Investigating the Reaction Between Sodium Thiosulphate and Hydrochloric Acid

GCSE Science

Investigating the Reaction Between

Sodium Thiosulphate and Hydrochloric

Acid

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References

In order to understand the experiment better, and to get more accurate results, I used the following sources for extra information:

Science to 16- Pople and Williams, Page 19
GCSE Chemistry- Earl and Wilford, Chapter 11
www.schoolchem.com/rc4.htm

Planning

Research

Measuring the Rate of Reactions

The rate of reactions can be measured by either tracing the lessening of the reactant, or the development of a product.

What is the collision theory?

A reaction can only happen between particles when they collide. Particles have to collide with enough force in order for a reaction to be possible, as if they do not collide with enough power, the particles merely rebound off each other, and so no reaction can take place. Particles have more energy the faster they are moving, and therefore the collision rate increases. Particles move faster when the temperature is increased.

How can we change the rate of a chemical reaction?

There are five ways to increase the reaction rate. These are:

1) Increasing the temperature
2) Increasing the concentration

4) Increasing the surface area of a solid (e.g. a powder has a larger surface area than a block)

5) Use a catalyst.

The reaction rate can be decreased by:

1) Decreasing the temperature
2) Decreasing the concentration

4) Decreasing the surface area of a solid (e.g. a block has a smaller surface area than a powder)

5) Not using a catalyst.

What is a ‘catalyst’?

Catalysts are often used to increase the rate of a reaction. However, the catalyst itself does not take part in the reaction, as this would give inaccurate results. Catalysts are not affected in any way by the reactions, and so may be re-used. A catalyst works by providing a suitable surface for the reaction to take place on. The particles collect on surface of the catalyst and collide more frequently with each other, therefore increasing the reaction rate. A catalyst is often used as a powder so that the surface area is larger.

Which reactions could be used?

There are three main reactions which can be looked at to show how the rate of reaction can be changed. They are:

The reaction between calcium carbonate and dilute hydrochloric acid:

Formulae:

Hydrochloric acid + calcium carbonate calcium chloride + carbon dioxide + water

HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)

The rate of reaction can be measured by following the speed of the formation of the carbon dioxide, which can be done by carrying out the experiment in an open flask positioned on a balance. As the carbon dioxide is formed and escapes into the atmosphere, the mass of the flask will decrease.

The reaction between sodium thiosulphate solution
and dilute hydrochloric acid:

Formulae:

Hydrochloric acid + sodium thiosulphate sodium chloride + sulphur dioxide + sulphur + water
HCl(aq) + Na2S2O3(aq) NaCl(aq) + SO2(g) + S(s) + H2O(l)

The sulphur made in this reaction causes the colourless solution to go cloudy, and eventually to become opaque.

The reaction is usually carried out in a clear flask or beaker positioned on a piece of plain white paper which has a black cross drawn onto it. The cross can be easily seen through the solution at the beginning of the reaction. However, as the sulphur is formed the solution becomes cloudier, until eventually the cross is no longer visible.

The disintegration of hydrogen peroxide solution.

Formulae:

Hydrogen Peroxide oxygen + water.
2H2O2(aq) O2(g) + 2H2O(l)

This experiment is performed in a closed flask. A gas syringe is linked to the top of it. The reaction can only begin with the addition of a catalyst to the hydrogen peroxide. The catalyst used in this situation consists of manganese oxide. The amount of oxygen in the gas syringe increases as the reaction develops.

Planned Method

What am I going to call my experiment?

Investigating the Reaction between Sodium Thiosulphate ...

This is a preview of the whole essay

The disintegration of hydrogen peroxide solution.

Formulae:

Hydrogen Peroxide oxygen + water.
2H2O2(aq) O2(g) + 2H2O(l)

Planned Method

What am I going to call my experiment?

Investigating the Reaction between Sodium Thiosulphate and Hydrochloric Acid

What am I going to do?

I wish to find out how altering certain variables will alter the reaction rate between sodium thiosulphate and hydrochloric acid. I plan to change the volume of the sodium thiosulphate, whilst keeping the volume of hydrochloric acid, the concentration of the hydrochloric acid, and the temperature consistent. I will increase the amount of sodium thiosulphate by 10cm3 each time, and will investigate using between 10 and 100cm3 sodium thiosulphate. I will perform each experiment three times, in order to ensure accuracy of results. If I get time, I wish to study the effect that changing a different variable has on the reaction rate.

