What affects the rate of reaction between dilute hydrochloric acid and sodium thiosulphate solution.

1. Hydrochloric acid (2.M)
2. Sodium thiosulphate (1.5M)
3. 10cm3 cylinder
4. 20cm3 cylinder        
5. 50cm3 cylinder
6. 250ml conical flask
7. Stopwatch
8. Piece of Paper
9. Distilled water
10.  A fine black pen
11.  Pipette  

The reaction I will be investigating is:

Sodium  +       hydrochloric            sodium +water + sulphur +sulphur                   Thiosulphate            acid                   chloride                            dioxide

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

Prediction

I predict that as the concentration of Na2S2O3 is increased, the rate of reaction will increase. I predict this, because if there are more molecules, they are more likely to collide, and therefore react. If the concentration of Na2S2O3 is doubled, I predict that the rate of reaction will also double, and if the concentration were to be zero, then the rate of reaction to be zero. In the graph, I expect there will be a negative correlation.
Fair Test

To allow this to be a fair test, possible variables within control, will be controlled and remain constant. This experiment will take place at room temperature, usually around 20C. Also, several readings will be taken, in order for average results. Results taken will be made as precise as possible. It has been decided that a concentration of no less than 10cm3 will be tested because any less than this would probably take too long. It has been decided that 10ml of HCL and 30ml of Na2S2O3 will be used.

Diagram:

Before the apparatus was set up the experiment had to plan as fairly as possible. This involved only changing one variable at a time, therefore all other variables are kept constant. Firstly the volume of the sodium thiosulphate was kept as similar as possible, I did this by measuring the volume as 30cm3 in a 50cm3 cylinder. Another variable I took into account was the volume of Hydrochloric acid I will use. I will change the amounts of acid in each concentration. I could not control the room temperature would stay around the same duration of the experiment. The temperature would affect the concentrations of the acid. I would start the stopwatch immediately when I pour the acid.

To obtain these results I then measured and recorded how. I predict that the solution with the high concentration of acid will have a much faster rate of reaction than the weaker solution. I have predicted this from the last experiment I did on marble chips.

Procedure

Put 30cm3 of sodium thiosulphate in to the conical flask. Draw a small cross on the piece of paper and put the flask on top of it.

81331556. Pour the acid into the sodium thiosulphate solution and start the clock.
81331557. Record the temperature.
81331558. Time (in seconds) how long it takes for the cross to disappear

Wash out the flask thoroughly.

Repeat using the same volume of sodium thiosulphate, and the concentration of hydrochloric acid and water as given in the table.

Obtaining Evidence:

 

Analysing and Considering Evidence:

Experiment 1 was repeated because it was an anomalous on my graph and I got it in 30secs.From looking at my results and graph I am able to conclude that the more concentrated the quicker the reaction happened. There was a dramatic change in the amount of seconds it took for the cross to disappear as more water was added to the solutions. For example: If you look at the graph, it shows how much longer it takes for the cross to disappear during the experiment. I find it hard to explain. As I look at my results, the more diluted the solution is the longer it takes the cross to disappear. Before I did the experiment I made some predictions they were, that the more concentrated the acid was the quicker the reaction would happen and more sulphur dioxide would be produced. This was correct because what I thought would happen, did happened.
         At this point I am able to answer my main hypothesis, which was “What affects the rate of reaction between dilute hydrochloric acid and sodium thiosulphate solution”. The answer to this would be that changing the concentration of the acid will either make it go slower pr quicker. These factors affect the number of collisions of reactant particle, and this affects the rate of reactions. If the number of collisions per second increases then the rate of the reaction will increase.
Overall this was a very successful experiment, everything went very smoothly, that was why my results were very good, and I am able to make a conclusion like this

Evaluating:

I feel that this was a successful experiment. My results seem to indicate a clear pattern from which I was able to confidently draw a conclusion. This conclusion confirmed my hypothesis. This method produced results accurately and quickly enough to finish the experiments in the allotted time. My results were reliable, as the experiment was a fair test. I ensured that this was true by keeping all of the variables not involved in my experiment constant. I could not control the temperature at any time in the reaction expect for at 0 seconds. This is because the reaction released thermal energy. I can say that no other energy was deliberately put in or taken out of the contents of the conical flask during the experiment. I used same volume of sodium thiosulphate throughout the whole experiment, this kept the surface area constant, but something unexpected happened.
        I had a problem with my method in that I had to pour the acid and the water into the conical flask and start the stopwatch. I think this is the reason for the abnormalities in my results, as I did not start the watch at the correct time when I pour the acid and water into the conical flask. The truth of my conclusion is not undetermined by this however, as the same effect happened to each test.

To provide more evidence for my investigation I could use a thermometer to measure the temperature during the experiment, this would show whether my theory of the lower concentration not heating up the reactants is true or not. For further investigation I could use stronger acid to see if there is limit to how quickly the reaction can take place.

Reference:

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