A Investigation into the Rate of Chemical Weathering of Marble
Georgia Smith 11V January 2002
Chemistry Coursework
A Investigation into the Rate of Chemical Weathering of Marble
Planning
Chemical weathering is where acids in the atmosphere weather away certain materials, such as marble. The acids often come from pollution like car fumes and smoke. This acid is absorbed into the atmosphere and then, as part of the weather cycle, it is 'rained' back down and erodes the surfaces of materials. The acid involved in chemical weathering can slowly weather away at statues and buildings.
Rate is a measure of how fast or slow something is. Rate of reaction describes how fast or slow a chemical reaction is. For chemists who are making a product, it is important to know how long the reaction takes to complete before the product is produced. Rate is a measure of a change that happens over a single unit time. This unit of time is most often a second, a minute or an hour.
This investigation is going to be carried out in the chemistry lab so it is essential to imitate the process of chemical weathering in the most accurate way possible. The particles which actually react with the marble are the H+ ions, these reactant ions originally come from hydrochloric acid. The best way to imitate the chemical weathering of marble is by using acid to replicate acid rain.
Metal Carbonate + Acid ----- Metal Salt + Water + Carbon Dioxide
CaCO + 2HCl ----- CaCl + H O + CO
This experiment is based on the Collision Theory. Reactions occur when particles in the reacting solid, liquid or gases collide with each other. Here, the reaction occurs when the acid particles collide with the calcium carbonate particles in the marble. Not all the collisions result in a reaction; sometimes the particles just bounce off each other. For the H+ ions to react they must have a minimum amount of energy called the activation energy. The molecules' energy comes from the kinetic (movement) energy they have. When you warm a substance its particles gain more kinetic energy and their average velocity increases. Therefore reactions are more likely to take place at higher temperatures.
Temperature is not the only variable to consider in the investigation. There are many others:
* Type of Acid - An acid is an acid because it produces H (hydrogen ions) in solution. The acids that are available are; hydrochloric acid, nitric acid, sulphuric acid, citric acid and ethanoic acid. Some of these are strong acids which means that they completely split up into ions to give the highest possible concentration of H+ ions. Others are weak acids and do not completely split up into ions. The effect of varying the acid would show which acids are most destructive on the marble.
* Concentration of Acid - The concentration of an acid is usually quoted in 'moles per litre'. The hydrochloric acid in the in the lab is 2mols/litre i.e. 2mols HCl (73g) per litre of acid solution. The other acids vary in their concentration, for example, sulphuric acid is quoted at 1mole/litre.
The effect of varying the concentration of one of these acids, by diluting it further with distilled water to create a range of concentrations, would display whether or not the concentration of the acid effects the rate of chemical weathering of marble. Altering the concentration of the acid can affect the rate of reaction because, in theory, increasing the H+ ion concentration will increase the frequency of collision. The table below illustrates how an experiment could be performed using this variable:
Acid (ml)
Water (ml)
Total Vol (ml)
Acid %
30
0
30
00
20
0
30
67
0
20
30
33
* Temperature of the Acid - The temperature of the acid could be lowered by adding ice to the solution or increased by heating it over a flame. It is important to remember that 20 C is not twice as hot as 10 C so the particles do not necessarily move twice as fast. This is a scientific experiment so the best scale to use would be the Kelvin Scale. If the temperature were varied it would show whether or not temperature has an effect on the rate of chemical weathering of marble.
* Surface Area of Marble - This is the most difficult variable to control because to compare the effect of varying surface area it would be necessary to estimate the surface area involved. The only scale that can be used is small/medium/large and this is highly inaccurate because one person's opinion may be different from another and the marble chips available are generally very similar sizes, there is only a few millimetres difference between 'small' and 'large'.
Using smaller pieces of marble would increase the reaction rate because this increases the total surface area which is where ...
This is a preview of the whole essay
* Surface Area of Marble - This is the most difficult variable to control because to compare the effect of varying surface area it would be necessary to estimate the surface area involved. The only scale that can be used is small/medium/large and this is highly inaccurate because one person's opinion may be different from another and the marble chips available are generally very similar sizes, there is only a few millimetres difference between 'small' and 'large'.
