Plan
Temperature is the only variable in this reaction, as the aim is to look at the effect of temperature on rate of reaction. All other factors will need to stay constant, including volume of reactants, method of heating up, equipment used, amount of time the mixture is shaken for, and when the cross can be said to be obscured. Here is a step by step plan, taking into account issues of reliability and accuracy.
1. Measure out 35cm3 of hydrochloric acid using a measuring cylinder into a conical flask first. The volumes of the reactants were decided by looking at the preliminary results, as I found a mixture of 35cm3 of hydrochloric acid and 25cm3 of sodium thiosulphate to react at around 2:18 mins, which I believe to be enough time to time the reaction accurately, but not so much time as to make the experiment last too long. To make sure we get an accurate and consistent volume of acid, we will hold the measuring cylinder level to our eyes and read from the meniscus.
2. To get the acid to the temperature needed, we will place the conical flask onto a wire gauze, on a tripod, over a bunsen burner. We will leave the thermometer in whilst it heats up, keeping this level to our eyes to read it too.
3. Whilst one person is heating up the acid, the other person in the two will measure out 25cm3 of sodium thiosulphate after washing the measuring cylinder to prevent premature reaction which could make results unreliable.
4. When the acid reaches the temperature, we will take the flask off the heat and onto the cross on paper. We will pour the sodium thiosulphate into it as soon as it is put down, and start the timer from the minute all the sodium thiosulphate has been poured in.
5. We will shake the mixture to speed up reaction time (shaking causes more particles to collide and therefore more particles that react with each other). To increase reliability, the same person will do the timing, shaking the mixture and reading off the temperature each time. We will shake the flask for roughly five seconds, then stop to see whether it will obscure the cross, then shake it again if it hasn’t, repeating until we can’t see the cross. We will only stop the timer when the cross has been completely obscured.
6. We will do three repeats (with an average) for each temperature, increasing in steps of 10ºC from room temperature (20ºC) to 70ºC, because I think this is a good range of temperatures. Anything under 20ºC would take too long because lower temperatures mean particles have less energy to move around and react with each other, and anything over 70ºC would react so fast as to make it very hard to time accurately.
Unfortunately, because the unheated sodium thiosulphate which is added afterwards will cool it down and it will cool down while reacting, it will be very hard to get the exact right temperature. I predict that the actual temperature will be about three degrees lower than what we aim for. One more thing that could decrease the reliability of our results is if we use different apparatus for the different temperatures and repeats, for instance a more powerful bunsen burner or a slightly faulty thermometer could affect our results. For this reason, we will make sure we use the same apparatus each time.
Results
The graphs I plotted support my prediction to an extent and support the background information. However, my prediction was not accurate because I predicted that with every 10ºC increase, the time taken to obscure the cross would decrease by 4 seconds. In reality, the rate of reaction increased by less and less with each 10ºC increase. For instance when we increased the temperature from 20ºC to 30ºC, the time taken to react decreased by 30.31s, whilst when the temperature was increased from 30ºC to 40ºC, the time decreased by 7.03s, nowhere near as dramatic an increase in rate as the 20ºC to 30ºC jump. By my original prediction, in both the 20ºC to 30ºC increase and the 30ºC to 40ºC increase, the time taken to react would have decreased by 4s, consistently. I drew a graph of time plotted against temperature, and the results formed a steep downwards curve, presenting visually the fact that each 10ºC increase does not decrease the time taken by the same amount.
To illustrate the direct relationship between temperature and rate of reaction, I also plotted a graph of time (s-1) against temperature. The table above shows the results plotted on the graph. I can see from the graph that the points form an almost perfectly straight, upwards pointing line, showing that increasing temperature increases the rate of reaction. Looking at the table, I would say that each 10ºC increase in temperature increases the rate by roughly 0.03s-1.
In conclusion, I was right in my general prediction that increasing temperature increases the rate of reaction. This is because the higher the temperature, the more energy particles have and the more they move around, causing more collisions and so more particles reacting with other, which means that the reaction happens faster.
Evaluation
I think that the method we used to carry out the experiment was reliable, because we took into account the fact that we had to keep all other factors besides the variable constant. This included volume of hydrochloric acid and sodium thiosulphate, equipment used, when exactly to start and stop the timer, how vigorously and long the mixture was shaken for, and other things which could possibly affect the accuracy and reliability of our results, such as making sure to clean reactants out of the measuring cylinders so that they didn’t start reacting before we’d pressed the timer.
You can tell our results are reliable, as they form almost a perfect curve on the graph showing temperature against time taken to react, and almost a perfect straight line on the graph of 1/time. For this reason I would say that our results are definitely good enough to support a firm conclusion, i.e. increasing temperature increases rate of reaction. However, the temperature/time taken graph shows a slight anomaly at 60ºC. This could have been for a number of reasons:
• We could have pressed the timer at the wrong moment, for example not started it just as all the sodium thiosulphate was poured in, or not stopped it as soon as the cross was obscured.
• We might not have shook it consistently, i.e for five seconds, then a pause, then five seconds and so on.
• We might have shook it too gently or too hard, altering the
number of collisions.
• We could have heated it to the wrong temperature, or after it had been heated, waited too long before pouring in the sodium
thiosulphate and starting the timer.
• We could have read the temperature from the thermometer
wrongly.
Despite all these possible things that could decrease the accuracy of our results, it was only a very slight anomaly, so I am pleased with the results and will not repeat the 60ºC test.
One possible way of improving the method could be refining the temperatures we did the experiment at. As I have mentioned in my plan, it was very hard to get the exact temperature written down due to cooling whilst reacting and the cooling effect of the sodium thiosulphate (added afterwards). To get the exact temperature we could have heated it to 3ºC or so higher than the desired temperature so that it would balance out the cooling. In reality, the actual temperature was probably about 3ºC lower than we wanted.
Another area which could have been improved is our shaking of the mixture to make the experiment quicker. This may have decreased reliability, as we had to hold the beaker still to check whether the cross was obscured or not, and as we shook it for five seconds each time, the cross could have been obscured whilst we were shaking the beaker so our times might be slightly off. There is also the danger of us shaking more vigorously for one temperature than another (for whatever reason), which would make the results for which the beaker was shaken harder have a faster reaction time than the others. To combat this, we could have of course just waited for the reactants to obscure the cross without touching the beaker, but I believe this would have taken too long, considering the amount of time available to us. I think stirring would be the best option as this allows you to see whether the cross has been obscured, but would still speed up the reaction enough to be practical.
If I was to look into this area further, I could stay with hydrochloric acid and sodium thiosulphate, and try a different range of temperatures. For instance, it would be interesting to see whether differences of about 2ºC, for instance from 30ºC to 40ºC increasing in 2ºC jumps, would give as strong results - how much rate of reaction is affected by small fluctuations in temperature. I could also try the experiment whilst both reactants were on the heat. This would get rid of the problem of the unheated sodium thiosulphate and the surrounding air cooling down the hydrochloric acid, but it would also mean adjusting the bunsen burner constantly to keep them reacting at the needed temperature. This could be difficult to manoeuvre, and therefore unreliable.