes place. First of all, the particles of the reacting substances must collide with each other and, secondly, a fixed amount of energy called the activation energy must be reached if the reaction is to take place.

If a collision between particles can produce sufficient energy (i. e. if they collide fast enough and in the right direction) a reaction will take place. Not all collisions will result in a reaction. A reaction is speeded up if the number of suitable collisions is increased. Effect of Concentration on the Rate of Reaction The more concentrated the reactants, the greater will be the rate of reaction. This is because increasing the concentration of the reactants increases the number of collisions between particles and therefore, increases the rate of reaction.

This also explains why the greatest rates of reaction are usually as soon as the reactants are mixed i. e. they are both at their highest concentrations. As the reaction precedes the concentrations of the reacting substances decrease and the rate of reaction decreases. The effect of concentration is shown in this experiment using potassium persulphate and Iodine showing a colour change and cloudiness due to the precipitation of potassium. Effect of Pressure on the Rate of Reaction When one or more of the reactants are gases an increase in pressure can lead to an increase rate of reaction.

The increase in pressure forces the particles closer together. This causes more collisions and increases the rate of reaction. Effect of Temperature on the Rate of Reaction When one of the reactants is a solid, the reaction must take place on the surface of the solid. By breaking up the solid in to smaller pieces, the surface are is increased, giving a greater area for collisions to take place and so causing an increase in the rate of reaction. This explains why mixtures of coal dust and air can cause explosions. Effect of Light on the Rate of Reaction.

The rates of some reactions are increased by exposure to light. Therefore light has a similar effect to increasing temperature. Effect of Catalysts on the Rate of Reaction A catalyse is a substance which can alter the rate of a reaction but remains chemically unchanged at the end of the reaction. Catalysts usually speed up reactions. A catalyst, which slows down a reaction, is called a negative catalyst or inhibitor. Catalysts speed up reactions by providing an alternative pathway for the reaction, i. e. one that has much lower activation energy.

The factors, which affect the rate

More collisions will, therefore, have enough energy for this new pathway. So the factors, which affect the rate of a chemical reaction, include:

1. Concentration
2. Particle Size
3. Pressure (for reactions involving gases)
4. Temperature
5. Light
6. Presence of a catalyst

Certain aspects must be kept constant to ensure a fair test, these include:

1. Repetition of experiment with averages taken
2. Concentration of the 0.05M (10cm3) KI
3. Amount of starch (3ml)
4. The same cross each time
5. Temperature
6. Amount of light entering the beaker
7. The distilled water must be the same.
8. The beaker the same
9. The amount of agitation
10. Time you start the stop watch
11. The person who starts the stop watch and times the reaction
12. The height of the eye/viewing above the coin

Line graph of expected results Obtaining Evidence During the experiment all equipment was used safely with precision and skill giving successful results. Below shows the time taken for the cross to disappear when a varied concentration of Potassium Persulphate was reacted with Potassium Iodine and the starch. Concentration of Potassium Persulphate (K2S2O8) (%).

• Concentration of Potassium Iodine (KI) (10cm3)
• Amount of starch (cm3)
• Time (Seconds)
• Analysing Evidence

A basic explanation of what has been found out is that as the Potassium Persulphate concentration increases, the rate of reaction increases. The line of best fit is drawn through virtually all the points so there are no obvious anomalous results. We can say that the concentration is proportional to the time and for a given concentration it can be found out how long it will take to react by investigating the actual rate of this reaction shown in the graph above. If the time for the cross to disappear is short this indicates a fast rate of reaction. If it takes a long time this indicates a slower rate of reaction.