Investigate the reaction between sodium thiosulphate and acid at different temperatures.

Chemistry Coursework

Aim – The National Rivers Authority (NRA) is one of a number of organisations which fine companies if the companies are released prohibited chemicals into the environment. One chemical, which is regarded as harmful, is sodium thiosulphate. The chemical used in photographic processing industry to remove unwanted silver bromide from exposed film. Photographic developers are therefore looking into alternative approaches to dealing with the leftover sodium thiosulphate. One answer might be to convert the waste sodium thiosulphate solution into some thing harmless, and preferably useful. In this assignment you have to investigate the reaction between sodium thiosulphate and acid at different temperatures.

Prediction – In this investigation I expect to find as I increase the temperature the reaction will take place faster. This is because as the temperature increases it gives more energy to the sodium thiosulphate and hydrochloric acid particles causing them to collide more often and with more force, this increases the rate of reaction. As the temperature rises a greater number of sodium thiosulphate and hydrochloric acid particles have energy greater than the activation energy therefore leading to more successful collision, and increasing the rate of reaction.

Reaction Equation –

Na2S203 (aq) + 2HCl (aq) 2NaCl (aq) + SO2 (aq) + S (s) + H2O (l)

Rate of Reaction Equation

Rate of reaction =Amount of Sulphur formed to make cross disappear Time taken for cross to disappear

The amount of sulphur formed to make the cross disappear should be the same in all experiments therefore can be substituted for a constant value such as 1.

Rate of reaction Constant 1

Time Taken Time Taken

The rate of reaction represents the speed that the reactant changes into its product. This is represented as one over the time taken for the reaction to take place.

Variables – Concentration of sodium thiosulphate and Hydrochloric acid – The concentration of sodium thiosulphate and hydrochloric acid will be kept the same, as to make it a fair test, because if you change the concentration of one reactant it changes the number of particles making the reaction unfair and not reliable. If you create batches of the reactants you reduce the percentage error of volume measurement and of the concentration. E.g. when you measure 25ml of water from a 25ml measuring cylinder a certain amount of water will stay in the cylinder, Then instead of water it was hydrochloric acid and some was left behind, it would change the total concentration because the number of particles has been reduced therefore there is less particles for the other reactant to collide with, also the chance of the amount left behind being the same will be small. To make sure that the concentration and volumes are the same you should create a batch of the substances. To do this you need to prepare a total volume of sodium thiosulphate and distilled water. This will cover four reactions when I’m only doing three repeats. I’m doing this so that I have enough of the solution to carry out the experiments. As I have a concentration of 6ml of sodium thiosulphate and 19ml of distilled water, I will be creating 24ml of sodium thiosulphate and 76ml of distilled water, giving a total volume of 100ml.

This is a preview of the whole essay

Temperature of Sodium Thiosulphate and Hydrochloric Acid – As the temperature increases so to does the rate of reaction. So temperature is important to keep as constant as possible in each repeated experiment but carry out at the different temperatures sets. This is because if there is a slight change in the temperature the faster the reaction takes place. E.g. a 1°C increase provides enough energy to the particles to increase the rate of reaction by 10%, as for most reactions a 10°C increase doubles the rate of reaction. To stop this from happening I will need to put the two test tubes into a water bath as it has a greater volume of water therefore the temperature drop in the two test tubes will be smaller then without. I will also take the start temperatures of the Sodium Thiosulphate and Hydrochloric Acid and the finish temperature of the reaction mixture for each experiment in turn to then take an average so that I increase the reliability of each experiment.

Cross – This is a control variable. I won’t be changing this at all. The person that looks at it and that the cross is the same and in the same place. If I change the cross in any way or the person looking at it or where the person looking at it is looking from, it could change the result of the investigation.

