Investigating the effects of concentration on the rate of a chemical reaction, hydrochloric acid + sodium thiosulphate

Keith Thompson 30/04/07

Plan

Introduction: I am going to investigate the effects of concentration on the rate of a reaction, hydrochloric acid + sodium thiosulphate.

Chemical reactions normally fit into three categories, these are chemical reactions that take weeks or years to occur, chemical reactions that happens so fast that it can not be seen (spontaneous reactions), or, and there are very few in this category, reactions that take minutes or seconds to occur. The chemical reaction hydrochloric acid + sodium thiosulphate fits into the latter of these categories. It is because of this property that this will be the chemical reaction that will be observed in this investigation. When the two chemicals are added together there is no immediate reaction but over a course of a few seconds sulphur is slowly precipitated into the water and suspended in the water forming a slightly yellow cloud. As the reaction progresses this cloud becomes so thick that it is impossible to view through the water (the water becomes opaque).

This property can be used to investigate the effects of concentration on the rate of the chemical reaction. If a cross, drawn on a piece of paper, is placed beneath the beaker, or conical flask, the time it takes for the cross to become obscured can be timed. However depending on the rate of the chemical reaction this time will change. This allows me to find the rate of this reaction at different concentrations.

Safety

Safety is paramount in all scientific investigations and this will be no exception. Chemicals will be handled with care. People will not run in the laboratory. Blazes will not be worn. Ties will be tucked away. Glass will be handled with care. Safety goggles will be worn while handling acids. Food will not be consumed in the laboratory nor will drinks be drunk. Long hair will be tied back. Hands will be washed after experiments.

Scientific knowledge

For reactions to occur atoms must first come into contact with each other, and have sufficient energy for the reaction to take place, this is known as Collision Theory. It is therefore theoretically possible to alter the rate of reaction by increasing or decreasing the number of collisions in a given time. There are five things that can be done to increase the rate of collisions. These are as follows:

Increasing the pressure. By reducing the volume in which the same amount of particles exist the pressure is increased. Once the same number of particles is in a smaller area there is less space in which to move and so the particles are more likely to hit each other. It is therefore possible to predict that increasing the pressure will result in an increase in the rate of reaction. I will not test this variable because Budmouth doesn't have the facilities to test it.
Using a catalyst is another method I could use. A catalyst is a separate substance that speeds up a reaction. However, after the reaction has happened it is left behind. This makes this variable unsuitable for the type of experiment I am going to do. A catalyst is also a very specific chemical with only one likely useful catalyst property emerging.
Increasing the energy. By giving the particles extra energy they will move faster. This means that they cover more ground and are therefore more likely to hit each other, which in turn makes the reaction faster. The best way to give energy to a particle is as heat and so I can predict that raising temperature will increase the rate of reaction.
Increasing the concentration. Just as increasing the pressure will increase the number of particles colliding, so will the concentration. By putting more particles into the reaction, the chance of them colliding increases and so the rate increases. I shall test this variable.
Increasing the surface area. Particles can only collide when the two sorts can meet. So a reaction can only occur on the surface of the material. Therefore by increasing the area of the material that is available to collide the speed of the reaction will increase. I shall not test this because it is hard to control the exact surface area of the two reactants as they both come in an aqueous solution.

I will test the effects of concentration on the rate of reaction. I predict that increasing the concentration of sodium thiosulphate will directly and proportionately increase the rate of the reaction.

Chemicals in this investigation

Sodium thiosulphate is a colourless, transparent, monocline crystal. Mainly used as a photographic fixing agent (commonly known as hypo). It is also mainly used in electroplating and tanning industries. 0.01% maximum is water insoluble and 99.99% is water-soluble minimum.

Na2S2O3. 5H2O

Hydrochloric acid is an acid largely used in industry, it dissolves in water to make hydrogen and chlorine ions. The hydrogen ion ...

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Chemicals in this investigation

Na2S2O3. 5H2O

Hydrochloric acid is an acid largely used in industry, it dissolves in water to make hydrogen and chlorine ions. The hydrogen ion is just a proton and is very unstable so it ‘ leeches’ (electron sharing) itself to a water molecule to make:

H3O

This is a very unstable molecule but is more stable than the hydrogen proton. A hydrogen ion is not normally referred to in this way when as an ion it is simply known as:

H+(aq)

Hydrogen aqueous

This hydrogen ion will take an electron from almost any element it can come into contact with, giving acid its dangerous properties, and will bubble out as hydrogen the gas.

For example:

Acid + metal → Salt + hydrogen

Hydrochloric acid + Copper → Copper chloride + Hydrogen

Now I have briefly discuss the properties of the two reactants chemicals I will discuss the products.

Sulphur (spelt s u l f e r Yankee style) (S8(s)) is a pale yellow, odourless, brittle solid, which is insoluble in water but soluble in carbon disulphide. Sulphur is essential to life. It is a minor constituent of fats, body fluids, and skeletal minerals.

Sulphur dioxide (SO2 (g)) is a colourless gas that has a very pungent smell. It dissolves in water to make sulphurous acid, a weak acid.

Water (H2O)

Hydrochloric acid+Sodium thiosulphate→Sulphur+Sodium chloride+Sulphur dioxide+Water

The balanced equation for the above word equation is as follows:

16HCl(aq)+8Na2S2O3(aq) .5H2O(l) →13H2O(l)+16NaCl(aq)+S8(s)+8SO2(aq)

This equation shows the products of this reaction are: 13 water molecules, 16 sodium chloride molecules, 8 sulphur dioxide molecules (normally a gas, it dissolves in the water) and one sulphur molecule (which is precipitated into the water).

