Aim: to investigate how varying the temperature of hydrochloric acid in a reaction with sodium thiosulphate has an effect on the rate of reaction Prediction: as the temperature increases, the rate of reaction will also increase; this is because the heated hydrochloric acid atoms will have more energy, and therefore have a higher rate of bonding when the hydrochloric acid and sodium thiosulphate atoms collide.
Background Information
In my experiment I am mixing different temperatures of hydrochloric acid (5°C, 10°C, 20°C, 30°C, 40°C, 50°C) with sodium thiosulphate; this can be represented in the equation:
This reaction causes the solutions (sodium chloride and water) to turn cloudy: this is due to the formation of the solid sulphur.
Collision Theory
This theory states that a chemical reaction can only occur between particles (atoms, ions, or molecules) when they hit/collide. When reactant particles collide, there is a minimum amount of energy needed in order for a reaction to occur: if the colliding particles have less than this required minimum amount of energy, then the particles will just bounce off of each other and no reaction will occur; this minimum amount of energy is called activation energy. Once the reactant particles have this activation energy and have collided, they work to break the existing bonds and form new bonds, resulting in the products of the reaction.
The faster the particles are going, the more energy they will have, and they will therefore be most likely to react when they collide. Particles can be made to move more quickly if they are heated up (by raising the temperatures) as the heating will give the particles more energy. Also, if the reactants are more concentrated, then the rate of reaction will increase as there will be more particles per dm³. The more particles there are in the same volume, the closer to each other the particles will be. This means that the particles will collide more frequently with each other, causing the rate of reaction to increase. When a catalyst is involved in the collision between the reactant molecules, less energy is required for the chemical change to occur; hence more collisions.
Key Variables
Temperature of hydrochloric acid: this factor can affect the time taken for the hydrochloric acid and sodium thiosulphate to react with each other.
Volume of hydrochloric acid: the volume of hydrochloric acid that is mixed with the sodium thiosulphate could affect the rate of reaction.
Volume of sodium thiosulphate: the volume of sodium thiosulphate that is mixed with the hydrochloric acid could affect the rate of reaction.
Concentration of hydrochloric acid: I am using 2M hydrochloric acid - if this is changed, the rate of reaction could change.
Temperature of room: this could affect the energy of the molecules in the acid, thereby affecting the rate of reaction.
Independent Variable
Temperature of hydrochloric acid: if the temperature was increased, then the hydrochloric acid atoms would have more energy, which would cause them to collide with the sodium thiosulphate atoms harder and more frequently; thereby increasing the rate of reaction. However if the temperature was decreased, then the hydrochloric acid atoms would not have as much energy, and would collide into the sodium thiosulphate atoms less frequently, and with not as much force; therefore the rate of reaction would be slower. The temperatures used were 5°C, 10°C, 20°C, 30°C, 40°C and 50°C; I made sure the temperatures of the hydrochloric acid were correct by testing them with thermometers.
Controlled Variable
Volume of hydrochloric acid: it is important that this is kept the same; this way I can ensure that I am only testing the effect of hydrochloric acid temperature without any other factors affecting the results - using a measuring cylinder I would accurately measure out 50ml.
Volume of sodium thiosulphate: again this has to be kept the same so I can make sure that my results have only been affected by hydrochloric acid temperature and not volume - I made sure that the same volume of sodium thiosulphate was used each time by accurately measuring out 70ml using a measuring cylinder.
Concentration of hydrochloric acid: if the concentration of hydrochloric acid was increased or decreased, then the rate of reaction would be affected: this would be because either there were too little particles per dm³ (in which case the particles would collide with each other less frequently, so the rate of reaction would decrease) or there would be too many particles per dm³ (the particles would collide with each very frequently, thereby increasing the rate of reaction) which would end up affecting my results; I made sure that the hydrochloric acid we were using was always 2M.