Method

Place Apparatus in middle of desk: Boiling tube, test tube, 600ml beaker, kettle, Distilled water bottle, Sodium Thiosulphate, Hydrochloric Acid, Stop Clock, Paper Cross, 25ml measuring cylinder, 100ml measuring cylinder and 10ml measuring cylinder.
Prepare Batch of sodium thiosulphate and distilled water using both a 100ml and 25ml measuring cylinders.
Place 5ml of Hydrochloric acid into test tube using 10ml-measuring cylinder.
Place 25ml of sodium thiosulphate/distilled water solution into boiling tube using a 25ml-measuring cylinder.
Put water in kettle and switch on
Place a cross on the outside of the 600ml beaker
Place 150ml of cold water into 600ml beaker
Mix the hot and cold water in beaker
Then add 300ml of boiling water from kettle
Use Thermometer to take the temperature of the sodium thiosulphate and distilled water and Hydrochloric acid with two thermometers in each test tube
Wait for the temperature of both the Solution and Hydrochloric Acid to stop changing before recording it
Record temperature of non-change in results table
Pour Hydrochloric acid into solution and start stop clock immediately
Wait until cross disappears and the stop the stop clock
Record the time in table
Take the temperature of the mixture and record in table
Pour away as soon as possible
Wash boiling tube out with cold tap water then rinse with distilled water
Take average of the start and finishing temperatures and times
Repeat Experiments 3 times for each temperature to improve reliability or to make them reliable.
Plot on graph

The temperatures that I will carry out the experiments at 62,52,42,32,22°c.

Safety -

From Cleapps – 1995 – Hazcards

Preliminary Experiment –

Measured 25ml of neat Sodium Thiosulphate (0.1 mol/dm3)
And 5ml of Hydrochloric Acid regardless

Then I Did the experiment at room temperature 21.0°c.

Concentration Temperature Time

0.1mol/dm3 21.0°c 42.06 secs

This was too quick and therefore at higher temperatures the reaction will be quicker. I need a bigger time range. For the reaction at higher temperature to give a time that will not be so fast that it can be recorded accurately.

I then measured 6ml of sodium thiosulphate and 19ml of distilled water.

Concentration Temperature Time

0.024mol/dm3 21.5°c 233.15secs

60.5°c 17.27secs

Time range= 215.88secs

Across a temperature range of 21.5°c 60.5°c

This is a great time range as when the experiment is done at a higher temperature it will give a suitable result so that it can be plotted on a graph.

I also did experiments with a concentration of 0.05mol/dm3 and 0.03mol/dm3

Concentration Temperature Time

0.05mol/dm3 21.5°c 134.89secs

0.03mol/dm3 21.5°c 173.26secs

but were both too smaller time range between the highest temperature and room temperature.

When recording the temperature record start and finish temperatures to take an average. This is because the temperature will decrease while the reaction takes place. Therefore to plot a graph we plot the average of the two temperatures against 1/time taken.

Conclusion

From my results, the table and the graph, clearly show that the temperature of the reaction increased the reaction took place faster as the graph and table show that at 23°c to 62°c the reaction took place 247.24secs and 33.67secs respectively. This proves that as the temperature increases the reaction took place much faster. This matches my prediction.

This is as I expected because as the particles of hydrochloric acid and Sodium Thiosulphate are heated, they gained more energy and collide with more force, so the collision frequency between Hydrochloric Acid and Sodium Thiosulphate increased.

To show the changes in the rate of reaction, I plotted a graph showing the reciprocal of the time taken for cross to disappear against average temperature of reaction mixture. This is because the reciprocal of the time taken is proportional to the rate of reaction. So the precipitate formed in the reaction will be the same in every experiment so therefore can be substituted for 1, leading to: 1

Time taken

Rate of reaction = Amount of Sulphur formed to make cross disappear Time taken for cross to disappear

The amount of sulphur formed to make the cross disappear should be the same in all experiments therefore can be substituted for a constant value such as 1.

Rate of reaction Constant 1

Time taken Time taken

This table of Rates of Reaction tells me that in a 10°c rise in temperature the rate of reaction has increased by 1.8. This is close to my prediction. As I said that for most reactions a 100C rise in temperature leads to a double in the rate of reaction. I also find that as the table shows a fairly close together this tells me that I controlled my main sources of error sufficiently so that the 1/T graph can be produced. Therefore my results were reliable.

The variables controlled that made my results accurate and reliable were the temperature, where I used a water bath to stop the temperature of the reaction mixture from decreasing to quickly. Concentration, where I used a batch of Sodium Thiosulphate and distilled water to decrease the percentage error of measuring the same amount each time as some liquid is left in measuring cylinder. And the Cross, where I kept the person looking at it the same and from the same angle and distance therefore to keep a fair test.

For my Experiment a 10°c rise in temperature allows the number of particles that have energy greater than the Ea (Activation Energy) 1.8 times larger than the number before. This tells me that for this reaction the rate of reaction is almost double for a 10°c rise, therefore shows that the variables were controlled to a sufficient degree of accuracy to allow the reaction to take place at an optimum rate. Therefore leads to a valid conclusion.