"What Affects How Quickly Sodium Thiosulphate (thio) Goes Cloudy?"

Concentration can change the speed of the reaction in two ways:

* The higher the concentration of the thio, the faster the rate of reaction, and the shorter the time span.

* The lower the concentration of the thio, the slower the rate of reaction, and the longer the time span.

I will be using first-hand experience evidence, as it will be a more reliable investigation, if I were to use secondary sources, the results may not be accurate enough to analyse.

I will record my data in a table and then plot it onto a graph; I will use a wide range of equipment.

For my experiment, I will use the ‘disappearing cross’ method. To allow me to make this test a fair test, I will use the same cross for all the tests. The disappearing cross method is whereby I have a cross drawn on a piece of card, and is laminated, (to help stop the liquid solution from damaging the card).

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The cross is placed under the conical flask. As the reaction takes place, the solution becomes cloudy and the cross gradually disappears.

Being as I am using Hydrochloric Acid and Sodium Thiosulphate, I must ensure that I have protection from the liquids; therefore I will wear goggles. In order to make my experiment an accurate experiment, I will observe the cross and wait for it to disappear. When I think that the cross has disappeared, I will blink once, (with the eye that I use for observing); in order to ensure that the cross has actual disappeared.

In this experiment, to help make it a fair test, I will keep the volume of Hydrochloric Acid the same. I will change the volume of thio and distilled water. As you may have noticed, I have not mentioned temperature; this is because, I will not be using any immediate heat on the solution, but in fact, the room temperature will change, concluding that the temperature of the solution may have an effect on the results. I will decrease the thio from 25cm�, (1M), to an absolute minimum of 1cm�, (0.04M), thio. The equation for finding out the strength of the thio was 0.04 x (volume of thio). By doing this, it will make the thio react slower and turn the solution cloudy slower: this is because of the collision theory.

If there are lots of acid particles and thio particles, there will be more collisions, as there is not enough space for the atoms to move around meaning that the atoms would collide more often into one another. If there were fewer thio particles and the acid particles are unchanged, resulting in fewer collisions as the atoms would have more space to move around in and they would not collide as often.

Prediction

For this experiment, I predict that the higher the concentration of sodium Thiosulphate, the faster the rate of reaction will be, this is because, there would be a larger surface area for the Hydrochloric acid to react with. If there were to be only 0.04M of thio, the reaction would be far lower than if there were to be 1M of thio.

Below is a diagram of Hydrochloric acid and thio:

1. (1M) 2. (0.04M)

As you can see from the diagrams, Diagram 1 has 20 atoms, 10 of each, this means that there is less space and there would be more collisions between the atoms. In Diagram 2, there are fewer thio molecules than there are HCl; this means that there is more space but fewer collisions, as the atoms would be able to move more freely.

Below is the equation of the Sodium Thiosulphate and Hydrochloric Acid:

Na S O (aq) + 2HCl (aq) 2NaCl (aq) +H O (l) + S (s) + SO (g)

I obtained this equation from a program used at school known as Focus.

Plan

Below is a labelled diagram of the equipment I used:

My eye

Stopwatch

Thermometer

Conical Flask

Measuring

HCl Cylinders

Thio

The cross Distilled Water

Method

I used a measuring cylinder to measure the amount of thio and water; this would allow me to change the volume of these to liquids. I poured the liquids into the conical flask, after doing so, I got a stopwatch, as soon as I poured the hydrochloric acid into the flask, and I started the stopwatch. After having examined the solution, I inserted the thermometer to take a reading of what the temperature of the solution was at the end. This way, I would be able to note don the changes in temperature when drawing my graph. I used the same eye to observe the solution; this will make it a fair test, as one eye may be stronger than the other. I used 3 measuring cylinders, 1 for the Hydrochloric Acid, the other for the thio and the third for the water. By doing this, I will not be allowing any pre-reacted substances into the experiment. Using a laminated cross allows me to use it over and over again as it would not be affected by any liquids falling onto it.

I will take 10 readings initially, however, any results that are not close to the line of best fit, I will redo. I will record my results in a table, after recording the results into a table; I will transfer them into a graph. My results will be reliable, as I will have made all the experiments a fair test, the only thing that would be beyond my control would be the temperature of the room, and this may vary my results slightly.