Temperature of the room: if the temperature of the room was not kept constant at all times, then it could cause the hydrochloric acid or sodium thiosulphate atoms to have too much (or too little) energy at certain times: this would severely affect the accuracy of my results.
Dependent Variable
Time taken until solution turns cloudy: the rate of reaction should change depending on the temperature of the hydrochloric acid; specifically I will be measuring (using a stopwatch) how long it takes (in seconds) to no longer see the cross at the bottom of the conical flask, due to the cloudiness which occurs in the reaction between hydrochloric acid and sodium thiosulphate. I am going to make sure that I always start my stopwatch when all of the hydrochloric acid has been added: this way I can ensure that I start the stopwatch at the same point during every single experiment.
Preliminary
I carried out two experiments for my preliminary work, so I could have a range and see which one I preferred. The first one I carried out was:
Investigating how the volume of hydrochloric acid affects the loss of mass (in marble chips) as a gas forms
In this experiment I first used 50ml of hydrochloric acid; the second time I repeated the experiment, I used 75ml of hydrochloric acid - my results are shown below (highlighted in purple are my starting masses):
Time (minutes)
Beginning mass
(g)
Volume of
hydrochloric acid (ml)
0
32.7
50
0.5
81.05
50
1
80.83
50
1.5
80.72
50
2
80.7
50
2.5
80.62
50
3
80.6
50
3.5
80.59
50
4
80.58
50
4.5
80.5
50
5
80.57
50
Time (minutes)
Beginning mass
(g)
Volume of
hydrochloric acid (ml)
0
35.9
75
0.5
142.6
75
1
142.1
75
1.5
141.8
75
2
141.5
75
2.5
141.5
75
3
141.4
75
3.5
141.3
75
4
141.2
75
4.5
141.2
75
5
141.2
75
Even though I got good results, I decided not to choose this experiment, as it was very difficult to get my beginning masses to be equal (I wanted my beginning mass of marble chips to be 33g) and this could affect the accuracy of our results. The second experiment I carried out was:
Investigating how the volume of hydrochloric acid affects the rate of reaction with sodium thiosulphate
Volume of
hydrochloric acid (ml)
Volume of
sodium
thiosulphate (ml)
1st run
2nd run
3rd run
Mean average time (seconds)
15
70
33.56
33.94
34.44
33.98
35
70
44.50
47.62
36.84
42.97
50
70
35.09
33.03
38.31
35.48
70
70
45.01
44.18
42.20
43.80
100
70
44.54
51.87
48.75
48.39
This experiment was fairly easy to carry out: however, I found that by changing the volume of hydrochloric acid, I was actually changing two variables: the volume of the acid, and the point of contact; this could affect the accuracy of our results (I also thought that changing the volume of acid was a bit 'basic' so we needed to make it more complex). I then decided to change my experiment - so instead we were testing how changing the temperature of the hydrochloric acid affects the rate of reaction; my new experiment was:
Investigating how the temperature of hydrochloric acid affects the rate of reaction with sodium thiosulphate
Temperature of hydrochloric acid (°C)
Volume of hydrochloric acid (ml)
Volume of sodium thiosulphate (ml)
1st time (seconds)
2nd time (seconds)
3rd time (seconds)
Mean average time (seconds)
40
50
70
31.15
37.16
26.09
28.13
20
50
70
39.25
35.85
33.26
36.12
I decided to settle with this experiment as it gave me good results and was also fairly uncomplicated to carry out. The concentration of the hydrochloric acid I used was 1M. However, I found that the times were too high (which could potentially cause difficulty for me when I would plot my graphs as the axis would be quite high), so I decided to increase the concentration of the hydrochloric acid to 2M - this would decrease the time taken for the reaction to occur as there would be more particles per dm³, causing the particles to collide more frequently, and overall make the reaction go faster. This would also make it easier for me to analyse my results more accurately, and I would have a better gradient for my graph (as I would have a smaller range of results).