Fair test

This will be a fair test by all other variables, within my control, being kept constant. The temperature for this investigation will be room temperature, approximately 23°C. The pressure for this investigation will be 1 atmosphere. Measuring cylinders and conical flasks will be thoroughly washed to avoid contamination. The same conical flask will be used because of slight differences in the shape of different conical flasks, making the depth of the water/solution different and therefore affecting the thickness of the cloud and altering the time it takes the cross to become obscured. The same person will be watching for the point where the cross becomes obscured because of slightly different vision from person to person. The same piece of paper and cross will be used because of different intensities of black ink. The same bottle of hydrochloric acid will be used as the schools technicians have a tendency to produce hydrochloric acid with slightly different moralities with each bottle they produce.

Prediction

I predict that increasing the concentration of sodium thiosulphate will directly and proportionately increase the rate of the reaction. Because increasing the concentration means that there are more molecules to collide and course reactions (collision theory) in the same space. The range for this investigation is the concentration of sodium thiosulphate from 0.15 M to 0.03M.

Diagram

Equipment

Equipment that I will need for this investigation include:

1 Conical flask.
2 Measuring cylinders (cm3).
1 Pair of safety goggles.
1 Stopwatch.
1 Piece of paper with a cross-drawn on it.
1 molar hydrochloric acid.
1 molar sodium thiosulphate.
Distilled water.

Method

Set up the apparatus as shown in the diagram.
Measure out 10 cm3 of 1 M hydrochloric acid in one of the measuring cylinders.
Measure out to 50cm3 of 0.15 M sodium thiosulphate in the other measuring cylinder.
Add the two measuring cylinders together in the conical flask and start timing with the stopwatch.
When the cross becomes unviewable stop the stopwatch.
Record the time it took the cross to become unviewable on the results table.
Wash all apparatus.
Set up the apparatus again.
Measure out 10 cm3 of 1 M hydrochloric acid in one measuring cylinder.
Measure out 40 cm3 of 0.15 M sodium thiosulphate and 10 cm3 of water in the same measuring cylinder.
Add the two measuring cylinders together in the conical flask and start the stopwatch. Repeat steps 5, 6, 7, 8 and 9.
Measure out 30 cm3 of 0.15 M sodium thiosulphate and 20 cm3 of water in the same measuring cylinder. Repeat steps 4, 5, 6, 7, 8 and 9.
Measure out 25 cm3 of 0.15 M sodium thiosulphate and 25 cm3 of water in the same measuring cylinder. Repeat steps 4, 5, 6, 7, 8 and 9.
Measure out 20 cm3 of 0.15 M sodium thiosulphate and 30 cm3 of water in the same measuring cylinder. Repeat steps 4, 5, 6, 7, 8 and 9.
Measure out 10 cm3 of 0.15 M sodium thiosulphate and 40 cm3 of water. Repeat steps 4, 5, 6 and 7.
Repeat all 16 steps three times to get an average.

Observation

This is the results table for my first set of results:

This is the results table for my second set of results:

And this is the results table for my two results averaged:

From my results I have plotted a graph of rate of reaction against concentration to see if there's a relationship between the rate of reaction and the concentration of sodium thiosulphate. The concentration can be found by:

Taking the amount of Na2S2O3 in cm3 and divide by the total volume 50cm3.
Multiply this by the morality of the Na2S2O3 0.15M to get the concentration.

Analysis

The graph shows that the relationship between the concentration of sodium thiosulphate and the rate of reaction is constant, they have a uniform relationship. The results show that as the concentration of the sodium thiosulphate decreases so does the rate of the reaction, thus the time taken to complete the reaction at lower concentrations are longer than the time taken to complete the reaction at higher concentrations. This proves that an increase of concentration in a solution increases, or a better word ‘affects’, the rate of reaction. This is because, referring back to collision theory, the greater the concentration of a solution the more particles there are in the same space, particles are closer together and more likely to collide with enough force to start the reaction. If the particles are more likely to ‘hit’ each other then we can say that the rate at witch the partials collide has increased then we can also say that the rate of reaction has increased because the rate of the reaction is directly controlled by the rate of successful collisions. From what I can determine there are no obvious anomalous results but I would like to have checked with a secondary source to be sure. From the graph it amperes that below a certain point no reaction occurs at all. This is probably because the rate of reaction is taken from the time it takes for a cross to become unviable, which means that if you go low enough, there is not enough reactants to make the solution go completely opaque. So there probably is a reaction but one not sufficient to register on the graph. The gradient for the graph to 3 decimal places is:

m=ΔY/ΔX

m=0.01-0.0035/0.03-0.0552

m=0.0065/0.0252

m=0.258

The results do support my prediction, but I would like to conduct the experiment several more times with a greater range for improved reliability. I would also like to refer to a secondary source to be completely sure.

Evaluation

The experiment in theory is sound, but in practise, it could offer a lot of errors. The reason is that the time for the cross to become obscured by the precipitated sulphur is down to human judgment and is not very trustworthy for accurate scientific investigations. So, the results do not have a very good reliability factor to start with. To improve this, light sensitive computer controlled sensors could be used where instead of the human eye judging if a cross is still visible or not, the cross is replaced with a light source and the person is replaced with a light sensor. The computer could then be programmed to have a timer and to stop it the moment the light drops below a certain intensity and to record this time on a results table. By doing this, human error is removed almost completely (we still got to hit the start button). I could not find any anomalous results but that could be an error on my behalf. Until I have done more experiments or I can compare with a secondary set of result, I cannot say with complete certainty that my evidence is 100% reliable.

Investigating the effects of concentration on the rate of a chemical reaction, hydrochloric acid + sodium thiosulphate

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