Equipment needed:

Sodium thiosulphate
Dilute hydrochloric acid
Thermometer
Beaker
Paper with a clear marking (such as a cross) on it
Goggles
Measuring Cylinders (x2)
Droppers (x2)
Stopwatch

Why have I chosen to use these particular instruments?

Sodium thiosulphate: I will be altering the volumes of this chemical, in order to see when the reaction works best.

Dilute hydrochloric acid: This chemical makes up the second part of the reaction that I will be examining. This is a corrosive substance, and should be handled with care.

Thermometer: This provides a reliable measurement of temperature, and will assist me in ensuring that the solution remains at a constant temperature.

Beaker: I will be using a 250ml beaker, as this size is adequate for my purpose. Beakers are fairly robust, and will only shatter under extreme conditions. Extreme amounts of heat will damage the beaker, but they are, for he most part, heat resistant. It will also be transparent, enabling me to see through the sides of it to see when the cross disappears.

Paper with a clear marking on it: This will be placed beneath the beaker. I will look through the solution, and once the marking on the paper can no longer be seen, the reaction is complete.

Goggles: These are a vital part of every experiment, ensuring that the sensitive eye area is protected from dangerous or harmful substances e.g. hydrochloric acid.

Measuring cylinders (x2): These are a fairly accurate method of measuring the correct amount of substances. I will need two measuring cylinders, one for each chemical, as the chemicals must not come into contact with another chemical until the experiment begins.

Droppers (x2): These are the best method of transferring small amounts of chemicals from one place into another. Again I will need two of these as the chemicals must not be mixed prior to the experiment.

Stopwatch: Used to measure time accurately. Very useful when measuring time in seconds or milliseconds, as the stopwatch itself is very accurate, and it is only by human error that the time taken would be wrong.

Method:

Set up equipment as shown in the diagrams below.
Measure the required amounts of sodium thiosulphate and hydrochloric acid into separate measuring cylinders.
Position the beaker directly over the marking on the paper.
Stand in a position so that you can easily see the marking through the beaker.
When you are ready, pour all of the sodium thiosulphate and hydrochloric acid simultaneously into the beaker.
When both chemicals are entirely in the beaker, begin the stopwatch.
Watch the marking carefully. When you can no longer see it, stop the stopwatch.
Record your results.
Repeat the experiment for various volumes.

Diagrams

Predictions

I predict that the rate of reaction will increase as the volume of sodium thiosulphate is increased. However, I think that as the volume of sodium thiosulphate is decreased, the reaction rate will also decrease. I think this because it is the sodium thiosulphate which makes the solution become opaque, and therefore the more of this substance is included, the faster the solution should go cloudy. When the amount of hydrochloric acid exceeds the amount of sodium thiosulphate, the sodium thiosulphate will be unable to work as quickly, as it will take longer for the hydrochloric acid particles to combine with a sodium thiosulphate particle.

Fair Testing

Only one variable should be changed per experiment
The same conditions should be kept, where possible, throughout experimentation e.g. heat (if the variable being altered is not temperature), light etc
Keep the intervals between the amounts of each chemical different i.e. change the amount of each by 5cm3 or 10cm3 each time
Do not mix the sodium thiosulphate and hydrochloric acid together until there is the required amount of each substance, and the required conditions have been met
Start the stopwatch at the same point in each experiment
The same person should watch the cross each time (if the experiment is to be conducted in groups)

Safety Rules

Goggles must be worn at all times
Long hair must be tied back
All stools and chairs must be pushed under desks or out of the way
All coats and bags should be put away properly and out of the way
No running in a science lab at any time
Much of the science equipment could be dangerous if not used or put away safely
Do not put anything from the science laboratory near or in your mouth
Be careful when handling chemicals as some may be poisonous or may be harmful if got on the skin

Results

When I carried out the experiment, I originally changed the temperature in order to see how much the reaction time would increase, as this seemed the easiest variable to keep constant and dependable. However, as the results below show, this did not work, and the results which I achieved were unreliable. Therefore, I re-did the experiment, changing the variable which I originally intended to, this time keeping the temperature constant and changing the volume of the sodium thiosulphate.

When altering the temperature, I raised it by 10°c each time. The volumes of sodium thiosulphate and hydrochloric acid were kept the same, as was the concentration of the acid. The stop watch was started at the beginning of each experiment, and stopped when I was sure that the cross was no longer visible.