Skill O- Obtaining evidence

Test

Volume of thio (cm�)

Volume of Water (cm�)

Volume of Hydrochloric Acid (cm�)

Time Taken (secs)

Temperature (�C)

Concentration of thio (M)

1

25

0

10

8.97

25

1

2

22

3

10

12.82

21.2

0.88

3

20

5

10

11.62

25

0.8

4

17

8

10

16.99

20.5

0.68

5

15

10

10

19.15

22

0.6

6

12

13

10

19.20

24.5

0.48

7

10

15

10

26.31

22

0.4

8

7

18

10

32.87

25.5

0.28

9

5

20

10

40.87

22

0.2

10

2

23

10

77.19

24.5

0.08

Here are the results to my experiment. I had done 10 preliminary tests to see if the results would fit on my line of best fit appropriately. Being as my results did not all fit the line of best fit, I redid those test numbers:

2

22

3

10

10.53

21

0.88

3

20

5

10

12.75

22.4

0.8

4

17

8

10

16.27

25.5

0.68

5

15

10

10

17.97

23.1

0.6

6

12

13

10

18.67

22

0.48

I also did two extra results, which would allow me to look further into the line of best fit:

11

2.5

22.5

10

60.75

23

0.1

12

3

22

10

53.12

23

0.12

Skill A- Analysing and considering evidence

As you can see from the table above, I was right in saying that the higher the concentration of the thio, the faster the rate of reaction. Below is a graph to show the results:

As you can see form the graph, there is a curve in the graph indicating that the lower the concentration of the thio, the longer it takes to react, the pattern does agree with my original prediction. The results in blue are my preliminary results and the ones in pink are my secondary results, as you can, if I draw a line through both sets of data, I get a nice smooth curve. Some of the blue results are not a part of the smooth curve; these results have been retaken. There are two red data points; these results are ones that I did to see if they fit in line of best fit, in this case they do. The results that are not very reliable as when they were taken, there could have been some kind of differentiation, like I said earlier, the only variable that I could not control was the temperature of the room, this could have affected the results.

According to my line of best fit, if I were to have 9cm� (0.36M) of thio, it should take about 28secs to react. On the other hand, if I wanted thio to react for 1min, I would need the thio to be 3cm� (0.12M).

Skill E – Evaluating

My investigation went quite well, I didn’t have many problems whilst doing the experiments. Any problems that I did have whilst doing this investigation was the temperature of the room; being as the tests were done on different days during the summer/autumn time, the temperature was warm on some days and cold on others changing the temperature of the room, the temperature varied the results. I could have changed the temperature of the solution, by heating it up, to allow the results to be fairer, but this was not part of the coursework. I didn’t have any other problems. The results that were off the line of best fit by quite a bit, I had to redo in order to allow the results to be shown in a more accurate perspective.

My results seemed to be quite accurate and it followed my prediction, all those data points that were far from the line of best fit had to be redone to show that due to a variable that was beyond my control, the results seemed odd.

In order to make my results better, I could have repeated the investigation three times and then out of those three timings, I could have worked out an average and plotted them on to the graph. My results were accurate in the sense that all of my results were done fairly being, as I made sure that I didn’t mix any pre-reacted substances to the solution, the results are not very accurate as the temperature of the room varied from time-to-time, therefore, I could not ensure that the test was done very accurately. My results are reliable in the sense that they all follow my prediction, all of my data points fit nicely on the line of best fit. At the same time, my results are not very reliable as I only performed the investigation once, I didn’t find an average, and the results could have been fluke.

In order to obtain my results, I could have used the help of ICT. Programs such as Focus could have enabled me to work out more accurate timings/results. I could have used a computer animated program to show me when the cross disappeared, and I could have had had it done at a certain temperature; this would have enabled me into getting accurate answers and I would have no need to perform the investigation several times.

Being as I did the investigation manually, I drew the graphs and other things on the computer, this would give me an accurate graph. I drew the line of best fit manually as it would allow me to make my own analysis and allow me to give the best possible answers.

I could have done the investigation in different ways:

* Changed only the temperature of the solution

* Not added distilled water but change the amount of thio I mix with hydrochloric acid.

* Changed the strength of acid and the thio.