Using smaller pieces of marble would increase the reaction rate because this increases the total surface area which is where the reaction takes place. Increasing the surface area must increase the number of particles of the marble coming into contact with the acid solution.
I have chosen to vary the concentration of the Hydrochloric Acid because this acid (HCl ) contains H+ and Cl- ions in solution. When it is put in a reaction with Marble (CaCO ) the Cl- ions cancel out so the particles that are actually reacting with the marble chips are the H+ ions:
CaCO + 2H + 2Cl - ----- Ca + 2Cl - + H O + CO
So the reaction is: CaCO + 2HCl ----- CaCl + H O + CO
I will vary the concentration by diluting the acid with water. In each experiment, an excess of acid will be used. This means there will be more than enough acid to react with all the marble chips.
Now that I am sure it is the H+ ions in the acid that control the reaction I can use the collision theory (which states that the greater the concentration of H+ ions, the greater the rate of reaction) to investigate whether varying the concentration of acid has any effect on the rate of the reaction.
The rate of the chemical weathering can be measured in one of two ways:
. By measuring the volume of Co collected. This can be done by either collecting the Co in a gas syringe and measuring it at regular intervals, or collecting the Co by displacing water from an inverted measuring cylinder. The volume of Co could also be measured by using a datalogger and linking it to the gas syringe to automatically datalog the volume of Co at regular time intervals.
2. By measuring the change in mass - This can be done by measuring, at regular intervals, the mass of the marble chips as it changes. This is very easy to do if the experiment is carried out on an electronic balance. Or the electronic balance could be linked up to a mass datalogging program on the computer to automatically log the mass changes at regular intervals.
I have chosen to measure the reaction rate using the first method because I am familiar with the use of the gas syringe and there is not enough electronic balances or dataloggers for the entire class to use. I also feel that a gas syringe will obtain a reliable result if the experiment is performed accurately. I can do this by taking precise measurements (to the same decimal point) and repeating the experiment more than once.
This picture displays the set-up and apparatus that I will use in the experiment:
I carried out a preliminary experiment to help plan a suitable range of measurements and to ensure that I am using the best method. For this 'pilot test' I used 3g of medium sized marble chips (that's about 13-14 chips) and hydrochloric acid. Here are my results:
Volume of Carbon Dioxide Collected (cm )
Time
Water: 0ml
Water: 10ml
Water: 20ml
Water: 30ml
(secs)
Acid: 30ml
Acid: 20ml
Acid: 10ml
Acid: 0ml
0
36
20
1
0
20
48
28
5
0
30
60
38
20
0
40
73
49
26
0
50
82
60
33
0
60
97
70
37
The main drawback of my preliminary experiment was that I only timed it for one minute, this did not allow me to see a wide enough range of results and the reaction did not finish (fill the gas syringe with CO ) for all of the concentrations. I think I will need to time for up to 5 minutes to allow all reactions to complete. It was also very difficult to measure accurately at such short time intervals so I am going to extend these intervals to 15 seconds. I am also going to increase the volume of solution to 60ml so that I can make a greater number of variations and obtain more accurate results. I can also see that the reaction involving no acid will be too slow to include in the experiment so in my actual experiment I will leave it out. I am going to use the same mass of marble chips as in the preliminary experiment but a larger amount of solution so I am also going to leave out the Water = 50ml / Acid = 10ml solution because I am sure this would take too long.
Prediction
I predict that if I double the concentration of acid the rate of the reaction will also double. The Collision Theory states that the greater the concentration of H+ ions, the greater the rate of reaction and my preliminary experiment concurs with that:
The figures below that I am doubling are what were obtained in my preliminary experiment when there was 10ml of acid, when they are doubled they equal roughly what was obtained in my preliminary experiment for 20ml of acid - the figures in the brackets.
1 x 2 = 22 (20)
20 x 2 = 40 (38)
33 x 2 = 66 (60)
This illustrates why I predict that by doubling the acid (H+) concentration the frequency of collision will double and therefore the rate of reaction between the H+ ions and the marble chips will double. When the acid is more concentrated there are more particles of the acid present in the same volume. This means that particles will be closer together and more likely to collide with the marble chips and react. When more water is added, the acid becomes more dilute. Acid particles become more spaced out, and are less likely to react.