When altering the volume of sodium thiosulphate, I raised the amount by 10cm3 each time. The temperature was kept at a constant of 60°, and the volume of hydrochloric acid and its concentration also remained the same. The results I achieved here were much more reliable, and followed my prediction.

Results for changing the temperature:

2. Results for changing the Volume of Sodium Thiosulphate

Was my prediction right?

My research had already showed me that when the temperature of the solution was increased, the speed of the reaction would also increase. My experiment proved that this was correct, as when the temperature increased, the particles sped up, and so there was a dramatic increase in collisions. Therefore the reaction time was reduced considerably, as the results above show.

My prediction stated that as the volume of sodium thiosulphate was increased, the reaction time would decrease. My results show this to be the case. When there was more hydrochloric acid in the solution, the reaction time was much slower, suggesting that the hydrochloric acid was almost diluting the sodium thiosulphate. Conversely, when the volume of sodium thiosulphate was more than that of the acid, the reaction time decreased. This shows that my prediction was correct, and that it is the sodium thiosulphate which causes the solution to turn opaque.

Do my graphs show any patterns?

Each time I performed the two experiments, the pattern remained the same; the reaction rate increased as either the temperature or the volume of the sodium thiosulphate was increased. The times in each of the three runs of the two experiments were fairly similar.

The second experiment worked particularly well in this respect, as each result was only one or two seconds different to the other runs. The lines on the graphs are therefore parallel, and show the reaction rate increasing at a steady speed.

When I changed the temperature, the results were far less reliable, and showed no consistent pattern. On several occasions, the reaction time first decreases, but then increases once again, meaning that the lines on the graph are rather serrated.

Evaluating the Experiment

In what ways did the procedure help you to get good results?

I carried out the experiment on the computer, which eliminates most human errors, such as mixing the substances before they should have been, or not keeping the solution at the right temperature etc. However, it does introduce computer errors, which I may be unable to detect. However, the results which I achieved seem to adhere to my research and predictions, for the most part.

I carried out all of the experiments in precisely the same way, by keeping all variables, except for the one which I was required to change, constant. Also, the person who watched the cross remained the same, which reduces possibilities of human error.

The experiment was performed under the maximum amount of fair conditions achievable, which means that my results were as fair as I could possibly make them. This is reflected in the results which I achieved, as the results followed the expected pattern, which the research indicated.

Were the results what you expected?

The results followed the same pattern that I had expected: the reaction rate increased as the temperature or sodium thiosulphate increased. This also meant that the graphs followed the expected pattern.

I was, however, surprised at the times taken for the cross to disappear. The diversity of times amazed me, as, although I knew that the times would obviously change, I had expected them to be closer together. I also thought that the experiments would be a lot slower.

Describe ways of improving the procedure?

When performing the experiment on the computer, it was on occasions unclear whether the cross had disappeared. Especially later on in the experiments, there was almost an illusion that the cross was still there, when in fact it was not.

Of course, the best way of improving the experiment would be to greatly reduce, or preferably eliminate, any human error. Light sensors or lasers would be particularly good for this experiment, as they eradicate human error, and so, provided that other fair conditions were kept, would offer the best, fairest, and most reliable results.

Explain how the evidence collected is good enough to make an accurate conclusion?

The results achieved were as fair as possible, and as human error was greatly reduced, quite reliable. Only one variable was altered for each experiment, and this was changed in equal amounts. The stop watch was stopped the second tat the cross was no longer visible, and so in precisely the same point on each test. The substances were, obviously, not mixed prior to the experiment, and so worked to their maximum potential.

Describe, in detail, further work that you could do to provide additional evidence?

In order to provide more evidence, I could perform the experiment manually. This way, I would have slightly more options open to me, as there were limited choices on the computer. For example, if I performed the experiment manually, I would have the opportunity to lower the temperature using ice, or heat it up to 100°c to get a wider range of results. I would also be able to experiment with different volumes of sodium thiosulphate more freely.

I could also experiment with altering different variables, such as different concentrations of hydrochloric acid, in order to see what would happen to the reaction time.

Also, I could perform other experiments to see how reaction rates change. I could do the other two experiments discovered in my research: the reaction between calcium carbonate and dilute hydrochloric acid, and the disintegration of hydrogen peroxide solution. This would tell me if the patterns for rates of reactions remain the same for all experiments, rather than just one, or what the differences between the rates of reactions are, nd why they are different.

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