What factors affect the rate at which hydrochloric acid reacts with sodium thiosulphate?

Planning:

This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction. They react as in the equations below:

Sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water

Na2S2O3(aq) + 2HCl (aq) -> 2NaCl (aq) + S(s) + SO2(g) + H2O(l)

For a reaction to occur the reactants must a) collide

b) Collide with sufficient energy in

order to make a successful

collision.

The collision theory is useful when carrying out this experiment because it explains why rates of reactions differ. The concentration of a solution effects the rate of reaction because the rate depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules in a given volume than a more dilute substance. Therefore the frequency of successful collisions is greater, and the reactions will occur more quickly. The 4 factors that control the rate of a reaction are: Temperature, concentration, surface area and the use of a catalyst.

Variables:

Temperature- I will conduct my experiments at room temperature as an increase could speed up the reaction. This is my controlled variable.

Concentration- I will be altering the concentration of Sodium thiosulphate and this will be my independent variable.

Stirring the solution- I will try to keep jogging of the solutions to a minimum so as not to alter the rate of reaction for any of my experiments

In order to make sure that I carry out a safe experiment I will wear goggles at all times and take care when I am pouring out the liquids.

Prediction

I predict that as the concentration of sodium thiosulphate increases the rate of reaction will also increase as there will be more particles and therefore more successful collisions between Sodium thiosulphate and Hydrochloric acid particles.

This is a preview of the whole essay

I think that if I double the concentration the reaction rate will also double and the time will half, as they are directly proportional. I expect this to happen because using the background knowledge it becomes apparent that if the number of particles doubles, then the chances of successful collisions will also double, making the concentration and rate directly proportional. The reason I predict this is; in a dilute solution there are not so many particles to collide with. This means there is not as much chance that a sodium thiosulphate particle will collide with a hydrochloric acid particle so it takes longer for all the particles to have a successful collision and the reaction will be slower. However in a concentrated solution there are more particles pact together so the chance of a successful collision occurring is very high and the more successful collisions means there will be a faster the reaction.

Method

I am going to measure how changing the concentration of sodium thiosulphate affects the reaction time between it and hydrochloric acid.

When they react they form sulphur, which slowly forms as a precipitate. I will be timing the reaction to see how long it takes for the mixture to turn cloudy.

Does changing the concentration of a liquid affect the reaction rate? I will test this by putting Sodium Thiosulphate + Hydrochloric acid in a conical flask, which has a piece of paper under it with a black cross on. As soon as I mix the Sodium Thiosulphate with the Hydrochloric acid I will time how long it takes for the cross to disappear. The faster the reaction the quicker the solution will precipitate, so therefore, the quicker the cross will disappear. I will then do the same experiment but add water to the Sodium Thiosulphate to dilute it and time the experiment again to see if there is any difference. I am going to test this on 9 different concentrations of Sodium thiosulphate ranging 50cm- 10cm.

1) Measure out 5cm3 hydrochloric acid in a measuring cylinder. 2) Pour the hydrochloric acid in conical flask. 3) Measure out sodium thiosulphate in a measuring cylinder (from 50 cm3 and no water to 10 cm3 of sodium thiosulphate and 40cm3 of water.) 4) Pour in to the conical flask with the Hydrochloric acid and start the stop watch. 5) Time how long it takes for the cross to disappear completely. 6) Repeat experiment 8 times using different concentrations

After completing a preliminary experiment I was able to use it to inform my plan- see planning sheet.

Fair test

To make this experiment a fair test I will only vary one thing - the concentration of the Sodium Thiosulphate solution. I will conduct all the tests at room temperature because temperature has an effect on the rate of the reaction. The measures of Hydrochloric acid will all be the same (5cm) and measured with droppers at eye level to get the most accurate readings possible. I will also use the same method for mixing all the solutions, rinsing all the conical flasks out after each experiment in case some of the liquid is left, making the volumes irregular. I will also make sure that I add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could affect the results of the experiment. I am going to start the stopwatch at the exact time as I put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in, and one person can start the stopwatch.

