Investigating how the concentration ofthiosulphate solution affects the rate of reaction.

Investigating how the concentration of thiosulphate solution affects the rate of reaction.

Aim: My aim is to investigate how different concentrations of sodium thiosulphate solution affect the rate of reaction.

Hypothesis: As you increase the concentration (molarity), the rate of reaction will increase.

I think this because a high concentration means a large number of particles in a given volume. If there are more particles and the activation energy is right, then there will be more collisions. More collisions mean a large number of successful collisions. Therefore the rate of reaction will increase.

Introduction: In this investigation I am going to carry out an experiment to determine the effects of different concentrations of thiosulphate solution on the rate of reaction. I will do this experiment using the following formula:

Na2S2O3 + 2HCl 2NaCl (aq) + H2O + SO3 + S(s)

I will only change the concentration of the sodium thiosulphate solution to see if this effects the reaction i.e. if it speeds it up. I will use 50cm3 of thiosulphate solution. I will have to do some calculations to be able to work out an appropriate volume of hydrochloric acid (HCl)

Scientific knowledge:

With high concentrations the rate of reaction will be fast. This is because according to the collision theory the more you increase the concentration of a substance then the rate of collision increases. If the solution is made more concentrated it means there are more particles of the reactant colliding with the water particles. Hence the collision between the sodium thiosulphate and the Hydrochloric acid particles becomes more likely. If a low concentration of sodium thiosulphate were used then there would be less collision. The more the collision with particles the faster the rate of reaction will be. Whereas with low concentrations the rate of reaction will be slow.

With high concentrations there is more collision between the particles therefore they will react quickly and largely because the number of particles increase when the concentration is increased. Hence as the amount of solution is kept the same and only the concentration is varied, the time taken for the reaction to process will be shorter. Therefore the cross will disappear quickly. The cross will disappear quickly because when the concentration is increased the numbers particles are also increased causing more precipitate to form in a smaller length of time.

I have simplified my explanation in the diagram below to show the effect of concentration on particle activity.

As you can see the higher the concentration the more the particles are present and hence the more the collision will take place in less time causing more precipitate to form in a short length of time.

Activation Energy

Even if the species are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction.

Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction, the energy profile looks like this:

If the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. You can think of the activation energy as a barrier to the reaction. ...

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Activation Energy

Even if the species are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction.

If the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. You can think of the activation energy as a barrier to the reaction. Only those collisions, which have energies equal to or greater than the activation energy result in a reaction.

Any chemical reaction results in the breaking of some bonds (needing energy) and the making of new ones (releasing energy). Obviously some bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds.

Where collisions are relatively gentle, there isn’t enough energy available to start the bond-breaking process, and so the particles don’t react.

Apparatus:

Below is a list of all the equipment needed for the experiment to be carried out.

Sodium thiosulphate solution- to use it for obtaining results and to see how different concentrations of this affects the rate of reaction.
Hydrochloric acid- to add to the sodium thiosulphate for a reaction to take place.
Burette-to measure the small amounts of liquid measurements
Clamp stand- to hold the burette into place.
Water- to make different concentrated solutions of thiosulphate
Measuring cylinder- to measure larger amounts of a solution accurately (at eye level) i.e. hydrochloric acid
Conical flask- for the reaction to take place in it
Piece of paper- to draw a cross onto it and place the conical flask on the paper so that you can watch the cross until it disappears, due to the reaction
Pen- To draw a cross on the piece of paper
Stop clock- to time how long it takes for the cross to disappear
Goggles- for eye protection
Paper towels- To clean any mess, if any, after the experiment
Results table- to write down your results that you get from the experiment

A diagram for the apparatus is shown overleaf

Preliminary Test:

I will use different concentrations for the thiosulphate solution but I will need to know what range to work from therefore I will have to do a preliminary test. This will tell me if my chosen range is a suitable range to work from and gives me a measurable/ sensible time when the reactants react.

Method:

Firstly collect all the equipment mentioned above

You then have to make different concentrated solutions. A table to show how to make the concentrations is shown below:

You can do each of the different concentrations one after the other to make it easier.

I have shown the working out that I have used to obtain the results in the above table on the previous page.

You measure 13cm3 of hydrochloric acid using a measuring cylinder and then put it into the conical flask. (I have rounded 12.5cm3 to 13cm3 so that it will be easier for me to measure it more accurately.)
You then draw a cross onto the paper and place the conical flask directly onto it.
Then you make your first dilution using a measuring cylinder
When you have done this, you quickly add it into the hydrochloric acid (conical flask) timing it as soon as you put it in.
You then start swirling the conical flask to allow the solution to mix keeping your eyes over the flask.
As soon as you are unable to see the cross any more, stop your stop clock.
Record your first result in a results table.
You use the same procedure for each of the different concentrations making sure that you wash the equipments before reusing them so that no errors can occur.

