Acid Rain is rain with a pH of 4.2 or lower which indicates its acidity. It is believed that the main contributors to the acidic properties of this rain are oxides of sulphur and oxides of nitrogen.
Oxides of Nitrogen are found in the atmosphere and are produced due to the following reasons:
- High temperature combustion in automobiles
- Decomposition of Nitrogen containing bacteria
- Lightning
- Industrial processes such as Nitric Acid manufacture
- Agricultural Burning and Forest Fires
These above methods produce Nitrogen Dioxide (NO2) which is an Oxide of Nitrogen. This NO2 in the absence of Oxygen react with the water vapour present in the atmosphere to produce Nitrous and Nitric Acid.
2NO2 + H2O → HNO2 + HNO3
However, Nitrogen Dioxide can directly be converted to HNO3 directly in the presence of oxygen and moisture.
4NO2 + 2H20 + O2 → 4HNO3
Limestone has long been preferred as building material for constructing durable buildings and monuments. Limestone consists of CaCO3 (Calcium Carbonate). Even though they are highly durable they are being eroded by acid rain. The Nitric Acid in the acid rain reacts with this Calcium Carbonate which results in formation of a soluble solution which washes away with the rain leading to erosion.
CaCO3(s) +2HNO3 (g) → Ca (NO3)2(s) +CO2 (g) +H2O (g)
Hypothesis:
Nitric Acid is highly acidic and reacts with alkalis. Due to its high acidic properties, it is highly corrosive and leads to corrosion of various building materials. In the experiment, three different concentrations of Nitric Acid are taken. One is dilute Nitric Acid, one is moderately concentrated Nitric Acid and the third is doubly as concentrated as the second Nitric Acid.
If the concentration of the acids is stronger, 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 there is more collision, speeding up the reaction.
If the concentration of the Nitric Acid is doubled (as it is done in the third case) then the rate of the reaction as compared to the previous reaction will be halved. Thus one can predict that the increasing concentration of Nitric Acid will lead to an increase in the rate of the reaction.
Variables
Independent: Concentrations of Nitric Acid.
Dependant: Time taken to completely dissolve/consume limestone.
Controlled:
- Mass of Limestone taken
- Volume of Nitric Acid
- Room Temperature and Pressure
Method
- Take 3 equally sized test tubes
-
Pour 0.025dm3 of Nitric acids of different concentration in each test tube respectively.
- Make sure that each tube of the acid is more concentrated than the previous one
- Take 3 pieces of limestone, all of the same mass and weigh them
- Record the weight of the limestone.
- Place each piece into one of the three test tubes.
- Take three timers (one for each test tube) and start them all at the same time
- Observe until entire piece of limestone has been eroded away.
- Stop the individual timers every time an entire piece of limestone has been eroded away.
- Check the time from start to stop time and record the time taken for complete erosion of limestone in all three test tubes.
Precautions
The mass of the limestone must be exactly the same
The concentrations of Nitric Acid must range from lowest to highest.
Apparatus
- 3 Test tubes of equal sizes
-
0.025 dm3 of dilute Nitric Acid (2M)
-
0.025dm3 of moderately concentrated Nitric Acid (4M)
-
0.025dm3 of concentrated Nitric Acid (6M)
- 3 pieces of limestone, all of the same mass of 5 grams
- Electrical Balance (error of ± 0.95g)
- 3 digital timers
Data Table
