Calcium carbonate + hydrochloric acid Calcium chloride + carbon dioxide + water
CaCO3 + 2HCl CaCl2 + CO2 + H2O
The amount of fizzing does not need to be measured very accurately because these preliminary experiments are qualitative experiments, not quantitative, just to show that the factors which I have researched do actually have some affect on reaction rate in the way in which I expect them to. I will just record which sample from each experiment is the fastest and which is the slowest from observation, times or other detailed results are not necessary.
Temperature
I will put pieces of similarly sized calcium carbonate into beakers of hydrochloric acid of the same concentration, but different temperatures, and record the rate of reaction in comparison to the other samples.
10ml acid and 10ml hot water 10ml acid and 10ml cold water
10ml acid, 5ml hot water and 5ml cold water
Results:
Concentration
I will put pieces of similarly sized calcium carbonate into beakers of hydrochloric acid of different concentration, but with the same temperature. I will record the reaction rate in comparison to the other samples.
10ml acid 5ml acid, 5ml water 2ml acid, 8ml water
Results:
Particle Size
I will put pieces of calcium carbonate of various sizes into beakers containing 10ml of hydrochloric acid, which are the same concentration and the same temperature. I will record the reaction rate in comparison to the other samples.
Large calcium carbonate crystal Small calcium carbonate crystals calcium carbonate powder
Results:
Conclusion
My background research is correct. Surface area, temperature and concentration do affect reaction rate, in the way in which I expected. This means I can now use one of these variables in the main investigation.
Sodium Thiosulphate and Hydrochloric Acid
I will now look at the reaction between sodium thiosulphate and hydrochloric acid. I will be using this reaction for the main investigation, looking at how concentration affects this reaction rate. I am going to test this reaction to check that something does happen, what if anything does happen, and to find out how I can measure the main investigation.
I will put sodium thiosulphate and hydrochloric acid into a conical flask with a cross underneath. I expect the cross to disappear as the reaction takes place.
It took 17.2 seconds for the cross underneath the flask to disappear. It disappeared because the solution went a cloudy yellow colour. It also gave off a smelly gas. This proves that a reaction does occur between sodium thiosulphate and hydrochloric, and I can measure the rate of it by timing how long it takes to go cloudy. I can use this reaction to test the affect concentration has on reaction rates.
Effect of Concentration on Sodium thiosulphate and Hydrochloric acid
The equation for the reaction between sodium thiosulphate and hydrochloric acid is:
Sodium thiosulphate + hydrochloric acid sodium chloride + sulphur dioxide + sulphur + water
Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + SO2(g) + S(s) + H2O(l)
From this I can see that the cloudiness of the water is due to the formation of sulphur. Unlike the other products of this reaction sulphur is a solid, and it is the particles of this solid which are present in the water making it turn cloudy. So in actual fact when measuring this reaction I am measuring the rate of formation of sulphur.
Prediction
In the main part of my experiment I am going to test how concentration will alter the rate of the reaction between sodium thiosulphate and hydrochloric acid.
I think that if either or both of the solutions is less concentrated the reaction rate will be slower. This is because there are fewer particles in the same volume of the solution, so they will collide less. This means it will take longer for the particles to gain enough energy for a reaction to occur, it will take longer for the particles to overcome the activation energy barrier so there will be fewer reactions.
I think that reducing the concentration of sodium thiosulphate will have more effect on the reaction rate than reducing the concentration of hydrochloric acid. This is because the sodium thiosulphate is the compound that contains sulphur. Since we are measuring the rate of formation of sulphur, if there is less sulphur it will take longer for the solution to turn cloudy, making the reaction rate slower.
Although changing the concentration of hydrochloric acid will affect the rate of reaction, it will not have as much affect as the sodium thiosulphate because there will still be the same amount of sulphur present.
Plan
Fair Test
During the whole of this investigation I will make tests fair by keeping everything apart from one variable exactly the same. This includes:
- Using the same size and shape of flask each time, so the timing for when the cross disappears can be measured more accurately and fairly.
- Trying to keep the room temperature the same so temperature doesn’t affect my results.
- The same person doing all of the timing, because their reactions for when the cross disappears should be constant.
- Using the same overall volume of liquid in each test.
- Using separate measuring cylinders for each solution, sodium thiosulphate, hydrochloric acid and water, so they do not contaminate each other.
Method
- Set up the equipment as shown in the diagram below:
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Measure out 25 cm3 of sodium thiosulphate in one measuring cylinder, and 25 cm3 of hydrochloric acid in another measuring cylinder, try to make these measurements as accurate as possible to the nearest ml.
- Add the two chemicals into the conical flask at the same time.
- Start the stopwatch as the chemicals are added together, and time how long it takes for the cross to disappear. Measure the time to the nearest 0.01-second, this should be fairly accurate.
- Record the result.
- Wash out the conical flask ready for the next reaction, so there is no contamination from the previous one.
- Repeat the reaction three times at each concentration. This will make the results more reliable, and will show any anomalous results, this makes the experiment much more accurate.
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Dilute the sodium thiosulphate, using water to keep the volume constant at 25 cm3. This will make the experiment a fair test. Keep the concentration of hydrochloric acid the same at 25 cm3. Dilute the sodium thiosulphate to each of the concentrations in the table below, and fill it in with the results of the reaction.
