What affects the rate of reaction between Sodium thiosulphate and Hydrochloric acid?

Apparatus

• 500ml Beaker
• Stopwatch-- So I can accurately time the rate of a reaction to 1/100th of a second.
• Measuring Cylinder-with accurate scale, there needs to be two different cylinders because if you measure both of the solutions in the same cylinder it might contaminate it.

What will I keep constant?

• The volume of sodium thiosulphate
• Use the same 250ml beaker

What I will vary?

• I will vary the concentration of hydrochloric acid. (by adding the correct amount of water, stated below.

What I will measure?

I will measure the time taken for Hydrochloric acid and sodium thiosulphate to become opaque. I know this when the black cross which will be drawn on a piece of paper, will no longer be visible. I will take the results in seconds and then calculate the average time for the reaction to happen.

Method  

1. On a plain piece of paper draw a black cross.
2. Measure 25ml of hydrochloric acid into the measuring cylinder.
3. Then place the beaker with the acid on top of it.
4. Add the 25ml of sodium thiosulphate acid  
5. Start timing when all the sodium thiosulphate has been poured into the beaker.
6. Wash the beaker thoroughly and repeat steps 2-5 another two times.
7. Repeat steps 2-5 with different with different hydrochloric acid molar strengths :

Results Table

Fair Test

• I will use separate measuring cylinders for both chemicals so I don’t contaminate either of them until I’m ready to time the reaction.

• I will not use a catalyst in the experiment.

• I will not increase the temperature of the liquids, and the area which I shall be working in will be kept constant at room temperature(22oC)
  

Results Table

Analysis

Referring to graph A, which shows that the concentration of acid increases the rate of reaction, which means my conclusion, agrees with my prediction.

E.g. when the acid was 50ml and the was 50ml sodium thiosulphate 50 ml the average time was 34 seconds for the full reaction to take place.

When the acid was 30ml and the sodium thiosulphate was 50ml the average time taken was 38 seconds.

Looking at graph A you can see that there is a positive correlation between the concentration of hydrochloric acid and the reaction rate.

When hydrochloric acid and thiosulphate react together the sulphur is stronger of the two. This means that as the reaction goes on the solution becomes yellow and will change from being transparent to translucent to opaque. The more concentrated the acid the more particles there are which means the more collisions.

On my graph, the line of best fit shows that the time taken for the reaction between hydrochloric acid and sodium thiosulphate is proportional to the concentration of the hydrochloric acid.

Evaluation

I think that my results were fairly accurate, as there wasn’t any anomalous points. If I were to repeat this experiments I think that I could improve my results even more by

• I don’t think there was a large enough range of results taken  so I would Increase the number of experiments another twice for each category

• Repeat experiment but with a different person to judge when the reaction has finished.