An investigation of the factors which affect the reaction of acid rain on limestone

Aim:
To investigate the factors which affect the reaction of acid rain on limestone.

Background Knowledge / Scientific Knowledge:

• Acid reacts with limestone to produce Carbon dioxide. (CO2 is the product of the reaction).
• Temperature:- As more heat is given to the molecules of the reactants, they gain kinetic energy, and move more quickly, and violently.
• Acid rain causes the erosion or weathering of limestone.
• Equation for Hydrochloric acid is: HCl
• Equation for Limestone is: Ca CO3
• The reaction between limestone and acid rain is exothermic - it gives out heat.
• Limestone is a calcium carbonate, and it is alkali.
• Greater surface area = quicker reaction, because there are more surfaces for the other reactant to work on.

The Collision Theory:
A reaction occurs when the molecules of two or more reactants collide into one another. Then the reaction takes place. After the collision and subsequent reaction has taken place, 'products' are produced. In the case of this experiment carbon dioxide is the product of the reaction between limestone and acid rain - basically hydrochloric acid and water.

The equation for this reaction is

Prediction

1. I predict that if the temperature goes up by 20°C then the rate of reaction will go up by 4 times. I believe this because the rate of reaction doubles every 10°C rise in temperature, as the molecules gain more kinetic energy, and will collide more.
2. I predict that if the concentration of reactants is stronger then the rate of reaction will occur more quickly, because it increases the amount of molecules the molecules of the other reactant, can collide with. Therefore they all collide more.
3. I believe that the molecules of the reactants would work more effectively and collide more, if the temperature was higher, meaning they would react more. I predict this because the higher the temperature, the more kinetic energy the molecules have.
4. I think that you would need twice as much acid, if the size of the piece of limestone was doubled, to make the reaction occur as quickly as the previous experiment.
5. I predict that if the concentration of the acid was stronger by 2 times as the concentration before, then the reaction between the limestone and the acid would take place in half the time taken by the previous experiment.
6. I predict that if the limestone is crushed then the reaction will occur more rapidly because there is a greater surface area, for the reaction to occur.
7. I predict that the greater the volume of the gas, the greater the reaction has taken place.

Apparatus: Hydrochloric acid, 9 test tubes, 3 water baths, Bunsen burner, Beaker, Water, Thermometer, Stop clock/watch, Tripod, Gauze, Test tube rack, 18 pieces of limestone, Conical flask, Special bungs, Syringe.

Diagram: (for Method 1):

Variables/factors:

My variables will be the following:

• temperature
• concentration of the acid

Method:

Set up 3 water baths each at different temperatures - Water bath:1 at 10°C / Water bath:2 at 20°C / Water bath:3 at 30°C.

Then place the acid "rain" (hydrochloric acid and water), into 9 separate test tubes and place 3 into each water bath, so the acid can be heated up. Make sure that each test tube of the acid is more concentrated than the one before. Each heated water bath should contain 3 test tubes, each with a different concentration of acid in them.

Then collect 9 pieces of similar sized limestone, weigh, and record the results in a table.

Then place each of the pieces into the test tubes, with the warm HCl acid. Time from this point, for 1 minute. Then remove the piece of limestone from the acid and reweigh, and record the results. - Remove the two reactants from each other by putting them through a sieve. Then wash the limestone with distiled water and dry - this is to make sure that the limestone does not continue to react with the HCl acid.

After weighing the pieces of limestone - subtract the first weight value from the last weight value to find out how much mass of the limestone has been lost, whilst the reaction took place. Then you can see how each of the variables affected the reaction.

Method 2:

Set up the apparatus as shown in diagram (below). Warm the acid up first before adding it to the experiment as shown in the diagram (below). In this experiment the result will be the amount of CO₂ which is produced.

Warm the acid by using heated water baths.

Gently release the tap so acid pours on top of the limestone. Then CO₂ gas shall rise from where the reaction is taking place, and it will go up the right hand tube into the syringe, where the volume of the gas can be measured.

Again use the time period of one minute.

Fair Test? (for Method 1):

To keep my test fair I shall...

• try to make sure that I have the right concentration of acid in each of the 9 test tubes.
• turn the Bunsen burner off, 5°C before reaching my temperature target, so I do not over shoot it. The reason why I have to do this is because after turning off the Bunsen burner, heat energy is still transferred to the water bath, from the tripod, gauze, atmosphere and the beaker.
• weigh each piece of limestone 3 times and get an average weight - just in case the scales are inaccurate.
• remove all the HCl acid from the limestone, after the allowed time, otherwise the reaction will continue, and my results will be inaccurate.
• shall ensure that my variables are how I want them to be, and when I need to change them, that they are ready.
• shall not allow the reactions to go on after their allowed times.

Fair Test? (for Method 2):

To keep my test fair I must...

• make sure that none of the gas escapes from the syringe.
• make sure that none of the acid is added to the limestone, before the timed period.
• make sure that the acid is at the correct temperature, before starting the experiment. If not, then my results will be inaccurate.
• make sure that I replace each piece of limestone before each experiment.
• make sure that all the equipment I use is in the correct, and in the most efficient state before starting the experiment.