The effect of concentration on reaction rate:
Particles are far apart and are less likely to meet and react.
More particles are present in the same volume, so they are closer together and more likely to meet and react.
Method
. Firstly I will measure out five lots of 3g of medium sized marble chips using the electronic balance for accuracy. I will ensure that there are roughly 13 to 14 chips in each lot so that they all have about the same surface area.
2. Next I will measure out the different concentrations of hydrochloric acid and distilled water solution using the measuring cylinder, each to exactly 60ml of solution in total. It is necessary to use distilled water because tap water may have other chemicals or an acidic trace in it which would effect the results of the experiment.
3. I will then put a set of the marble chips into the bottom of the conical flask which is attached to the gas syringe by a rubber stopper and a rubber tube. Before I actually begin the experiment it is important to push all of the air out of the gas syringe otherwise it will not begin from the actual starting point.
4. As soon as the first solution is poured into the conical flask I will put the rubber stopper in to ensure no carbon dioxide is lost. I will also start the stop clock at the same time.
5. Every 15 seconds I will record the volume of carbon dioxide that has collected in the gas syringe.
6. I will carry out the experiment for each concentration and also repeat it to gain more results for a more precise conclusion.
Obtaining
These are the results that I obtained during my actual experiments. In bold the average of the two experiments is shown.
Experiment 1, Experiment 2, Average - Vol. of CO Collected (cm )
Time
Water: 0ml
Water: 10ml
Water: 20ml
Water: 30ml
Water: 40ml
(mins)
Acid: 60ml
Acid: 50ml
Acid: 40ml
Acid: 30ml
Acid: 20ml
00:15
7, 19, 18
4, 10, 12
3, 10, 12
8, 8, 8
5, 2, 4
00:30
32, 32, 32
26, 30, 28
20, 21, 21
1, 13, 12
8, 6, 7
00:45
49, 48, 49
42, 46, 44
40, 39, 40
9, 19, 19
1, 11, 11
01:00
67, 64, 66
60, 62, 61
57, 55, 56
25, 27, 26
5, 17, 16
01:15
86, 86, 86
78, 78, 78
75, 74, 75
35, 37, 36
21, 24, 23
01:30
99, 99
96, 92, 94
90, 91, 91
42, 49, 46
27, 32, 30
01:45
50, 60, 55
36, 40, 38
02:00
59, 72, 66
44, 50, 47
02:15
69, 85, 77
52, 60, 56
02:30
79, 96, 88
60, 70, 65
02:45
90, 90
68, 80, 74
03:00
96, 96
75, 88, 82
03:15
82, 97, 90
03:30
91, 91
03:45
95, 95
04:00
00, 100
Analysis
This graph displays the volume of CO collected against the time taken
I have plotted smooth lines through the points instead of straight best-fit lines, because the reaction is not steady all the way to the end. I can just see it beginning to slow down towards the production of 100cm of CO . The line decreasing in gradient and beginning a plateau effect displays this. I think that the points in circles near the top of the graph are slightly anomalous - this is probably due to an error in judgement as I was trying to finish the experiment or possible parallax error because I had to judge how much CO had been collected by sight alone.
From looking at my results and graph I am able to conclude that the more concentrated the acid is the quicker the reaction takes place. There was a dramatic change in the amount of time it took to collect the gas as more water was added to the solutions. For example, if you look at the graph it shows how much longer it takes for the gas to be collected in the more diluted of the concentrations (right side) than the more acidic (left side).
I can see if my quantitative prediction was correct by working out the gradient of the initial rate (the beginning straighter section), this will give me the rate of reaction;
Rate of reaction/Gradient = Volume of CO
Time
By working out the rate of reaction I can then plot this against the concentration of the solution on a graph to check whether or not my prediction was accurate.
I am going to look at the gradient of each concentration after 30 seconds because this is part of the initial reaction for all of the solutions:
In my prediction I stated that by doubling the acid concentration, the rate of reaction would double too. The last graph I drew proves this prediction to be wrong because when the concentration of acid is doubled the rate of reaction actually triples:
My prediction was not entirely wrong though. Due to my use of the Collision Theory I was correct in predicting that the rate of reaction would increase as the concentration of acid was increased. Changing the concentration of the acid will either make the rate of reaction slower or faster. It is clear that when the solution is made more concentrated with acid, then there are more reactant particles per set volume. This makes collisions between the reactant particles more likely. Therefore there are more collisions per second, so more particles reacting per second - and the rate of reaction is increased.
