8.1 Ions in aqueous solution  (Page 4/4)

Acid rain

Acid rain refers to the deposition of acidic components in rain, snow and dew. Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the soil and affects the chemical balance of lakes and streams.

1. Carbon dioxide Carbon dioxide reacts with water in the atmosphere to form carbonic acid (H ₂ CO ₃ ).
   CO ₂ + H ₂ O $\to$ H ₂ CO ₃
   The carbonic acid dissociates to form hydrogen and hydrogen carbonate ions. It is the presence of hydrogen ions that lowers the pH of the solution making the rain acidic.
   H ₂ CO ₃ $\to$ H ⁺ + HCO ₃ ^$-$
2. Nitric oxide Nitric oxide (NO) also contributes to the natural acidity of rainwater and is formed during lightning storms when nitrogen and oxygen react. In air, NO is oxidised to form nitrogen dioxide (NO ₂ ). It is the nitrogen dioxide which then reacts with water in the atmosphere to form nitric acid (HNO ₃ ).
   3NO ₂ (g) + H ₂ O $\to$ 2HNO ₃ (aq) + NO(g)
   The nitric acid dissociates in water to produce hydrogen ions and nitrate ions. This again lowers the pH of the solution making it acidic.
   HNO ₃ $\to$ H ⁺ + NO ₃ ^$-$
3. Sulphur dioxide Sulphur dioxide in the atmosphere first reacts with oxygen to form sulphur trioxide, before reacting with water to form sulphuric acid .
   2SO ₂ + O ₂ $\to$ 2SO ₃
   SO ₃ + H ₂ O $\to$ H ₂ SO ₄
   Sulphuric acid dissociates in a similar way to the previous reactions.
   H ₂ SO ₄ $\to$ HSO ₄ ^$-$ + H ⁺

Although these reactions do take place naturally, human activities can greatly increase the concentration of these gases in the atmosphere, so that rain becomes far more acidic than it would otherwise be. The burning of fossil fuels in industries, vehicles etc is one of the biggest culprits. If the acidity of the rain drops to below 5, it is referred to as acid rain .

Acid rain can have a very damaging effect on the environment. In rivers, dams and lakes, increased acidity can mean that some species of animals and plants will not survive. Acid rain can also degrade soil minerals, producing metal ions that are washed into water systems. Some of these ions may be toxic e.g. Al ³⁺ . From an economic perspective, altered soil pH can drastically affect agricultural productivity.

Acid rain can also affect buildings and monuments, many of which are made from marble and limestone. A chemical reaction takes place between CaCO ₃ (limestone) and sulphuric acid to produce aqueous ions which can be easily washed away. The same reaction can occur in the lithosphere where limestone rocks are present e.g. limestone caves can be eroded by acidic rainwater.

H ₂ SO ₄ + CaCO ₃ $\to$ CaSO ₄ $.$ H ₂ O + CO ₂

Investigation : acid rain

You are going to test the effect of 'acid rain' on a number of substances.

Materials needed:

samples of chalk, marble, zinc, iron, lead, dilute sulphuric acid, test tubes, beaker, glass dropper

Method:

1. Place a small sample of each of the following substances in a separate test tube: chalk, marble, zinc, iron and lead
2. To each test tube, add a few drops of dilute sulphuric acid.
3. Observe what happens and record your results.

Discussion questions:

• In which of the test tubes did reactions take place? What happened to the sample substances?
• What do your results tell you about the effect that acid rain could have on each of the following: buildings, soils, rocks and geology, water ecosystems?
• What precautions could be taken to reduce the potential impact of acid rain?