The Effect of Concentration on the Rate of a Chemical Reaction

The Effect of Concentration on the Rate of a Chemical Reaction
Aim ~ to investigate the reaction between Hydrochloric Acid and Sodium
Thiosulphate.

2Hcl[aq] + Na2 S2 O3[aq] àH20[l] + SO2[g] +S[s] +2NaCl[aq]. The sulphur Precipitate causes the reaction mixture to go cloudy. We will use a light sensor to measure the progress of this reaction. Theory ~ the theory for reacting particles depends on how often and how hard the reacting particles collide with each other. For the particles to be able to do this they need activation energy, as they need enough energy to react. There are four factors that affect the number of collisions these are: ~ The temperature, which increases the amount of collisions. ~ The Surface area, which create more collisions. ~ A Catalyst, which increases the number of collision, as the particles have a surface to stick to where they can bump into each other. ~ The Concentration. If a solution is made more concentrated it means that there are more particles. This means that there are more particles knocking around between the water molecules, this makes the collision between the reacting particles more likely.
A low concentration A high concentration Prediction ~ I predict that if I dilute the acid then the reaction will be slower, therefore it will take longer to change colour. I think this because of the collision theory. If I half the concentration of acid then, I think that it will take double the amount of time to change colour. Fair Test ~ in this experiment I am going to change the concentration to change the reaction time. I will use 5 different concentrations, ranging from 1 molar to 0.2 of a molar. This will give me a good range of results. Each time a take a reading for a result I will take it three times so that it gives me a better set of results to use. I will not change anything other than the concentration, as this would not be a fair test. Although I can?t control the natural temperature and amount of light, These factors may effect my results. Method ~ In our test we are going to change the concentration of the acid by diluting it with water. First we set up all our apparatus, this included: A light sensor, which was connected to the computer. This is what took the readings. On top of that was a tripod, with a thin sheet of cardboard on top of it with a hole in it, to let the light through. On top of this was the beaker where the experiment takes place. The next step was to record all of our readings. In the first test we used pure Hydrochloric acid and Sodium Thiosulphate. We then took four more readings, changing the concentration of the reading each time. The second reading was with 20cm3 of acid and 5cm3 of water. The third was used with 15cm3 of acid and 10cm3 of water. The fourth reading was taken with 10cm3 of acid and 15cm3 of water. The last result was taken with 5cm3 of acid and 20cm3 of water. Each time we changed the concentration we took the test on that concentration three times to give us a better result. We started the timing (on the computer) when we put the hydrochloric acid/water into the beaker, we left it ten seconds then added the Sodium Thiosulphate. We stopped timing the reaction after one minute so that it had long enough to react. We then printed off a graph of the results. From the graph we took the time. This was taken from where we decided the reaction had started. After we had all our result for each concentration we gave each concentration an average time, which will be used on the graph and a rate time. 5 0.014 69 47 X - 90 0.2 20 5 0.031 32 32 30 X - X - 0.4 15 10 3 Average (secs) 0.024 42 44 39 24 X - 0.6 10 15 0.027 37 43 34 35 0.8 20 0.031 32 37 33 27 1 0 25 Rate ==== 1/(s) Acid ==== (cm3) Water = (cm3) Concentr-ation (cm3) 1 2 Time to change (secs) Results ~ Conclusion ~ In our graph it shows us that some of our results were not that accurate, due to weather conditions. Although this effected our results we can still se what the obvious pattern is. The pattern is that when you increase the concentration then the faster the reaction will take place. We know that as when the acid is at 1m then the rate time is 0.031 and when the concentration in of the acid is 0.2m then the rate is 0.014. This proves that this particular part of my prediction is correct. Although when the concentration was doubled the reaction time does not double, therefor this part of my prediction is incorrect. This has lead me to believe that there must be other factors which can effect the rate of the reaction. Evaluation ~ when we did the experiment there were a few factors which effected our results. The light sensor that we used was accurate, as this took out any human error, although natural surrounding may of affected it, these things we could not control. Our measuring cylinders were also not the best we could have had. We used pipettes, but there are better measuring cylinders. We also had a problem with the beakers. Each time we did an experiment we had to use the same beaker, this was a problem as the beaker stained which effected our results. The only way to resolve this problem would to use a different beaker each time. We also did the experiments on different days, which made a big difference. The weather conditions and temperature were different to the day before which may of affected our results. Most of these factors we could not do much about. The one thing that we could have done to get better results would be to take more results. We only took a set of three for each concentration. This is not really a lot to base our result on to, even if they do come out the same most of the time. If we did a set of five results to each concentration, I think it would be better as we would have more to look at and give us better results. We had one bad result. This was due to some one opening a blind while we did the experiment. The light sensor was highly sensitive and picked up on this light change. Although it was only one slight change it showed us how our results may be affected so easily. There was another factor that we could have experimented with, this was the temperature. We could have done this by taking a range of temperatures and taking the results a few times. A temperature would have been good to do, as you can be very versatile with the temperatures and would have been less effected by the natural surroundings, which can cause mistakes. I would have used a Bunsen burner, a water bath and ice to get the temperatures. I think that if I had done this experiment then the higher the temperature then the faster the reaction would take place. I feel this would happen as the particles would have more energy and move around more, causing more c0ollisions, therefore the reaction takes place faster.

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