Results for method 1:

Temperature  Mass before/g   Average/g Mass after/g    Average/g  Concentration
    /°C      1     2     3             1     2     3                of acid/M
     10   | 0.26  0.26  0.26 | 0.25 |  0     0     0   |  0   |       0.5
     10   | 0.42  0.41  0.43 | 0.42 | 0.41  0.41  0.41 | 0.41 |       1.0
     10   | 0.28  0.26  0.27 | 0.27 | 0.24  0.25  0.24 | 0.24 |       2.0
     20   | 0.37  0.42  0.41 | 0.40 | 0.37  0.38  0.36 | 0.37 |       0.5
     20   | 0.41  0.40  0.41 | 0.41 | 0.40  0.38  0.39 | 0.39 |       1.0
     20   | 0.24  0.25  0.24 | 0.24 | 0.15  0.18  0.20 | 0.18 |       2.0
     30   | 0.21  0.23  0.25 | 0.23 | 0.25  0.22  0.23 | 0.23 |       0.5
     30   | 0.25  0.26  0.28 | 0.26 | 0.17  0.16  0.18 | 0.17 |       1.0
     30   | 0.17  0.19  0.20 | 0.19 |   0     0     0  |   0  |       2.0

Table 1 - Summary of results for method 1:

Temperature  Mass lost during  Concentration
    /°C       the reaction/g     of acid/M
     10             0.25           0.5
     10             0.01           1.0
     10             0.03           2.0
     20             0.03           0.5
     20             0.02           1.0
     20             0.06           2.0
     30              0             0.5
     30             0.09           1.0
     30             0.19           2.0

Table 2 - Results for method 2:

Temperature      Volume of        Concentration
    /°C       carbon dioxide/ml     of acid/M
     10              6                0.5
     10             18                1.0
     10             13                2.0
     20             22                0.5
     20             25                1.0
     20             27                2.0
     30             47                0.5
     30             50                1.0
     30             54                2.0

Method 2 (modified):
Keep the old method 2, but add the following modification: Use a beehive rather than a syringe, because there was not enough pressure to push the syringe up, and therefore I could not record the volume of carbon dioxide.

Method 1:
Results:
The results for method 1 are given in table 1. You can see that the reaction occurred more effectively at the lower temperatures. I believe that those results are inaccurate because because I know from my other graphs that the higher the temperature the more reaction takes place. For my results in the 0.5 Molar experiment they may have come out inaccurately because on the day of doing the experiment the acid was 20 - 25°C. This meant that in order to do the 10°C experiment I had to add ice to the water bath to cool it down. It is my opinion that because of the extreme atmospheric heat in the laboratory - about 25 - 30°C at times - that the acid was reheated up after I had cooled it, with the ice. For the acid at molarity of 0.5 and at the temperature of 30°C I can offer no explanation why no mass was lost, during the reaction.

I predicted that if the temperature goes up by 20°C then the rate of reaction will go up by 4 times. I believe this because because the rate of the reaction doubles every 10°C rise in temperature, as the molecules gain more kinetic energy and collide more. My results - excluding the results in graph 1, which are totally inaccurate - prove this prediction is approximately right - within a small fraction grams.

I also predicted that if the concentration of the reactants is stronger then the rate of reaction will occur more quickly, because it increases the amount of molecules the molecules of the other reactant can collide with. Therefore they all collide more. My results also prove this by showing how much more mass is lost during the experiment with a greater concentration (molarity) of acid. The table shows an approximate doubling of the mass loss as the concentration of the acid goes from a molarity of 1 to a molarity of 2.

In method 1, you can see how inaccurate the value is for the 0.5 molarity acid, this may have been because of the temperature of the acid or the molarity of the acid was inaccurate.

Method:
As a whole I do not think some parts of the experiment were very fair. In some places the reaction continued after the allowed time as I was taking the limestone to the sink for the acid to be rinsed off. The acid left on the limestone will have continued to react. Also the temperature of the acid could have varied due to the extreme heat in the laboratory.

The molarity of the acids may not be very accurate if the molarity had been mixed, which may have occurred with so many people using the same acid source. This may have been caused by contaminated pipettes, and perhaps I should have taken more care by washing out the pipettes beforehand.

The values where the mass totally disappeared may have occurred if the limestone had been lost, for example during the rinsing stage. If I had had more time I would have repeated those particular parts of the experiment, so I could get more accurate results.

I do believe that the equipment, such as the thermometer and scales, were accurate and not the sources of my error.

Method 2:
Results:
The results for method 1 are given in table 2.

I predicted that the greater the volume of the gas, the greater the reaction has taken place. My results for method 2 prove this prediction is correct. You could see from the size of the limestone how much reaction has taken place. You can also see the graphs that the higher the temperature, the greater reaction occurred and the greater the volume of carbon dioxide gas was produced.

I predicted that if the temperature goes up by 20°C then the rate of reaction will go up by 4 times. I believe this because the rate of reaction doubles every 10°C rise in temperature, as the molecules gain more kinetic energy, and will collide more. My results prove this as you can see between 10 and 30°C.

My results in Table 2 show that with a greater concentration of acid the greater the amount Carbon dioxide is produced and therefore a greater reaction has taken place.

In method 1 the reaction doubled when the concentration of the acid doubled, but this is not happening in method 2 as the volume of Carbon dioxide has only slightly increased when the molarity of the acid has been doubled. I can offer no explanation for this.

Method:
For this experiment I had to modify the method as there was not enough pressure produced by the carbon dioxide to move the syringe, so I had to use the beehive method instead. This method worked reasonably effectively, but occasionally before starting the experiment an air bubble went into the measuring cylinder, so I had to re-dip the measuring cylinder - that is probably why there are some inaccuracies in the results, especially in the results when temperature = 10°C / Molarity of acid = 1.0.

Summary:
Method 1 is the more accurate method although it is much more complicated than method 2. There are more things to observe and ensure that they are correct.