Measurements

I am going to take a range of readings, ranging from 50cm of sodium thiosulphate (and 0ml of water) to 10cm of sodium thiosulphate (and 40ml of water.) In my preliminary experiment I stopped taking readings as 5cm of sodium thiosulphate but I found that the reaction took over 10 minutes and that it would be more sensible to stop at 10cm of sodium thiosulphate.

I will also take 2 readings for each concentration to make sure my results are more reliable. If I obtain any anomalies then I will repeat the experiment again so that I can get a more accurate average.

Apparatus

2 measuring cylinders 1 stop-watch, 1 conical flask, 1 piece of paper with a thick black cross, Droppers hydrochloric acid Sodium thiosulphate Distilled water Safety goggles.

Analysis

After plotting 2 graphs; one studying concentration against time and the other of rate against concentration I am able to analyze my results.

My first graph (concentration + time) showed that as the volume of Sodium thoisulphate decreased, the time increased supporting my prediction that the more particles of Sodium thiosulphate present, the more the rate of reaction would increase. My other graph, on the whole, showed that the rate of reaction was directly proportional to concentration, as I stated in my prediction.

This table shows the points on my line of best fit. This proves that as the concentration doubles the rate also doubles, but only to an extent. For example from 16g/dm – 32g/dm the rate multiplied by 2.2 (almost doubling), showing that my prediction was more or less correct. The same is shown from 12g/dm- 24g/dm where the rate multiplied by 2.3. The graph plotting rate against concentration has positive correlation as when the concentration increased the rate of reaction did as well. This meant the particles would collide more often and with more energy as the particles would have a sufficient Activation energy in order to react successfully. When the solutions of reacting particles were made more concentrated there were more particles to react with, therefore successful collisions between reacting particles were more likely to occur. I found that the results I collected proved the prediction I made to be true.

I also predicted that as the concentration doubled the time would half. This can also be shown using my line of best fit for the concentration against time graph. For example as the concentration of sodium thiosulphate doubled from 16g/dm- 32g/dm, the time almost exactly halved –e.g. by 2.2. The same can also be shown from 12g/dm- 24g/dm where it halved by 2.3.

Evaluation

I think that my results were accurate to an extent as I was able to use them to help back up my prediction. The line of best fit was very close to creating a smooth curve which shows that my results are fairly reliable. Although the rate did not exactly double when the concentration did, it gave me enough evidence in which to support my hypothesis. I did obtain 1 anomaly result at concentration,

In order to improve the experiment and the accuracy of my results I could have;

Repeated the experiment more than twice to obtain more accurate results
Used measuring cylinders for smaller volumes- e.g. so that the measurements of the liquids would be more precise.
Gotten 2 people to time the experiments at the same time so that I could take the average times.

I think that my experiment was successful because it helped to support my original prediction; that as you increase the concentration of Sodium thiosulphate, the rate will also increase and the time will decrease. I think that in general the experiment I conducted was fair as I made sure that I only altered the concentration of Sodium thiosulphate, and washed out all the conical flasks after each experiment, so that none of the liquids were left inside.

Before conducting my final experiment I did some preliminary work in order to familiarize myself with the project. This helped to improve the accuracy of my results because I was then able to make any changes in the way I conducted the experiment.

My results do seem, on the whole, to fit my conclusion and despite the anomaly I was able to produce a smooth line of best fit. I think that I have obtained sufficient evidence in which to support my conclusion, although taking more than 2 readings would have helped to increase their reliability. For example from 16g/dm- 32g/dm the rate almost doubled by 2.2, which shows that although they do tend to imply that as the concentration doubles, the rate also doubles, it would have been more informative to take the average of 5-6 readings and see if the results changed.

If I were to extend this investigation and carry out any further work I could look into the affects of catalysts (and other variables) on reaction times. I could also carry out the same experiment using different substances or by varying the amount of acid instead. It would also be interesting to substitute the ‘piece of paper with a black cross on it’ for a light sensitive detector which would go out when the reaction had completely clouded over. Therefore my times would be more accurate and my graphs would be more precise.

Overall I think my experiments were successful and I believe my evidence was sufficient enough in which to support my hypothesis.