Rate of Reaction between Hydrochloric Acid and Sodium Thiosulphate.
Katherine Allen
Rate of Reaction between Hydrochloric Acid and Sodium Thiosulphate
Aim
The aim of this investigation is to find out how the concentration of a solution affects the rate of reaction. I am going to investigate how the concentration of sodium thiosulphate affects the time it takes to react with hydrochloric acid.
Introduction
When hydrochloric acid and sodium thiosulphate solution react, one product of the reaction is sulphur. The sulphur is produced and appears slowly, eventually resulting in the whole mixture becoming opaque.
The word equation for this reaction is:
sodium thiosulphate
+
hydrochloric acid
-->
sodium chloride
+
sulphur dioxide
+
sulphur
+
water
The chemical equation is:
Na2S2O3 + 2HCl --> 2NaCl + SO2 + S + H2O
There are several factors affecting the rate of reaction. These include the surface area, the temperature, the concentration and catalysts.
Surface area
The larger the surface area, the more collisions can occur. The reaction is therefore faster.
This is not a factor that I need to worry about, considering it is liquids that I will be reacting.
Temperature
In most reactions, bonds in reactant molecules must first be broken before bond in product molecules can form. The minimum energy required to break the bonds is called the activation energy. If molecules hit each other slowly, they may not have enough energy to react. Heating will mean more there is more energy to be converted into kinetic energy, allowing the molecules to move faster, so more collisions are likely to result in a reaction. A thermometer will therefore be kept near to where the experiment is being carried out, to monitor the temperature and to ensure no change has occurred.
Catalysts
A catalyst speed up a reaction without being used up itself. It weakens the bonds in the reactant molecules, so less energy is needed in a collision to make the molecules react. More molecules will therefore have the energy required.
No catalysts will be added at any point throughout the experiment.
Concentration
It is the concentration that I will be investigating and is not therefore a variable that needs to remain constant.
Prediction
I predict that as the concentration of the sodium thiosulphate increases, the time taken for the cross to disappear will decrease. Furthermore, I predict that this will be inversely proportional, so as the concentration of the sodium thiosulphate is doubled, the time taken for the cross to disappear will be halved. For example, if the time taken for 40cm³ of sodium thiosulphate to react with the hydrochloric acid, making the liquid opaque was ...
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Concentration
It is the concentration that I will be investigating and is not therefore a variable that needs to remain constant.
Prediction
I predict that as the concentration of the sodium thiosulphate increases, the time taken for the cross to disappear will decrease. Furthermore, I predict that this will be inversely proportional, so as the concentration of the sodium thiosulphate is doubled, the time taken for the cross to disappear will be halved. For example, if the time taken for 40cm³ of sodium thiosulphate to react with the hydrochloric acid, making the liquid opaque was 20secs, I would expect it to take 40secs for 20cm³ of sodium thiosulphate to do the same. I think this because when the concentration of the sodium thiosulphate is doubled there will be two times the amount of molecules for the hydrochloric acid molecules to collide with and the reaction should therefore occur twice as fast.
Method
* Hydrochloric acid
* Sodium thiosulphate
* Water
* 2 burettes
* A pipette
* A clamp stand
* A 75cm³ beaker
* A stopwatch
* A test tube
* A thermometer
* A cross drawn on paper
The apparatus I will use includes:
The experiment will be set up as follows:
A large cross will be drawn onto a piece of paper and placed underneath a 75cm³ beaker. Two burettes will be filled, one with water and the other with sodium thiosulphate solution. A 10cm³ pipette will be filled with hydrochloric acid.
For each experiment that is carried out, there will be a total of 50cm³ of liquid in the beaker. The total will remain the same all the way through to ensure the experiment is fair. There will be 10cm³of acid in every experiment as changing the amount of acid would affect the results. It is the amount of water and thiosulphate solution that will change throughout the investigation. 5cm³ of thiosulphate solution and 35cm³ of water from the burettes will be measure out into the beaker. 10cm³ of acid will then be measured out from the pipette into a test tube, so that it may be poured quickly into the beaker. As it is poured in, the stopwatch will be started and time taken for the cross underneath the beaker to disappear from view will be recorded. This will then be repeated for accuracy and, if the two results vary greatly, they will be repeated again.
The same experiment will be carried out, each time increasing the amount of sodium thiosulphate by 5cm³ and decreasing the amount of water by 5cm³. This will continue until there is no water added, just 40cm³ of sodium thiosulphate.
