DIAGRAM: This diagram shows the set up of my investigation:
SAFETY:
To ensure safety throughout this investigation I will wear goggles at all times when handling the hydrochloric acid. When I have finished each experiment, I will pour the remaining solution straight down the sink, and run water down the pipes to make sure that the solution is entirely washed away. This is to make sure that the sulphur dioxide fumes that should result from these experiments do not harm anybody.
OBSERVATIONS TO BE MADE:
During these experiments I will time the reactions, and see if the speed of the reaction changes along with the concentration of the solution. I will see if there is any sort of pattern between the concentration and the rate of reaction, and I will also record whether or not my results are directly proportional. When I have completed my experiments, I will compare my results with those of at least two other groups. I will record my findings on both graphs and tables.
FAIR TEST:
To make this a fair test, I will use the same measuring cylinder each time, and the same piece of paper. As I will be working in a group, the same person each time will decide he the cross can no longer be seen.
PREDICTION:
For this investigation I predict that as the amount of water used increases, the reaction time will decrease. This is because as the amount of water increases the sodium thiosulphate will become more diluted. It will then have a weaker concentration, and so will take longer to react with the hydrochloric acid.
Therefore, my prediction is:
Fastest reaction time – 50cm3 sodium thiosulphate, 0cm3 water and 10cm3 hydrochloric acid
Slowest reaction time – 10cm3 sodium thiosulphate, 40cm3 water and 10cm3 hydrochloric acid
SCIENTIFIC BACKGROUND:
Aside from changing the concentration, the rate of reaction can also be altered in other ways. Light, temperature, particle size and pressure can all affect reaction rates and times. If the temperature is raised, the particles in the solution begin to move faster. This causes more collisions between them, and so the reaction happens more quickly. Light also speed up reactions – light is able to break molecules into free atoms, and so they react faster. When the pressure is increased, more collisions happen between the particles in the solution. When the particles size decreases, the particles have a larger surface area. This means that more of the solution can come into contact with the particles – hence, a faster reaction.
(Scientific information obtained from Letts textbook and Encarta ’98)
PRELIMINARY WORK:
I have conducted this investigation previously, but instead of changing the concentration to alter the reaction rate, I changed the temperature of the water that I used. My results were as follows:
These results show that altering the temperature also changes the rate of reaction – as the temperature increases, the rate of reaction decreases. This shows that other things apart from the concentration can be used to change the time taken for a reaction to happen.
OBTAINING EVIDENCE:
Whilst conducting these experiments, I timed the rate of reaction each time, until the cross could no longer be seen. To make this fair, the same person each time judged when the cross could no longer be seen.
These are my results:
After conducting my experiments and recording the results, I took results from two other groups, Sophie’s group and Laura’s group. Their results are as follows:
Rate of reaction
There is only one anomalous result in these results – that is the rate of reaction for experiment 4 in Laura’s group. In this case, the rate of reaction actually decreases. However, in all the other results, as the concentration decreases, the time of the reaction increases.
This table shows the average rate of reaction for each group:
OBSERVATIONS:
As the reaction took place, a yellow precipitate of sulphur dioxide formed in the beaker. This made the solution a cloudy, yellow colour. As more and more sulphur dioxide formed, the solution became increasingly hard to see through. When the solution became so cloudy that we could no longer see the cross, we knew that the reaction was complete.
ANALYSIS:
My results show me that as the concentration of the sodium thiosulphate, hydrochloric acid and water solution decreases, the time taken for a reaction to happen increases. This is because as more water is added, the solution becomes more diluted. As the concentration becomes weaker, the sodium thiosulphate will take longer to react with the hydrochloric acid.
My experiments showed me that as the sodium thiosulphate and HCL react with each other, they produce a cloudy yellow sulphur dioxide precipitate. This precipitate makes the solution increasingly hard to see through, and also makes the cross on the paper impossible to see once the reaction is complete.
CONCLUSION:
From this investigation I have drawn the conclusion that varying the concentration of a solution does indeed affect it’s rate of reaction. As the concentration of the solution became weaker, the reaction time increased, which shows that the concentration had indeed affected the time that it took for a reaction to happen.
My results support my original prediction. I had said that the fastest reaction would take place in the solution containing 50cm3 of sodium thiosulphate and no water, and that the slowest reaction would happen in the solution containing 10cm3 of sodium thiosulphate and 40m3 water. Both of these predictions were accurate.
EVALUATING EVIDENCE:
The results from my experiments were not completely accurate – if they had have been, they would have been directly proportional. However, the basic trend showed that the weaker the concentration, the higher the rate of reaction was. I didn’t have any anomalous results from my experiment or Sophie’s, but I did have one that resulted from Laura’s investigation. During experiment four, her reaction time went down when theoretically it should have gone up – it went from 23 seconds, up to 30 seconds and then back down to 28. There could have been various reasons for this. For example, the beaker may not have been washed out thoroughly from the previous experiment, or a different person may have judged when the cross could no longer be seen. Because not all the results are completely accurate, they are not sufficient enough to support a firm conclusion.
IMPROVMENTS ON FURTHER WORK:
I could improve on the accuracy of this investigation in a few different ways. For example, instead of decreasing the sodium thiosulphate by 10cm3 each time I could instead decrease it by 5cm3. This would give me a wider, and possibly more accurate range of results. I could also do this for the amount of water that I use.
Although the experiment was handled fairly, it could have been even fairer. This is because different members of the class judged when the cross on the paper could no longer be seen, and as all the members of the class have different levels of eyesight, this is not exactly accurate.
We judged that the reaction was complete when the cross on the pad could no longer be seen by the naked eye. This could have been made 100% accurate by using a device such as a light sensor. The light sensor would pass through the beaker, and when it could no longer reach the cross, the timing would be stopped automatically. This would make the experiment both accurate and exactly fair.
