The Aim of This Lab is to Show the Negative Effect Acid Rain Has on Building Materials

Normal rain has a pH of 5.6. This acidity is due to the reaction of water molecules that form rain droplets with carbon dioxide (CO2), resulting in the formation carbonic acid (H2CO3).

The reaction occurs as follows:

Carbon Dioxide + Rainwater → carbonic acid

CO2 (g) + H2O (l) → H2CO3 (aq)

This acidity does not substantially affect building material under normal circumstances since the pH is relatively high for the acid to have a major corrosive effect.

Nevertheless, rain in some cases is much more acidic, and is called acid rain. Acid rain is a form of precipitation that has an abnormally low pH (
Rain water sometimes reacts with Nitrogen oxides and Sulfur oxides in the presence of Oxygen, resulting in an even stronger acid, such as Sulfuric Acid (H2SO4) or Nitric Acid (HNO3).

The formation of acid rain can take place as depicted by the following reactions:

Sulfur Dioxide + Oxygen + Rainwater → Sulfuric Acid

2SO2 (s) + O2 (g) + 2H2O (l) → 2H2SO4 (aq)

Nitrogen Dioxide + Oxygen + Rainwater → Nitric Acid

4NO2 (s) + O2 (g) + 2H2O (l) → 4HNO3 (aq)

Like normal rain, acid rain causes the degradation of certain building materials. However, the rate of degradation of acid rain is incomparable with that of normal rain. Two examples of building material that are degraded by acid rain are limestone and marble (both of which are composed of Calcium Carbonate CaCO3). The latter is basic in nature and therefore an acid-base reaction takes place thereby neutralizing the acid in the following manners:

Sulfuric Acid + Calcium Carbonate → Calcium Sulfate + Water + Carbon Dioxide

H2SO4 (aq) + CaCO3 (s) → CaSO4 (s) + H2O (l) + CO2 (g)

OR

Nitric Acid + Calcium Carbonate → Calcium Nitrate + Water + Carbon Dioxide

2HNO3 (aq) + CaCO3 (s) → Ca(NO3)2 (s) + H2O (l) + CO2 (g)

The aim of this lab is to show the negative effect acid rain has on building materials.

Research question

Does the concentration of acid rain affect the rate of the reaction between limestone and acid rain?

Hypothesis

The more acidic rain is, the higher its corrosive ability. Therefore, the lower the pH of the rain, the more CaCO3 will react, and it will exhibit a decrease in its mass (erosion). As the reaction takes place and more H+ ions are reacting, the pH is going to increase progressively. The neutral-pH solution will not cause any corrosion. The slightly acidic rain (pH 5.6) water will cause some degradation of limestone, while Sulfuric Acid and Nitric Acid will cause the greatest amount of corrosion of limestone.

Variables:

Dependent Variables

Mass of limestone tablets at Day 2

pH of solutions at Day 2

Independent Variables

The control variables need to be constant in order to get valid and accurate results.

The independent variables are:

Initial pH of the solution in each beaker

The volume of the solutions

Temperature

Pressure

Planning b

Materials

4 Beakers (20ml)

Graduated Cylinder (25ml)

Distilled Water (neutral)

Natural rainwater (pH = 5.6)

Dilute H2SO4, sulfuric acid, solution (1%)

Dilute HNO3, nitric acid, solution (1%)

Limestone tablets

pH paper strips

pH color-range chart

Marker

Tweezers

Balance

Controlling the variables

The preparation of the solutions is one of the most crucial aspects in the experiment since it is the effect of the pH that is being studied. The solutions should not contain any impurities and after preparing them their pH should be measured for certainty.

Temperature is a factor that might affect the final results, therefore it is sensible to place all the beakers containing their respective solutions and tablets of limestone under a "normal" room temperature (between 20 and 30°C).

In order to try and simulate normal and "frequent" conditions under which rainfall occurs (i.e. no sunlight) the solutions should be placed in a location that has minimum or no light.

Upon inserting the limestone tablets into each beaker, the experimenter should make sure that no splashing of the contents of each beaker occurs, since the tweezers can become contaminated with the contents of a solution, and the tool itself could help contaminate other solutions by using the limestone tablet as a vector.

Since the purpose is to study the effect of the concentration of acid in each solution on the tablets of limestone, each piece should be completely submerged in the proposed volume of water. This prevents any kind of side reaction taking place between the tablet and the gases in the surrounding air and it gives a larger surface area of the tablet's appearance to be studied.

Procedure

1.Create a table that depicts the contents of 4 different beakers on two different days: Day 1 and Day 2. The beakers contain neutral distilled water, natural rainwater, dilute sulfuric acid, and dilute nitric acid. The table should include the initial pH of the liquid used in each corresponding beaker, and the mass of the limestone tablet to be introduced into each beaker.

2.Label the 4 beakers A, B, C and D, using a marker.

3.Using the graduated cylinder measure 50ml of neutral distilled water and place contents into beaker A.

4.Using the graduated cylinder measure 50ml of sulfuric acid at 1.26∙10-6∙mol∙dm-3 (pH » 5.6) and place contents into beaker B. This solution represents the usual pH of acid rain

5.Using the graduated cylinder measure 50ml of sulfuric acid with a concentration of 5∙10-5 mol∙dm-3 (pH»4) and place contents into beaker C.

6.Using the graduated cylinder measure 50ml of sulfuric acid at 5x10-4∙mol∙dm-3 (pH » 3) and place contents into beaker B. This solution represents the usual pH of acid rain

7.Place a strip of pH paper in each of the beakers to make sure solutions have their assigned pH. Using the pH color range chart, compare the color of the pH paper in each of the beakers to the chart.

8.Pick 4 pieces of limestone of similar size and record their weights in the table with respect to the beaker and the solution they are placed in. the tablets should be small enough to be competently submerged into the solution they are placed in. If the weight of a certain tablet is not close to the other, try chipping off pieces from the required tablet in order to obtain pieces of similar weights.

9.Place each piece in a beaker.

10.Place the beakers in an area where they are undisturbed, and leave them for 1 day

11.At Day 2, Place pH paper in each of the beakers and note the pH of each. Record the pH values in the Data table.

12.Note DpH (the change in pH) and record any changes in physical appearance of the tablets or beakers.

13.Compare the new pH values with the ones of the day before. Calculate the DpH by subtracting the new pH with the old pH. Since we are calculating a change, the answer must be positive: DpH = | pH(Day 2) - pH(Day 1) |

14.Remove the limestone tablets from each beaker, using tweezers. Wash thouroguhly with distilled water directly after removal.

15.Weigh the tablets and record the new mass in the Data table.

16.Calculate Dm, the change in mass of the tablets, by subtracting the new mass from the initial mass.