How Concentration Affects a Reaction.

Fair Test:

The experiment will be made fair by making sure that the light intensity will start and end at the same amount. The sunlight could affect the speed therefore the experiment will take place in the shade. Also the same amount of hydrochloric acid and sodium thiosulphate will be mixed each time so that the concentration changes will affect the same amount of hydrochloric acid and sodium thiosulphate. The concentration of the hydrochloric acid will be changing each time to test if it speeds up a reaction. The speed for the light intensity to drop by 10% will be measured after each attempt so that the affects of concentration changes can be recorded. Each concentration will be tested three times and an average will be worked out. If one result is out of line with the others then that will be easily identified using graphs and tables. The concentration has been picked so that there is a range.  

Prediction:

I predict that the higher the concentration the faster the reaction will take place meaning the light intensity will drop by 10%. This is because the more concentrated the hydrochloric acid is the more particles there are to collide, therefore more effective collisions to break old bonds and make new bonds. 0.25M concentration would be the slowest reaction and 2M would be the fastest reaction. I also predict that if it is hotter the reaction would also be quicker; this is because the particles will be heated up and would have more energy to make them move around faster.

Apparatus:

• 2 Beakers
• 2 Measuring Cylinders
• Hydrochloric Acid (0.25M, 0.5M, 1M, 1.25M, 1.5M, 2M)
• Sodium Thiosulphate
• Conical Flask
• Stand
• Palm Top
• Apron
• Goggles
• Thermometer

Method:

1. Put on apron and goggles
2. Collect chosen amount of HCl and STs
3. Collect all equipment
4. Measure the correct amount of HCl and STs
5. Set up palm top and stand
6. Measure starting temperature
7. Mix together HCl and STs
8. Start the timing and when LUX decreased by 10% stop timing
9. Measure ending temperature and record results
10. Repeat with different concentrations

Analysis:

The graph shows that as the concentration of hydrochloric acid increased, the time taken for a 10% decrease of light intensity decreases. This is because the more concentrated the hydrochloric acid is the more particles there are to cause an effective collision and break old bonds and make new bonds, which makes the reaction take place quicker. The temperature was kept the same as best as possible but if the reaction was hotter then that would speed up the reaction because the particles would move faster because they have more energy and more collisions will be made. There were a couple of anomalies within the experiment and they have easily been identified on the graph, but the majority of the results match with the best fit line. I am not quite sure why these anomalies have shown up but it may have been because the timer wasn’t started or finished at the correct time. The graph shows that the time decreased rapidly but then only gradually decreased until the final concentration. For a concentration of 1M the time should have been around 35-40seconds but was 33.94seconds. Some of the concentrations may have been stirred more than others which caused a different range of times.

Evaluation:

The repeats in these results showed a consistency and the same time was roughly given for each concentration of hydrochloric acid. The graph shows that the time rapidly decreased after the first concentration but then over the next three concentrations only gradually decreased and then on the final concentration rapidly decreased again. The results however didn’t fit that neatly into the line of best fit but they are clear and easy to understand. There were some anomalies within the experiment and this may have been caused by the stirring or the timer being started and stopped wrongly. I think that our group had a good method because our results showed what we were trying to find out, which was to test how the change of concentration affects a reaction. To get rid of any anomalies we could have either not stir the mixture or use a machine to stir it at the same speed each time. I think that the procedure that we had used was adequate to test and that it was reasonably reliable. In my prediction I had stated that as the concentration increases, the time taken for a 10% decrease in light intensity would decrease.This is because more particles would be in the higher concentration causing more effective collisions to happen. In future experiments we could increase the range of results as well as having more intermediate values.