To investigate what happens when sodiumthiosulphate and hydrochloric acid are poured into the same flask.

GCSE Science

Investigating the Rate of Reaction

Aim

Plan

There are many different variables that effect the rate of reaction.

Temperature
Pressure
Concentration of hydrochloric acid
Concentration of sodiumthiosulphate
Volume of hydrochloric acid
Volume of sodiumthiosulphate
Stirring the solution
Adding a catalyst

The variable that will be changed is the temperature.

Prediction

Rate of reaction is the speed or velocity in which a chemical reaction takes place. When a solution is heated it will speed up the rate of reaction.

Also once the reaction has taken place the sulphur will be precipitated.

Na2S2O3(aq) + 2HCL(aq) → S(s) + SO2(g) + 2NaCl(aq)

This is because there is a (s) and a (aq) together being produced together in the end result. This means that the sulphur will be precipitated.

For a reaction to take place, regent molecules must react. There has to be sufficient activation energy for this reaction to take place. When the solution is heated up, particles move faster because they have gained energy. So when the particles have gained energy, above activation energy, there will be more collisions in the same space (volume) more often. As well as more collisions there will be more successful ones.

Activation Energy (Maxwell – Boltzmann Graph)

T = Temperature

The faster the reation the more particles move faster.

Knowing that 1 in every 10/14 molecules cause a reaction in normal room temperature. So when it is heated the molecules must have more collisions.

Molecules at different temperatures

Low Temperature

Molecules have less energy than high temperature. There are less collisions, that are more successful.

High Temperature

The molecules have a lot of energy,

So move much faster causing more

Successful collisions.

So there are two theories that effect the temperature.

Kinetic Theory

- increase in temperature equals increase in energy

Collision Theory

Apparatus

A list of equipment needed to run the experiment.

Sodiumthiosulphate
Hydrochloric acid
Water
Flask
Measuring cylinder
Thermometer
Heat proof mat
Gauze
Bunsen burner
Tripod
Large beaker
Ice cubes
Stop clock
Paper and pencil
Protective eye wear
Aprons

Method

Take a flask and ...

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High Temperature

The molecules have a lot of energy,

So move much faster causing more

Successful collisions.

So there are two theories that effect the temperature.

Kinetic Theory

- increase in temperature equals increase in energy

Collision Theory

Apparatus

A list of equipment needed to run the experiment.

Sodiumthiosulphate
Hydrochloric acid
Water
Flask
Measuring cylinder
Thermometer
Heat proof mat
Gauze
Bunsen burner
Tripod
Large beaker
Ice cubes
Stop clock
Paper and pencil
Protective eye wear
Aprons

Method

Take a flask and place it onto a piece of paper which has a cross drawn on it big enough to see, but not too dark.

2) Measure out a reasonable amount of sodiumthiosulphate into one measuring cylinder and water in another. In this case 19cm³of sodiumthiosulphate and 31cm³ of water.

When pouring the sodiumthiosulphate, make sure any spillages are washed up straight away. This is because it is very corrosive. Put the sodiumthiosulphate into a flask with water and heat it up to a starting temperature. Once heated put the flask onto the piece of paper with the cross marked onto it.

Quickly pour the hydrochloric acid into the flask, once halfway poured in, start the stop clock. Use the same person to look at the solution so they can tell when it has gone cloudy and covered the cross underneath – then stop the stop clock.

Do this experiment with a range of temperatures. If you want to cool the temperature then place the souimthiosulphate in a flask then in a beaker of ice. Follow step 4. Record time and temperatures.

6) Make sure when doing this experiment everyone is wearing protective eye wear and aprons.

Observations

I will observe how long it takes for the solution to go misty. This means measuring the time and temperature, which affect the rate of reaction. For example, if the solution is at a low or high temperature the reaction will either speed up or slow down. This will take longer or shorter for the solution to change colour.

To measure the temperature then I will use a thermometer. To measure the rate of which the solution changes colour then I will use the same person to look directly down onto the solution and say we the cross below has disappeared.

Using the thermometer to the temperature I have chosen will be hard. So measuring the temperature to the nearest Celsius will be the most reliable. Also once the flask has been placed onto the cross it will start heating up or cooling down.

To see what happens when the solution is heated or cooled down, I will use a different range of temperatures. These will be: -

These are a reasonable range of temperatures that are not to near together of too far apart.

