An investigation to find out how the concentration of acid affects the rate of the reaction between HCl and CaCO3
An investigation to find out how the concentration of acid affects the rate of the reaction between HCl and CaCO3
Skill Area P: Planning Experimental Procedures
Introduction and Preliminary Work
I am attempting to find out what affect the concentration of hydrochloric acid has on the rate of the reaction between HCl and marble chips (calcium carbonate). The equation of this reaction is as follows:
CaCO3 (s) + 2HCl (aq) --> CO2 (g) + CaCl2 (aq) + H2O (l)
calcium + hydrochloric--> carbon + calcium + water
carbonate acid dioxide chloride
There are many factors which can affect the rate of a reaction. The temperature of the acid will have an impact on how quickly a reaction occurs. I carried out a preliminary experiment on the affect of temperature as the variable, by using water baths and thermometers to obtain four different water temperatures in which the flask containing acid and marble chips was placed. I used four different water temperatures from 20°C to 50°C, going up in 10°C each time. I carried out two tests for each temperature, and noted the change in mass every minute for ten minutes as the chips dissolved, and I took an average of the two sets of results. I found that, the higher the temperature of the water bath, the faster the reaction happened. This was because, as the kinetic theory explains, a high temperature gives the particles of the reactants more energy to move (and react) faster. I am not choosing to use this factor in my current investigation because it was quite difficult to obtain accurate results, as the water baths didn't always maintain the correct temperature. This caused a few errors in my results.
I also did a preliminary experiment in the same way on the effect of the size of the calcium carbonate chips on the rate of the reaction. This was an experiment that proved that the larger the surface area of the chips (i.e. the small sized marble chips), the faster the reaction took place. The mass decreased faster with a large surface area because the acid particles met the particles of the marble chips more often, causing an increased rate of reaction. I chose not to carry out an improved version of this experiment because there is only a small range of sizes of chips that I could have used, which are small, medium and large. This decreases the accuracy of results because the range of surface areas is fairly small, and the large chips, for example, could vary slightly in actual size.
I have chosen to do an investigation on the concentration of acid because I can use a good range of concentrations - five different acid concentrations ranging from 0.1 molar to 2 molar. This will increase the accuracy of my results. I have also done extensive preliminary work on this particular factor. I used only three concentrations of hydrochloric acid, so I will make my work better this time by investigating five. Here are the results from my previous experiment:
0.5M concentration:
Time (minutes)
Mass (grams)
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
29.73
29.69
29.65
29.63
29.62
29.61
29.61
29.60
29.60
29.60
29.60
29.03
28.97
28.92
28.91
28.89
28.88
28.88
28.87
28.87
28.86
28.86
29.38
29.33
29.29
29.27
29.26
29.25
29.25
29.24
29.24
29.23
29.23
Average total decrease in mass over 10 minutes = 0.15g
M concentration:
Time (minutes)
Mass (grams)
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
28.50
28.40
28.40
28.40
28.40
28.40
28.40
28.30
28.30
28.20
28.20
30.00
29.90
29.90
29.80
29.80
29.80
29.80
29.70
29.70
29.70
29.60
29.25
29.15
29.15
29.10
29.10
29.10
29.10
29.00
29.00
28.95
28.90
Average total decrease in mass = 0.35g
2M concentration:
Time (minutes)
Mass (grams)
st Test
2nd Test
Average
0
2
3
...
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0
28.50
28.40
28.40
28.40
28.40
28.40
28.40
28.30
28.30
28.20
28.20
30.00
29.90
29.90
29.80
29.80
29.80
29.80
29.70
29.70
29.70
29.60
29.25
29.15
29.15
29.10
29.10
29.10
29.10
29.00
29.00
28.95
28.90
Average total decrease in mass = 0.35g
2M concentration:
Time (minutes)
Mass (grams)
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
30.96
30.52
30.30
30.22
30.20
30.18
30.17
30.16
30.15
30.14
30.13
29.18
28.79
28.58
28.51
28.48
28.47
28.46
28.45
28.45
28.44
28.44
30.07
29.66
29.44
29.36
29.34
29.32
29.31
29.30
29.30
29.29
29.28
Average total decrease in mass = 0.79g
I discovered that, the higher the concentration of the acid, the quicker the reaction, because in a more concentrated solution there are more particles, causing more collisions and so a faster reaction takes place.
A few other factors can be used to affect the rate of a reaction. I have already done work on the temperature and concentration of acid and the surface area of the chips, but the mass of the carbonate, or using different acids or adding a catalyst (however, this is not possible for this reaction), can also affect the rate of reaction. In biology, the amount and intensity of light can alter the rate of the photosynthesis reaction and enzymes (biological catalysts) can also have an impact.
Safety
Safety is very important in this, and any, experiment so I will wear safety specs when carrying it out to protect my eyes from becoming damaged by the acid. I will also wear an apron to protect my clothes from the same. When I am using bottles that contain hydrochloric acid, I will remember to keep the lids on, when I am not using them, to prevent spillages. If any spillages or breakages occur, I will either clean them up myself or report them immediately. I will tie back my long hair to keep it out of the way and to stop it from getting damaged. I will also stand up whilst carrying out the experiment to avoid acid from spilling onto my lap.