Results:

From my first graph, you can see that as you increase the concentration of sodium thiosulphate, the time for the precipitate to form decreases.

From my second graph, you can see that as you increase the concentration of sodium thiosulphate, the rate of reaction increases.

Conclusion:

After having analysed the graphs and the results I can say that as the concentration increases the rate of reaction also increases and the precipitate is formed more quickly.

My results and graphs have also proved my prediction/ hypothesis correct. In my hypothesis I had said, “As you increase the concentration (molarity), the rate of reaction will increase.” This is proved correct from my results and graphs. From my results and graphs, you can see that as the concentration increases, the time for the cross to disappear and the rate of reaction also increases.

I can now give an explanation for the conclusions for being as they are.

I have concluded that as the concentration increases the rate of reaction speeds up and the time taken for the cross to disappear becomes quicker. By my scientific knowledge and understanding I will explain why this is so.

When the concentration is increased there are more particles present for collision. Hence, more collision takes place. There are more particles to react with so the reaction becomes faster. When the reaction becomes faster the time taken for the sulphur precipitation to be formed becomes quicker. Therefore the cross disappears quickly. The rate of reaction increases because there are more particles that collide more often. This causes the rate of the reaction to increase. This is simplified in the diagram below:

With low concentrations there are less particles and less collision therefore the rate of the reaction is slow. With high concentrations there are more particles and more collision. Therefore the rate of the reaction is fast.

Evaluation:

Method

The Method that I had used allowed me to collect all the data as evidence to allow me to prove my background knowledge to be correct.

The evidence collected was sufficient to enable a firm conclusion to be drawn. This is because the data that was collected was then analysed and concluded to say exactly what I had written as my background knowledge.

Accuracy

I had encountered some anomalous results. The main reason for getting these anomalies was that when I was looking at the cross. I was not sure whether the cross had disappeared or not, therefore, sometimes I had timed the reaction too long or too short. This caused my results to be different from each other.

I also think that I didn’t swirl the solution at a constant pace for all of the different readings. I may have had a slower pace in some cases and a faster pace in others.

I may have also delayed some time when adding the sodium thiosulphate to the acid. I may have started timing it before I started to mix the solution.

I also realised that I didn’t use the same pen and paper for the experiment. This can cause errors as the darkness of the pen and the thickness of the paper can cause inaccuracy. If the paper is very thin then this may not allow the pen to be more visible because the colour of the table underneath may be a distraction. A thicker pen or a pen with more ink (e.g. fountain pen) will be more clear to be seen whereas a thinner pen may not be as visible. This can cause inaccuracy in the experiment if the same paper and pen is not used.

I think that it was beneficial to have taken three readings for each of the different concentrations of sodium thiosulphate as it allowed me to identify the anomalous results that were present. The anomalous results would have decreased the accuracy of my results because they would not have been correct. Taking three readings for each concentration allowed me to increase the accuracy of the results as I had found the average of the three values.

Overall, I think that my results were reliable enough to support my conclusion. From my first graph you can see that nearly all the points follow the curve of best fit. From my second graph, you can see that all the points follow a straight line.

Improvements

If I were to carry out the experiment again I would propose some improvements to the investigation. I will make sure that throughout the experiment the same person will do the same job e.g. the same person will add the thiosulphate into the acid for each trial and the same person will judge when the precipitate is formed. This can cause an error if different people carry out the same procedure as everyone has different judgments and different eyesight’s. A better improvement for watching when the precipitate is formed is to use a light meter to obtain the timing of the reaction instead of using human eyesight. As I was carrying out the experiment I had found that I did not use the same pen. This can be improved by using the same pen to make sure that the same darkness of a pen is used.

Extension

Further work that could be carried out is that instead of using hydrochloric acid I could use different acids e.g. Nitric acid or sulphuric acid. To see its effects I could use a high concentration of sodium thiosulphate and hydrochloric acid.

I could also use a lower concentration for the acid, if I was to use the same equation, or I could increase the range of the concentrations that I used.

I could also use different concentrations of hydrochloric acid and keep sodium thiosulphate constant to see how this effects the reaction, if any.

Other extensions could include measuring the rate of reaction not by using the precipitation method but the change in mass method or by measuring the volume of gas given off.