When I was conducting my preliminary tests, I made sure to draw the cross on the same type of A4 white paper, with the same red board marker, and the same number of pen strokes each time (2). I also made sure that the cross was clearly visible when the conical flask was placed over it.
My experiment
Apparatus
Stopwatch - I used this to measure the amount of time taken for the reactions to occur
Conical flask - reaction took place in this
A4 white paper with an 'X' drawn on it
Red board pen - I chose the colour red as it could clearly be seen through the cloudy solution
Bunsen burner - I used this to heat up some water (which in turn was used to heat up the hydrochloric acid)
Tripod
Gauze - so we could balance the beaker of water on top of the Bunsen burner
Heatproof mat
Matches - used to light the Bunsen burner
2M hydrochloric acid - I used this concentration as the times for the reaction to occur using this concentration would be smaller than 1M: this would make it easier for me to analyse my graphs
Sodium thiosulphate
Thermometer - I used this to check my temperatures were accurate
3 beakers - one for holding hydrochloric acid, one for holding sodium thiophosphate, one for holding heated water
2 measuring cylinders - one used to measure the volume of hydrochloric acid, the other used to measure the volume of sodium thiosulphate
1 pair of tongs
1 pair of safety goggles
Method
First I measured out 70ml of sodium thiosulphate and poured it into a conical flask
I then measured out 50ml of hydrochloric acid:
I was changing the temperature of hydrochloric acid and my range was 5°C, 10°C, 20°C, 30°C, 40°C, 50°C
In order to get the hydrochloric acid at the temperatures of 5°C and 10°C I asked the lab technicians to refrigerate the acid for me at these temperatures
To heat the hydrochloric acid to my desired temperatures of 20°C, 30°C, 40°C and 50°C I poured 50ml of hydrochloric acid into a small beaker
I then heated a beaker of water via a Bunsen burner until it reached (slightly more than) the desired temperature
Using this beaker of water as a 'water bath' I placed the small beaker of 50ml hydrochloric acid within the water, until the hydrochloric acid also heated up to the desired temperature (this was partly due to the fact that the hydrochloric acid had been heated in water slightly hotter than the desired temperature, which ensured that the hydrochloric acid would have been heated right up to the correct temperature)
I used a thermometer to make sure the temperatures were accurate
I then drew on a piece of A4 white paper a cross ('X') using a red board pen; I then placed the conical flask on top of the A4 paper
I added the 50ml of hydrochloric acid to the previously measured out sodium thiosulphate, and started the stopwatch as soon as all the hydrochloric acid had been added (this was so I could measure how long the reaction takes once it has begun with the hydrochloric acid)
Wearing safety goggles to protect my eyes, I watched the solution turn cloudy, and made sure to pause the stopwatch as soon as I could no longer see the cross
I then recorded down my results, and repeated the above steps a further two times for each temperature; I tabulated my data and worked out mean averages for each temperature (see below)
When I was recording my times/calculating averages, I made sure to round my results up to the nearest second: this was because this is the only level of accuracy I could conduct my experiment to
Results
Temperature of hydrochloric acid (°C)
Volume of hydrochloric acid (ml)
Sodium thiosulphate (ml)
1st time (seconds)
2nd time (seconds)
3rd time (seconds)
Mean average time (seconds)
5
50
70
45
44
45
45
10
50
70
27
27
27
27
20
50
70
25
26
26
26
30
50
70
20
21
20.