A thermometer will be kept next to the apparatus and checked regularly in order to ensure no rise or fall has occurred.
Safety
Throughout the investigation safety goggles will be worn to protect eyes, and care will be taken when handling the hydrochloric acid.
Preliminary Work
Before the investigation is carried out, preliminary work was done. Experiments were carried out with the highest proposed concentration of sodium thiosulphate and the lowest proposed concentration, in order to ensure that the fastest time would not be too fast and the slowest time would not be too slow. It the reaction occurred too quickly, it would be hard to take an accurate reading from the stopwatch but if it took too long a limited amount of experiment could be conducted within the time allowed.
Preliminary Results
Acid (cm³)
Sodium Thiosulphate (cm³)
Water (cm³)
Time (secs)
0
5
35
300
0
40
0
28
From my preliminary results I can see that my proposed concentration boundaries will be suitable to time. The preliminary work has also highlight the importance of measuring the liquid out accurately and starting and stopping the stopwatch at exactly the right moment.
Results
Acid (cm³)
Sodium Thiosulphate (cm³)
Water (cm³)
Time 1 (secs)
Time 2 (secs)
Average time (secs)
0
5
35
294
308
301
0
0
30
34
35
34.5
0
5
25
78
79
78.5
0
20
20
58
58
58
0
25
0
43
43
43
0
30
0
36
37
36.5
0
35
5
31
31
31
0
40
0
26
28
27
Analysis
From looking at my graph I can see whether or not my prediction was correct.
The graph plotted to show the concentration of sodium thiosulphate against time is inversely proportional, which ties in with my prediction - that as the concentration of the sodium thiosulphate increases, the time taken for the cross to disappear decreases. This is because of the collision theory; when there are more molecules of sodium thiosulphate there will be more collisions with the molecules of hydrochloric acid, therefore speeding up the reaction and decreasing the time taken for the cross to disappear.
Also, in my prediction, I said that this relationship would be proportional. Allowing for experimental error, I can see that this part of my prediction was also correct. When there was 40cm³ of sodium thiosulphate, it took 27secs for the cross to disappear from view, and when there was 20cm³ of sodium thiosulphate the cross took 58secs to disappear. Although this is not exactly twice the time, the trend can be seen. However, when the concentration is lower, the trend is not nearly as accurate. When there was 10cm³ of sodium thiosulphate, the cross took 134.5secs to disappear and when there was 5cm³ it took 301secs. This may be due to human error or there may be another reason. I could repeat these experiments to test whether my original results were accurate or not. If they remain the same as the ones I have at the moment, I could experiment using a larger range of results, perhaps taking the concentration of sodium thiosulphate down even further, to see whether this break of trend continues.
On my graph showing the concentration against 1/time (rate) the results form a straight line. This means that the concentration of the sodium thiosulphate is directly proportional to the rate of the reaction. Again, this relate to the collision theory - as the concentration increases, so will the number of collisions and therefore the rate.
Evaluation
Although I conducted this investigation as accurately as possible, some anomalous results still occurred. There could have been many reasons for this.
I can be sure that no rise or fall in temperature occurred as I regularly checked a thermometer kept close to the apparatus. The chemicals were measured out using accurate equipment but there could possibly have been some error in ensuring the amounts were exactly correct. In a future experiment I may need to take more time in making certain the measured amounts are completely accurate. Another reason could have been contaminated equipment; although I washed out the beaker thoroughly after each experiment there may still have been a trace of acid, which could have affected the results. This is an experiment that relies heavily on human judgement. The exact moment that the liquid in the beaker became opaque may not have been entirely accurate, considering it was judged only by the human eye. Starting and stopping the stopwatch at the right moment may have also been an area of error, although it was done to the best of our ability.
2 results for each experiment are not really enough to ensure they are completely accurate. In a future experiment I would take 3 or perhaps four in order to check for anomalous results.
There are many different experiments I could do in order to investigate the rate of reaction between sodium thiosulphate and hydrochloric acid further. I could experiment using even less sodium thiosulphate to see whether or not there is a point whereby my conclusion are no longer true. Alternatively, I could change the variable and investigate how temperature affects the rate of reaction.
I would conduct these experiments by measuring out 20cm³ of sodium thiosulphate and adding 10cm³ of hydrochloric acid. I would then time how long it takes for the cross to disappear. Keeping these volumes the same in every experiment, I would heat the sodium thiosulphate by 10° then add the acid. I would keep heating it by 10° until it reached 70°, each time, timing how long the cross took to disappear and recording the results.