Method and Fair Testing
To begin with, I will measure out 10g of calcium carbonate and, for accurate results, I will ensure that this is as near as possible to 10g for each concentration of acid. I will put these marble chips into a conical flask and place it on an accurate top-pan balance. I'll then pour the hydrochloric acid into the flask, making sure that I use almost exactly 20cm³ each time, so that the experiment will be fair. I will use the same sized marble chips (medium) so that there is roughly the same amount of particles in the flask to collide with each other and react. All the experiments will be carried out at the same temperature (room temperature) so that all of the particles have an equal amount of energy with which to vibrate and move. The variable will be the concentration of hydrochloric acid.
I will record the mass at the beginning of each of the five experiments (as soon as I have poured the acid in) - this will be my reading for 0 minutes. I will proceed to take readings of the mass every minute for ten consecutive minutes, as accurately as possible. I am taking readings over ten minutes rather than five so that I can get enough results to be sure of when the reaction starts to slow down and stop. Alternatively, I am not going up to fifteen minutes so as to ensure that I have time to complete all of the experiments and the second tests. I will take the readings of the mass to two decimal places so that the readings are accurate but not too precise. I will ensure that the equipment is clean before going onto the next experiment - either a second test or a different acid concentration. I'll take both first and second tests, and then calculate the averages, to be sure of my results.
To help in my planning, and to gain scientific knowledge of the experiment, I have looked at three main secondary sources. These are GCSE Science Double Award Chemistry (John Murray publishers), Key Science 4 Book 2 (Stanley Thornes publishers) and http://www.uk-learning.net.
Prediction
According to my previous experiment on concentration, I predict the result to be that, the higher the concentration of the hydrochloric acid, the faster the reaction. This is because there are fewer particles in a less concentrated solution to collide with each other, so the reaction will be slower. The rate of a chemical reaction is increased as the concentration of the reactants also increase. This will be happening while the acid and the marble chips react, and is known as the collision theory.
As I am using five concentrations of acid, I am going to try to prove that the rate of reaction will be doubled as the concentration doubles. That is, I predict that the reaction will be twice as fast when I am using 2M acid than when I'm using 1M acid, and also twice as fast using 1M as using 0.5M.
Equipment list
Conical flask - to house the reaction between the acid and the chips.
Electronic top-pan balance - to ensure that I am able to record accurate readings of the mass in grams, to two decimal places.
00cm³ Measuring cylinder - to measure accurately the correct volume of hydrochloric acid that I will use.
Timer - to time one-minute intervals over a period of ten minutes accurately.
Reactants
0.1M, 0.5M, 1M, 1.5M and 2M hydrochloric acid - for varying concentrations. This is so I can obtain a good range of results, so I can prove or disprove my prediction, and so come to a definite conclusion. I will require 40cm³ of each concentration in order to carry out two tests for each.
00g medium sized marble chips - 10g for each of the five experiments and 10g for each of the second tests.
Skill Area O: Obtaining Evidence
Tables of Results
0.1M concentration:
Time (minutes)
Mass (grams) including flask
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
83.71
83.70
83.69
83.65
83.64
83.68
83.68
83.67
83.67
83.66
83.66
83.89
83.87
83.87
83.86
83.85
83.84
83.84
83.83
83.83
83.83
83.83
83.80
83.79
83.78
83.76
83.75
83.76
83.76
83.75
83.75
83.75
83.75
Total decrease in mass:
0.05g
0.06g
0.055g
0.5M concentration:
Time (minutes)
Mass (grams) including flask
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
84.48
84.47
84.41
84.39
84.36
84.35
84.35
84.35
84.33
84.33
84.32
84.45
84.40
84.39
84.37
84.37
84.36
84.36
84.36
84.35
84.35
84.35
84.47
84.44
84.40
84.38
84.37
84.36
84.36
84.36
84.34
84.34
84.34
Total decrease in mass:
0.16g
0.10g
0.135g
M concentration:
Time (minutes)
Mass (grams) including flask
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
84.33
84.21
84.17
84.15
84.10
84.08
84.06
84.05
84.04
84.03
84.03
84.59
84.44
84.41
84.37
84.35
84.32
84.31
84.31
84.28
84.27
84.27
84.46
84.33
84.29
84.26
84.23
84.20
84.19
84.18
84.16
84.15
84.15
Total decrease in mass:
0.30g
0.32g
0.31g
.5M concentration:
Time (minutes)
Mass (grams) including flask
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
84.35
84.11
83.98
83.94
83.93
83.93
83.92
83.91
83.90
83.89
83.89
84.36
84.15
84.01
84.01
84.00
84.00
84.00
83.99
84.05
83.99
83.99
84.36
84.13
84.00
83.98
83.97
83.97
83.96
83.95
83.98
83.94
83.94
Total decrease in mass:
0.46g
0.37g
0.42g
2M concentration:
Time (minutes)
Mass (grams) including flask
st Test
2nd Test
Average
0
2
3
4
5
6
7
8
9
0
81.94
81.55
81.41
81.37
81.35
81.32
81.31
81.31
81.31
81.30
81.30
83.57
83.08
83.02
82.98
82.97
82.94
82.93
82.93
82.92
82.92
82.92
82.76
82.32
82.22
82.18
82.16
82.13
82.12
82.12
82.12
82.11
82.11
Total decrease in mass:
0.64g
0.65g
0.645g
Skill Area A: Analysing Evidence and Drawing Conclusions
Results
I have drawn graphs of my averages on a separate sheet of graph paper, and drawn a curved line of best fit through each of the series of crosses. I have identified a few anomalous results, which I will write about in my conclusion.