20
40
50
70
18
18
16
18
50
50
70
18
14
14
15
Analysis of graphs
Overall, looking at my results and curves of best fit on my graph, I can see that there is a definite correlation between the temperature of hydrochloric acid and the rate of reaction: the higher the temperature, the faster the rate of reaction. In my graph with error bars, there are some noticeable error bars - however, this is more likely due to the fact that the axis on our graphs are fairly large, and slight variations would show up as being more conspicuous. Another explanation for the error bars could be a lack of precision (see evaluation). In my graphs, there are no anomalous result, which does reinforce the accuracy of my results. In my graphs we can easily identify a trend in the results: as the temperature increases, the time taken for the reaction to occur decreases (we can see this as the line of best fit is decreasing as the temperature is increasing). It can be seen that the longest time for the reaction to occur was when the hydrochloric acid was 5°C; likewise the fastest reaction was when the hydrochloric acid was at 50°C. The scientific explanation for this involves the collision theory: as the temperature of the hydrochloric acid rises, the molecules will have more energy; this will cause them to move faster. As the particles are moving around a lot more (due to their high levels of energy because of the heat) they are more likely to collide and react with the sodium thiosulphate molecules (as there is now much more energy and force behind the hydrochloric acid particles). However, when the hydrochloric acid particles are at a lower temperature, there will be less force behind the atoms as they will not have much energy - as a result the time taken for the particles to collide and react is longer. Therefore my initial prediction was correct - I can back up my prediction with scientific evidence and my curve of best fit.
Evaluation
Even though my results are accurate, the precision of my procedure could have been improved. One example is using the stopwatch: firstly it was difficult when to tell if the cross was no longer visible; bear in mind that it would have taken at least a second to verify that the cross was no longer visible. Once this had been verified, the stopwatch would then actually have to be paused (which again, would take up a second or so) - the result could have therefore been more precise. However, our results did not show a huge variation, and my error bars weren't large; this shows that our readings were generally accurate.
There are several ways I would refine and improve my method: one example is that when timing how long it takes for the reaction between hydrochloric acid and sodium thiosulphate to turn cloudy, I would have two people timing the reaction with a stopwatch instead of one. That way, two different times could be taken into account, a mean average could be calculated, and that time could be recorded; this would result in a much more precise time. I would have also employed the use of a colorimeter: a colorimeter is a device which shines a wave of light through a medium and measures light intensity. First, a light wave (which would have to be kept at a constant wavelength and intensity) is shined at a solution: this is called intensity light. The intensity of the light exiting the solution (transmitted light) is then measured on the other side of the solution by the colorimeter. By comparing the incident intensity to the transmitted intensity, you could calculate how much light was absorbed. In my experiment, I would stop timing the stopwatch when I thought I could no longer see the cross (as the solution has turned opaque); however, if I used a colorimeter, then rather than relying on my eyes (an unreliable source) I could just look at the reading on the colorimeter: when no light intensity could no longer be registered, then I would know that I should stop timing, (as no light was being allowed to shine through the solution, as it was opaque, meaning the cross would no longer be visible); therefore the colorimeter would greatly reduce any chances of error, as the point of measurement is very sharp. In my experiment, one of the greatest difficulties I faced was judging whether I could no longer see the cross; a colorimeter would have assisted me with this.
Another way I would have improved my method was to try to get the temperatures exactly right; occasionally, the hydrochloric acid would not be heated up to the exact temperature (it might have been one degree Celsius off). If I did this, then my results would be more accurate. Something else I would have done would have been to use the same red board pen and same piece of A4 white paper each time I conducted my experiment; this way the thickness of the paper, and the colour and thickness of the cross would stay constant - again this would improve the precision of my results as it would ensure that there were no other factors/variables affecting the reliability of my results.
However, despite all these potential improvements, I do believe that my results already are of a high accuracy: this can be represented by the fact that my error bars are reasonably small, and that there are no anomalous results. All of my results also fit in my curve of best fit, again reiterating that all of my data is accurate and follows the pattern that the higher the temperature, the faster the rate of reaction.
If I was to repeat the experiment again I would have increased the range of temperatures we used, and made the intervals between every single temperature testes 5 degrees Celsius. Another thing I would have done would have been to have varied the concentrations of the hydrochloric acid, and seen how the temperature would affect the different concentrations. This would result me in me having more data in my graph, and therefore more results to compare and analyse.