The difference between the reaction in my preliminary experiment (where doubling the concentration doubled the rate of reaction), and my actual experiment (where doubling the concentration tripled the rate of reaction), is probably explained by the proportionality between the solution and marble chips. I used only 30ml of solution in my preliminary experiment and 60ml in my actual experiments but I used the same mass of chips in both. This would mean there were more H+ ions to react with the same amount of surface area in my actual experiment - explaining why the rate of reaction tripled rather than doubled.
Despite my quantitative prediction being inaccurate I still feel I have gained sufficient reliable evidence to come to the conclusion that increasing the concentration of acid increases the rate of reaction - so when acid is more concentrated the rate of chemical weathering of marble is greater.
Evaluation
For accuracy of results I carried out the experiment twice to gain an average result. But I did this on two different days using different sets of equipment. In my table of results it is clear to see that towards the last couple of experiments there are quite large differences between the figures recorded for the volume of CO collected, and they finish at different times so the average is slightly inaccurate.
Water: 30ml
Water: 40ml
Acid: 30ml
Acid: 20ml
8, 8, 8
5, 2, 4
1, 13, 12
8, 6, 7
9, 19, 19
1, 11, 11
25, 27, 26
5, 17, 16
35, 37, 36
21, 24, 23
42, 49, 46
27, 32, 30
50, 60, 55
36, 40, 38
59, 72, 66
44, 50, 47
69, 85, 77
52, 60, 56
79, 96, 88
60, 70, 65
90, 90
68, 80, 74
96, 96
75, 88, 82
82, 97, 90
91, 91
95, 95
00, 100
The different weather conditions on the two separate days could possibly account for the difference in results. Temperature was one of the variables to consider and on a hotter day the reaction would be faster because the H+ ions would be moving about at a greater speed and colliding more often with the CaCO . My table indicates that Experiment 2 may have been carried out on a slightly warmer day because the figures are all higher than those recorded during Experiment 1.
Different equipment was also used and, although it was thoroughly checked, the rubber tubing connecting the conical flask and the gas syringe may not have attached properly or could have had a small leak allowing some of the CO to escape - this could account for the lower figures recorded in Experiment 1. Despite these possibilities I think it is more likely that the gas syringe I used began to stick a little in the later experiments, this could explain some of the larger 'jumps' in the figures.
A more accurate way of performing the experiment would have been to weigh the mass of the chips at regular intervals using scientific scales. With a gas syringe there is a large risk of parallax error when reading the volume amount from the side of the syringe because the eye may not be directly parallel. Also, the inner section of the syringe is moving at all times so it is difficult to get a highly accurate reading. These were probably the main sources of error in the experiment. It was impossible to stop some of the CO escaping from the conical flask between the acid hitting the marble chips and putting the rubber bung into the top of the flask - so readings at the beginning may have been slightly inaccurate.
Despite these potential inaccuracies in my method, my results a very useful because they still indicate a clear pattern from which I was able to confidently draw a conclusion, which concurred with the main part of my prediction. The method I used was simple to execute, could be carried out within the time available and produced results of reasonable accuracy. The quality of my results could be improved by doing more extensive experiments and investigating the effect of proportionality between the surface area and the volume of solution. A larger mass of chips could be used with a greater volume of solution so more variations could be made with the experiment.
Some of the variables were very difficult to control; like the surface area of the chips and the temperature. I feel that I did all that was possible to make these variables consistent though. The chips were weighed accurately and also counted to insure there was roughly the same number in each experiment - this could be made more reliable by using chips specially made to have the same surface area e.g. balls of CaCO . I could also have taken the temperature of the solution at the beginning of each experiment so I could know whether or not it was having any effect on the reaction, or even put everything into a water bath to sustain the same temperature. The experiments should ideally be carried out on the same day if they are going to be averaged because conditions and equipment should always be the same to make the investigation as fair as possible.
To gain more reliable results my results could be pooled with groups who did the same experiment so there is a greater number to gain an average from, but all the experiments included must be completed to the same standard of fairness and accuracy.
