Carry out an experiment investigating the effect of varying acid concentration on the rate of decomposition of limestone on the exterior of the Lincoln Cathedral.
Lincoln Cathedral Investigation
Planning:
Aim of Investigation:
To carry out an experiment investigating the effect of varying acid concentration on the rate of decomposition of limestone on the exterior of the Lincoln Cathedral.
Preliminary Work:
Aim: To find the most effective acid on the decomposition of limestone.
Equipment: Conical flask
Delivery tube
Water bath
Measuring cylinder
Water
Hydrochloric acid (50cm3)
Nitric acid (50cm3)
Sulphuric acid (50cm3)
Limestone (3 medium sized marble chips)
Stopwatch
Diagram:
Procedure:
Medium marble chip of limestone was added to the conical flask and the inverted measuring cylinder was filled with water. It was carefully tipped into the water bath using the 'up and over' method. 50cm3 of 2M hydrochloric acid was added to the conical flask with the limestone. The delivery tube was put under the measuring cylinder and the timer was started to measure the time taken to collect 50cm3 of carbon dioxide gas.
This procedure was repeated using two other acids; sulphuric and nitric. The acid that produced the fixed amount of carbon dioxide was the most effective acid on the decomposition of limestone and therefore was to be used again in the main investigation.
Results:
Acid
Time taken to produce 50cm3 of CO2 (seconds)
Hydrochloric 2M
317
Sulphuric 2M
No reaction
Nitric 2M
474
Conclusion:
Using the results obtained from the above table, it can be deduced that hydrochloric acid has the greatest effect on the decomposition of limestone. This is because it took the shortest time to produce 50cm3 of carbon dioxide. Therefore hydrochloric acid shall be used in the main investigation.
Variables:
Variable factor
Vary
Control
Mass of limestone
No
Yes
Temperature
No
Yes
Amount of acid
No
Yes
Concentration of acid
Yes
No
The table above shows that the concentration of acid will be an independent variable and all other factors will be controlled i.e., mass of limestone, temperature and the amount of acid.
Method:
Equipment: Conical flask
Delivery tube
Water bath
Measuring cylinder
Water
Hydrochloric acid (450cm3)
Limestone (15 medium ...
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Variable factor
Vary
Control
Mass of limestone
No
Yes
Temperature
No
Yes
Amount of acid
No
Yes
Concentration of acid
Yes
No
The table above shows that the concentration of acid will be an independent variable and all other factors will be controlled i.e., mass of limestone, temperature and the amount of acid.
Method:
Equipment: Conical flask
Delivery tube
Water bath
Measuring cylinder
Water
Hydrochloric acid (450cm3)
Limestone (15 medium sized marble chips)
Stopwatch
Diagram:
Procedure:
A medium sized marble chip was added to the conical flask and the inverted measuring cylinder was filled with water. It was carefully tipped into the water bath using the 'up and over' method. 50cm3 of hydrochloric acid was added to the conical flask with the limestone. The delivery tube was put under the measuring cylinder and the timer was started to measure the time taken to collect 50cm3 of carbon dioxide gas. This was measured by looking at when the water in the measuring cylinder decreased by 50cm3. In order to make it a fair test, all variables apart from the concentration of acid were kept the same, i.e. the limestone used were medium sized marble chips, all four experiments were carried out at room temperature and the volume of acid was kept constant at 50cm3.
This procedure was repeated using different concentrations of hydrochloric acid obtained by diluting 2M hydrochloric acid with water. The dilutions used are shown in the table below:
Volume of Hydrochloric acid (cm3)
50
40
30
20
0
Volume of Water(cm3)
0
0
20
30
40
Molar concentration (M)
2
.6
.2
0.8
0.2
To calculate different molars of hydrochloric acid, divide the amount of acid being used by the total volume of the solution, and multiply by the molar concentration of the initial acid used:
(50/50) x 2 = 2M
(40/50) x 2 = 1.6M
(30/50) x 2 = 1.2M
(20/50) x 2 = 0.8M
(10/50) x 2 = 0.2M
The reason for using five different concentrations of acid was to give a more accurate pattern of results. The range of concentrations used is 0.2M to 2M. This is sufficient since any concentrations below 0.2M will take too long to have an effect on the limestone and concentrations up to 2M will provide enough results to show the relationship between the concentration of acid and the rate of reaction. To ensure accurate and reliable results are obtained, the experiments will be repeated three times and an average of those will be taken.