All of my results have been taken to two decimal places, for adequate accuracy. The average total decrease in mass is sometimes calculated to more than that to avoid rounding up 0.5 values, giving a clearer accuracy and definition between test one and test two. I repeated the experiment, and took the averages of the two tests, in such a way, to make sure that I had correct results. I also did this to make sure that my results were as close as possible to their true value.
I also made sure to keep to the safety procedures (e.g. tying m hair back and wearing safety specs and an apron) whilst carrying out the experiment.
When doing the experiment, I contradicted my plan by using small marble chips instead of medium ones (because there were no medium chips available at the time). This probably increased the rate of reaction, for each individual concentration, because the larger the surface area, the faster the rate of reaction. However, this did not affect the outcome of my experiment because I was looking at concentration of hydrochloric acid. I was able to use small marble chips for every single concentration that I investigated, so, although each reaction occurred a little too quickly, it was still a fair test.
Patterns Shown
The higher the concentration of the acid, the steeper the corresponding lines on my graph are. This shows that the reaction occurred more quickly when the concentration of the hydrochloric acid was higher, which proves my prediction to be correct. This outcome can be explained using the collision theory, which is, the more concentrated the solution, the more particles there are, so more collisions take place. This means that the rate of the reaction was faster at 2 molar than at 0.1 molar because the concentration of the reactants was higher.
A solution containing 2 molar acid decreased the most in mass overall; this pattern continues until the least mass lost was at 0.1 molar. The 0.1M solution didn't lose that much mass because it had the fewest particles that were able to collide with the marble chips. This meant that the reaction was slowest at this concentration.
Skill Area E: Evaluating Evidence
Success of Investigation
The investigation turned out to be quite successful because it was able to prove my prediction to be correct in accordance with the collision theory. However, not every single one of my results fits the pattern. I have circled one anomalous result on my graph of 1.5M concentration, and two do not fit the pattern of the 0.1M concentration line. The 1.5M line of best fit doesn't go through the 8 minute reading of the mass, and the 5 and 6 minute reading for 0.1M don't touch the line of best fit for that series of data either. The mass can't go back up in a 'rates of reaction' experiment after it has fallen to its lowest possible value, so I know that these three results are wrong. One of the reasons that I could have obtained these incorrect results is that the table may have accidentally been jogged whilst I was carrying out my experiment, causing an imbalance of the solution in the flask on the top-pan balance. The display would then be altered so that it would appear that the mass had re-increased after falling to its lowest amount.
Another reason for my anomalous results could be that I was too careless in measuring out the exact volume of hydrochloric acid and the correct mass of marble chips, although it was fairly difficult to be completely accurate when measuring out the chips because they were all different sizes.
Overall, I think that my results would be to within about 10% of the true value, because they are fairly accurate, but they do vary from test to test and I did get a few anomalous results.
Changes to Method
If I had to repeat this investigation, I would certainly make some changes. First of all, I would modify the equipment I used by working with a gas syringe to measure the amount of gas given off rather than the decrease in mass. This would make the investigation better by giving me much more accurate results than a top-pan balance did. I would change my method slightly, again to obtain more accurate results, by measuring out the volume of hydrochloric acid more carefully when using a 100cm³ measuring cylinder, with the 20cm³ mark level with my eyes. These changes would also make my results more reliable because they would be more accurate.
My investigation was a fair test, apart from the acid and marble chips not being measured out completely accurately. I have mentioned that I used small chips instead of medium but, as I used them for every variable of the concentration of acid, the experiment remained a fair test.
The only real difficulty I met was not getting started soon enough to find that I had to wait in order to get the correct equipment, for example, a top-pan balance and all five acid concentrations, to begin obtaining my results. This was also why I had to use small marble chips.
Obtaining Further Information
To get more information on this topic before carrying out an investigation, I could have looked at a few more books and websites to be sure of the scientific ideas behind the experiment.
If I had more time at the end of this experiment, I could have carried out another one, using a gas syringe in order to obtain an extra set of (hopefully) better and more accurate results. This would make me surer of the conclusions I have drawn because I would have more evidence to back up what I was saying. This method would give me new information and not just more of the same because different, more accurate equipment would be used. Also, a gas syringe would enable me to measure the amount of gas given off by the reaction, so the results would also be of a much different nature to the ones I have already obtained.
Sarah Aldridge 11u3