Investigate the principal factors that affect the rate of reaction between Sodium Thiosulphate and hydrochloric acid.

Rates Of Reaction Coursework

Aim:

To investigate the principal factors that affect the rate of reaction between Sodium Thiosulphate and hydrochloric acid.

Theory:

The reaction between sodium thiosulphate solution and dilute hydrochloric acid:

HCl+sodium thiosulphatesodium chloride+sulphur dioxide+sulphur+water.
HCl(aq) + Na2S2O3(aq) NaCl(aq) + SO2(g) + S(s) + H2O(l)

The sulphur formed in this reaction
makes the colourless solution go cloudy.

The reaction is usually carried out in a flask placed on a piece of white paper which has a black cross on it.

At the beginning of the reaction, the cross can be seen easily.
As the flask becomes more and more cloudy, the cross gets harder to see.

Factors which affect the rate of reaction:

Increasing the temperature of a reaction increases the rate of a reaction. At the higher temperatures reactant particles move faster and collide more often and more violently. For every 10°c rise in temperature, the rate of reaction doubles.
Increasing the concentration of reactants increases the rate of a reaction. This is because there are more particles in the same volume so more collisions are possible every second.
Breaking up solids into smaller pieces increases reaction rate because this increases the total surface area. Increasing the surface area must increase the number of particles of the solid in contact with the other reactant in solution.

Using a catalyst increases the rate of a reaction. Catalysts are not used up in a reaction.

Collision Theory and Activation Energy:

Equipment:

Method:

Changing the concentration:

Prepare your reactants. You should have 10cm³ of HCl in a measuring cylinder and 50cm³ of Sodium Thiosulphate (with varying amounts of water to dilute it) in a conical flask.

Heat the sodium thiosulphate on a blue bunsen flame if you are investigating temperature.

Use a pencil to draw a large cross on a sheet of white paper.

Pour the 10cm³ of acid into the conical flask containing the ...

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Using a catalyst increases the rate of a reaction. Catalysts are not used up in a reaction.

Collision Theory and Activation Energy:

Equipment:

Method:

Changing the concentration:

Prepare your reactants. You should have 10cm³ of HCl in a measuring cylinder and 50cm³ of Sodium Thiosulphate (with varying amounts of water to dilute it) in a conical flask.

Heat the sodium thiosulphate on a blue bunsen flame if you are investigating temperature.

Use a pencil to draw a large cross on a sheet of white paper.

Pour the 10cm³ of acid into the conical flask containing the sodium thiosulphate.

Start the timer.

Swirl the solution making sure not to spill any onto your hands.

The solution should start to turn cloudy.

Hold the conical flask over the cross and look down from the top through the solution.

When the cross can no longer be seen, stop the timer.

Record your results.

Wash the apparatus thoroughly.

Repeat the experiment with the same factors and see if you get the same results.

If the two results are not corresponding, repeat the experiment until similar results are achieved.

Analysis Of Results:

Now that I have performed the experiment, I can look at my results and compare them with my prediction and theory.

I predicted that when I increase the temperature of the reactants, the rate of reaction would increase. My prediction was correct. In my theory I stated that, “For every 10°c rise in temperature, the rate of reaction doubles.” Although my results did not completely satisfy this statement, there is some evidence to prove it. At high temperatures, the rate of reaction is very rapid; therefore the time for the reaction to take place is quite small. If you lowered the temperature by 10°c you would expect the time for the reaction to double (20 secs for 50°c, 40 secs for 40°c). My results were very similar to these model results but not always exact. The theory that the rate of reaction doubles for every 10°c rise in temperature always gives a curved graph, it will never be a straight line. My graphs for temperature versus time show this very well.

My other prediction was that increasing the concentration of the reactants would increase the rate of reaction. My results agree with this prediction. Concentrated sodium thiosulphate reacted a lot faster than dilute sodium thiosulphate. The results go down fairly steadily on the graph but the 25cm3 result could be an anomalous result. This result could be correct. The line of best fit is wrong as it says that the solution will go cloudy in 209 seconds when there is no sodium thiosulphate present. This is impossible as the reaction would never take place. Therefore the line should never touch the y-axis but just get steeper as it goes towards it.

Conclusion:

My aim was

“To investigate the principal factors that affect the rate of reaction between Sodium Thiosulphate and hydrochloric acid.”

I have achieved this and gave evidence to prove that the theory is correct. I have investigated how temperature and concentration affect the rate of reaction and am happy with my results.

Errors:

All the timing and measuring in this experiment was done by hand and eye. There will always be slight errors in timing and measuring this way but these should not make much difference to the results as they will usually be consistent. E.g. Timer is always started 0.1 seconds late.

The stirring was done by hand so was sometimes inconsistent. This could lead to faulty results.

The point of completion (when the solution becomes completely cloudy and blocks out the cross) was decided by the same person each time. This made sure that it was a fair test.

When the reactants were spinning, there was less of the solution in the middle of the flask. This thinner layer meant that it was easier to see the cross while the reactants were spinning. This meant that to check to see if it had reached the point of completion, we had to let the reactants stop spinning so there was an even distribution of the reactants.

See diagram a

Diagram a

When heating the reactants, I had to heat the sodium thiosulphate to the right temperature then add the acid at room temperature. This caused a drop in temperature when the sodium thiosulphate and acid were mixed. This is not important as it is a systematic error (it happened every time and made the temperature drop by the same amount). This could explain the reason why the rate of reaction did not always double for every 10°c rise in temperature.

The beakers used were washed before each experiment. There was no chance of contamination from other chemicals as they were all washed with sodium thiosulphate before hand.

Future Investigations/Improvements:

If I could do the experiment again, I would make a few changes. These changes would give more accurate and also a wider range of results.

I would use a magnetic stirrer as opposed to hand if possible. The magnetic stirrer can be set to a certain speed and remains at that speed for the whole experiment.

I would use light sensors to decide exactly when the reaction had reached the completion point, as they are far more accurate than the eye.

I would repeat each experiment at least 3 times to ensure that the results are accurate.

I would heat the acid as well as the sodium thiosulphate when investigating temperature. This would mean that the acid wouldn’t cool down or heat up the solution.