Investigating The Effect of Concentration On The Rate Of Reaction of Sodium Thiosulphate & Hydrochloric Acid

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

A series of experiments will be carried out - changing the concentration (while everything else remains constant). Both the sodium thiosulphate and the Hydrochloric acid are soluble in water, so the concentration of either can be changed. However, the sodium thiosulphate can be varied in use as it is available in larger amounts, and various concentrations are prepared. Room temperature will be used as the temperature as it is is more practical and will not need to be monitored.

Prediction

I predict that in the experiment when the concentration is increased, the rate of reaction will be higher. This is because if there are more molecules, they are more likely to collide and react. However, the collision theory says that a very small percentage of these collisions results in a reaction. This is because of an energy barrier. Only those particles with enough energy to overcome the energy barrier will react when they collide. So, if the frequency of collisions is increased, the rate of reaction will increase. However, the percentage of successful collision will remain the same. The particles go through random collisions in which energy is transferred between the colliding particles and this leads to particles with differing energies.

Diagram

Equipment needed: Sodium thiosulphate (Na S O) of different concentrations, hydrochloric acid (HCl), a tile marked with a cross, a measuring cylinder, a beaker, a test tube and a stopwatch.

Method: First, I will measure the right amounts of sodium thiosulphate and hydrochloric acid into two separate test tubes. They will then be put into the beaker together. The experiment will be done with the substances at room temperature. This means that the beaker will be filled with Na S O and HCl via two measuring cylinders and placed on the tile marked with a cross. The amount of Na S O and HCl being put in the beaker will be determined by prior tests, but they will only need to be quite small amounts. As soon as the two substances are mixed together, the stopwatch will start timing and it will stop when the cross is obscured. This will act as a visual aid in seeing when a sufficient reaction has taken place. The results will then be written down and recorded. The same will then be done for the next amounts in the series of tests and so on. Each test will be repeated approximately 3 times for the

Different amounts so that a better range will be given and an average will be taken and sued for the graphs.

The amounts used will be 5cm3 of thiosulphate going up by 5 each time up to 50cm3. The volume of acid is kept at a constant during each one of these tests.

Safety: A pair of goggles will be worn during the whole experiment in order to protect the eyes from the danger of acids etc. An apron will also be worn to protect the skin and clothing. When handling beakers filled with acids and other mixtures and measuring cylinders a pair of tongs will be used.

Fair Test: In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up. The amount of HCl will be 5 cm3 each time, and the amount of thiosulphate will be fixed for its respected tests.

On the following page there are two different graphs (front and back) of what my results are expected to be. Rate/Time shows that the higher the temperature the higher 1/Time is. The second graph, "Time for 'x' to disappear" shows that the higher the temperature gets the less time it takes for x to disappear.

Results

Thiosulphate Volume (cm3)

Volume of Water (cm3)

Final Concentration of Thiosulphate (M)

Time

50

0

0.1

m 1s

45

5

0.09

m 4s

40

0

0.08

m 11s

35

5

0.07

m 27s

30

20

0.06

m 32s

25

25

0.05

m 37s

20

30

0.04

2m 20s

5

35

0.03

3m 28s

0

40

0.02

4m 25s

5

45

0.01

0m 27s

Above is a table of results showing the final concentration of the sodium Thiosulphate after the experiment. I have found that as the concentration is increased the time taken for the reaction to take place decreases. This means the rate of reaction increasers as it takes less time for a reaction to take place, so more take place per second.

Graph

The graph shows that the difference of rate between increasing temperatures was pretty much regular. However, there is a giant gap in the last concentration increase. For this to fully make sense it is necessary to recap the collision theory briefly: For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.

Mike McLoughlin