The results were plotted in a table and the rate of reaction was calculated using:
Rate of reaction = 1/t
Prediction:
Using the collision theory, I predict that as the concentration of hydrochloric acid is increased, the rate of reaction between the acid and limestone will also increase. This is because increasing the concentration of acid increases the number of acid particles which can collide into the limestone so the chances of a successful collision are greater.
Obtaining evidence:
Results from preliminary experiment:
Acid
Time taken to produce 50cm3 of CO2 (seconds)
Hydrochloric 2M
317
Sulphuric 2M
No reaction
Nitric 2M
474
As mentioned earlier, it can be deduced that hydrochloric acid has the greatest effect on the decomposition of limestone. It took the shortest time to produce 50cm3 of carbon dioxide, which means that it has the greatest effect on the limestone. Therefore hydrochloric acid was used in the main investigation.
Results from main experiment:
Concentration of Hydrochloric acid (M)
2
.6
.2
0.8
0.2
Time taken to produce 50cm3 of CO2 - Test 1 (seconds)
408
568
807
990
885
Time taken to produce 50cm3 of CO2 -Repeat 2 (seconds)
453
644
743
050
2028
Time taken to produce 50cm3 of CO2 -Repeat 3 (seconds)
498
596
608
958
824
Average time taken to produce 50cm3 of CO2 (seconds)
453
603
719
999
911
Rate of reaction (1/t)
0.0022
0.0017
0.0014
0.0010
0.0005
Analysing:
Using the results I obtained and the graphs plotted, I conclude that as the concentration of hydrochloric acid is increased, the time taken to collect a fixed volume of carbon dioxide (50cm3) is reduced. This indicates that the rate of reaction increases as you increase the concentration of acid. The graph showing the average time taken to produce 50cm3 of carbon dioxide against the concentration of acid gives a negative correlation. In this case, it means that as you increase the concentration of the acid, the time taken to produce the fixed amount of carbon dioxide decreases. The graph showing the rate of reaction against the concentration of hydrochloric acid shows a positive linear relationship, i.e., as the concentration of acid is increased, the rate of reaction also increases. This supports my prediction, relating to the collision theory. The collision theory states that increasing the concentration of a reactant increases the number of particles present which can collide into other reactants so the chances of a successful collision are increased. In this case, increasing the concentration of acid increases the numbers of acid particles which can collide into the limestone so the chances of a successful collision are greater. Relating this back to the Lincoln Cathedral, if the rain contains a greater concentration of acid, then the rate of decomposition on the exterior limestone would be greater.
Evaluation:
This investigation was fairly simple and safe to carry out and provided me with accurate and reliable results. The results obtained were reliable since the results of repeated experiments were close to those of the original tests. I used the average of the three results to obtain a more accurate pattern in my results. I feel that my results were fairly accurate, but they could have been more accurate if I had used the gas and syringe method since collecting carbon dioxide in the inverted measuring cylinder was slightly difficult and inaccurate to measure when the fixed amount of carbon dioxide was collected. Despite this, I found there were no anomalous readings in my results. This could be due to the fact that the procedure was simple and straightforward to follow. I think it would have been easier to have used a clamp stand as part of the equipment, as it would have kept our hands free, making the experiment safer. The experiment was safe to conduct since I used safety goggles to protect my eyes from the acid, and the maximum concentration of acid used was 2M, which is not very corrosive.
If I were to carry out the experiment again, I would use the gas and syringe method, as although it is slightly harder to set up than the up and over method, it would give more accurate results. In addition, I would use a wider range of concentrations of acid so that any pattern that emerges will become more evident.
Meena Jassal