Fair Test

There are certain things that will affect my experiment. So to make it a fair test I will: -

Use the same volume and concentration of sodiumthiosulphate. If there are different volumes and concentrations they will both change the rate of reaction. So I will keep the same volume and concentration every time I do the experiment.

Use the same volume and concentration of hydrochloric acid. This is the same reason as the sodiumthiosulphate.

Use the same amount of sodiumthiosulphate and water each time. This will stop the change in reaction.

Use the same person do stop and start the stop clock when seeing when the solution has gone cloudy. This is because of that person’s manual dexterity is different from other peoples.

Some factors I wont be able to control: -

Temperature of the room. The temperature of the room will be changing all the time. This may be because it could be a colder or hotter day than the last time I did the experiment. So the temperature will vary all the time.

The darkness of the cross. Keep the same cross all the time. If the cross id darker than the last time you did the experiment it will effect the time it disappears when the hydrochloric acid is added to the sodiumthiosulphate.

Light intensity. This could also effect the time. If sun is shining brightly onto the solution it will give an allusion that the cross has disappeared or is still there. To try and prevent this a light could be constantly shone on the solution so that the light intensity.

Viewing distance. If there is a different distance from the last time the experiment this could alter the way someone sees the cross. If you are further away from the cross it will dissapear quickly. If you are too near then it will dissapear slowly.

To find a reliable time when doing the experiment I will repeat the tests three times. This will help me find an average and to pick out any anomalies. It will also help draw a graph and a conclusion on the evidence.

Preliminary Work

Firstly I found out what concentration of sodiumthiosulphate.

After some consideration I decided on using 19cm³ of sodiumthiosulphate and 31cm³. This was because I wanted a reaction that would have less error. For example, if each time has a built in error of 1 second: -

Seconds fraction error % error

5 1/5 20%

100 1/00 1%

50 1/50 2%

This means that I don’t want a concentration that will take too long, but I don’t want a concentration that is too quick causing a higher error.

So the overall concentration will be 19cm³ sodiumthiosulphate and 31cm³ water with 5cm³ hydrochloric acid.

Results

Results of Sodiumthiosulphate reacting with hydrochloric acid

Conclusion

I have found that when the solution has been heated up it reacts quicker. This means that there is a difference in temperature. When the sodiumthiosulphate was heated up to a high temperature the solution quickly turned misty. The mist was small deposits of sodiumthiosulphte that eventually sank to the bottom of the flask, while some stayed suspended in the solution. When the sodiumthiosulphate was put into an ice bath it slowed the rate of the reaction right down - taking it longer to turn misty.

There is no definite pattern in the results and only a few anomalies where found. The only pattern that forms is that as the temperature gets higher the time level is shortened. It seems that heating the solution up increase rate of reaction time.

My prediction was right, “When a solution is heated it will speed up the rate of reaction.” This is because when a solution is heated up the molecules gain more energy. When they have gained more energy there is more successful collision. When looking at the graph there is quite a big jump from 11°C and 31°C. I think that is because there is a big jump in temperature. This means that the particles lose a lot of energy when cooled down.

Molecules only need a certain amount of energy to produce a reaction. This is called Activation energy. When heat is added, there is more energy needed over activation energy, therefore speeding the reaction up. All this is shown on the results graph. As the temperature gets hotter the rate of reaction speeds up. This proves that my theories were right and that the results support them.

Evaluation

The evidence showed quite clear that the rate of reaction speeds up or slows down once the temperatures have been changed.

The quality of my results is very high. There were only a few anomalies found, but these were only very minor. This could have been because I looked at the cross and waited too long to stop the clock.

The results were all very close together and showed a trend so that I could find an average quite easily. Looking at the graph there is quite a big jump from 11°C and 31°C. I think that is because there is a big jump in temperature. I think that a temperature at 20°C would have helped the graph to see why there is such a big jump.

The evidence was valid because I did nearly everything to make it a fair test. To improve the reliability of these results I could have done more tests and found a better average. I could have also tried some more temperatures. This would have widened the range and made it more interesting to see what happened when the temperatures was increased or decreased quite a lot. This could have helped me also understand why there was such a big jump from 11°C time and the 31°C time. It would also support my evidence a bit more.

If I could make some changes to the method I would use a computer-aided device to help calculate accurate times for when the cross was covered. This would become a very precise and accurate recording of time. It would also reduce the amount of human error.