The aim of my investigation is to investigate the rate of reaction, when the concentration of hydrochloric acid is changed, when added to a sodium thiosulphate solution.

Preliminary work

The preliminary work I carried out was, I set up the experiment as I would in the proper experiment and take 3 different readings from 3 different tests. I done this to see if the experiment works well and so that I can get an idea of long the experiment would take. This will help me so that I get a rough idea of how to set it up and so that I can do that quickly in the next lesson without wasting time. I can also use these results to get an accurate prediction of what the main experiments results will be like. I can also compare the results to get an average time.

Diagram

See sheet.

Precision & reliability

The values I plan to measure are, the time taken for the cross to be obscured and, the concentration of sodium thiosulphate solution mixed in with the hydrochloric acid. I will also take the temperature of the water the sodium thiosulphate and the hydrochloric acid. The highest value I will measure is the time taken for the reaction to take place. The smallest value I plan to measure will be the volume of acid added to the mixture because we keep this the same.

Method

1. Take out the equipment and set up as shown in diagram.
2. Pour into the flask 30ml of sodium thiosulphate.
3. Take the temperature of the sodium thiosulphate, the hydrochloric acid and the water and write them down.
4. Get the timer ready and add 5ml of hydrochloric acid as soon as the acid is added start the timer.
5. As soon as the black cross disappears stop the timer and record the time taken for the cross to disappear.
6. Do the experiment exactly the same again but instead add 25ml of sodium thiosulphate and 5 ml of water and carry on like this as shown here: thio-ml            water-ml

                              25                        5

                              20                        10                                                                                                  

15.  15

10                        20

5. 25

Safety

I will make my experiment safe by wearing safety goggles at all times because we are dealing with acid. I will be careful not to spill any acid and if I do so I will clean it up thoroughly. I must also always be careful not to break any glass.

See graph

Conclusions

I have found out from this experiment, that the concentration of sodium thiosulphate does add to the time taken, for a reaction to take place, when added to hydrochloric acid. The more of the sodium thiosulphate the quicker the reaction time. The pattern (trend) is that as the input variable is decreased, (concentration of thio) the out come variable increases (time taken for a reaction). The reason for this is when the concentration is increased, there is more particles of thio in the mixture, so there is more particles crowded close together, and collide more often with each other, resulting in an increased number of successful collisions. So that if there is more thio mixed with the acid. There will be more collisions and therefore the reaction time will increase.

Evaluating

The only problem we met during the investigation was that we did not know how much water to add to the sodium thiosulphate in our preliminary experiment. The variables we tried to control but couldn’t was the temperature of the three liquids. The results are pretty much accurate because it is substances we are dealing with and not other equipment, which could go wrong. I think that the experiment was a fair test because we kept everything the same except for the temperature, which could not be changed. Also every time we added the acid we did not stir it around with the thio, as this could affect the results of the investigation. Because if we stirred it around the two substances would be mixed into each other more quickly, and the particles would get a head start in causing collisions and making a reaction. But I think that the temperature made the test a bit unfair because, temperature also adds to the rate of reaction since the higher the temperature, the faster the particles move around and cause more collisions, which equals a faster reaction time. The changes that could be done to improve the results of the experiment are use a data logger to see how long the reaction takes, by using the light meter sensor of it. The reason the results are different is because the sodium thiosulphate solution once it is made up it starts to go of over time. And because we done the experiment on different days the thio may have different strengths and affect the results.