- After you have recorded three results for each concentration of sodium thiosulphate, change the concentration of the hydrochloric acid, using water to keep a constant volume. Make sure the sodium thiosulphate is always at a constant concentration using no water. Change the concentration of hydrochloric acid to fill in the table below with the results from these reactions.
- When the experiment is finished make sure all of the apparatus is cleaned and put away correctly.
Obtaining Evidence: Results
Changing concentration of Sodium thiosulphate:
Room temperature during experiment = 27 C
Changing concentration of Hydrochloric acid:
Room temperature during experiment = 22 C
Analysis
My results and graph tell me that the concentration of sodium thiosulphate and hydrochloric acid do affect the rate of the reaction between them.
My results back up my prediction in that the more concentrated each solution is, that faster the reaction rate is. This can be proved by results and graphs, which show that by increasing the concentration of sodium thiosulphate from 5ml to 25ml the reaction time decreases from 91.90 seconds to 14.07 seconds. The hydrochloric acid graph also shows similar results, proving my prediction, for example by increasing the concentration by 20ml the reaction time decreases from 31.00 seconds to just 16.85 seconds. The scientific facts that support my results are that if the concentration of either solution decreases the rate of reaction decreases because there are fewer particles present in the same volume of liquid. This means that the particles will collide less frequently so it will take longer for them to gain enough energy to overcome the activation energy barrier to start reacting. This makes the reaction rate slower.
I was also correct in saying that changing the concentration of sodium thiosulphate would have more effect on the reaction rate than changing the concentration of hydrochloric acid. This can clearly be seen by the graphs, because the sodium thiosulphate graph showing 1/time has a gradient of 0.00296, which is much steeper than the gradient of the hydrochloric acid graph, which is only 0.0014, less than half the gradient of the sodium thiosulphate graph. Also by changing the amount of sodium thiosulphate from 5ml to 25ml it made the reaction 77.83 seconds quicker, but increasing the amount of hydrochloric acid by the same amount only made the reaction 14.15 seconds quicker. The sodium thiosulphate has more effect on the rate of reaction than the hydrochloric acid because the sodium thiosulphate is the compound that contains sulphur. We were timing the reaction by watching for when the solution went cloudy. It went cloudy because sulphur was made during the reaction, and unlike the other products it is a solid, so the particles in the solution make it go cloudy and yellow. Since we were measuring the rate of formation of sulphur, if there is less sodium thiosulphate there is less sulphur so it will take longer for the solution to go cloudy, making the rate of reaction slower. The concentration of hydrochloric acid does have some effect on the reaction rate but not as much because the same amount of sulphur is still present in the reaction.
Evaluation
Our experiment was fairly accurate, because most of our results followed a pattern, as shown by the line of best fit on the graphs.
There were only a few anomalies, at 15ml the sodium thiosulphate graph has an anomaly, but this is only about 0.2 seconds away from the line of best fit. Also at 10ml the hydrochloric acid result is slightly anomalous, but again this is only about 0.2 seconds off. The 1/time graph for sodium thiosulphate shows a similar anomalous result as the normal sodium thiosulphate graph, bu the 1/time graph for hydrochloric acid is much less accurate, with anomalies at both 5ml and 20ml, particularly 5ml which is a long way from the line of best fit. Although there are these anomalies mostly they are not far from the line of best fit, showing the pattern of the results, so I still think my result are fairly reliable.
To make the experiment more accurate I could have done several things. Although I made my measurements to a fair degree of accuracy, they could be made more accurate by measuring the time to 0.000 seconds instead of 0.00, and by measuring each solution to 0.5ml instead of the nearest whole ml. I could also improve the accuracy by doing each test 5 times instead of 3, to get a fairer average. Another way in which I could improve the accuracy of my results and graphs is by taking a wider range of results. Instead of changing the concentration of each solution by 5ml each time I could change it by 2ml each time. I could also collect a broader range of results by extending the range to 50ml instead of 25ml, testing whether the same pattern still continues for more concentrated solutions. Also to improve the accuracy, both the sodium thiosulphate and hydrochloric acid experiments should have been done on the same day so the temperature remained constant. Changing the temperature could give the particles less or more energy, depending on which way it went, thus making the reaction faster or slower, making the overall results less accurate.
A different approach to this experiment that would still show similar results is using data logging equipment to test the rate of reaction. This could be done by putting a light source on one side of the flask in which the reaction will take place, and putting light sensor on the other side. As the reaction takes place the amount of light passing through the flask will slowly decrease, and by using data logging equipment this amount of light passing through could be plotted against time on a graph, to show a similar pattern of results as the concentration changes. This may be more accurate than getting someone to time the reaction, because there is no time delay because of the timing persons reactions, and also the experiment stops at exactly the same point each time.
Further work that would provide additional information about the reaction between sodium thiosulphate and hydrochloric acid and reaction rates in general, is an experiment to test a different variable. For example you could carry out an experiment to test how temperature affects the rate of this reaction. This would be done by heating up both reactants in a water bath before the reaction, and then when they are at the correct temperature, putting them together, and timing the reaction in a similar way as in this experiment. I would use a range of 5 temperatures, 25 C (room temperature), 35 C, 45 C, 55 C and 65 C. This would give me a good range. I would fill in a results table as shown below:
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