Acid deposition

8.5 Acid deposition (1 hour)

Pause for thought

The term 'acid rain' was first coined by Robert Smith (1817-1884) a Scottish chemist, in a book entitled 'Air and Rain: The Beginnings of a Chemical Climatology' published in 1872. Although the effect of polluting gases from the industrial revolution had been noticed even earlier than that, it was not until the 1960s that it was perceived as a serious problem. In everyday parlance 'acid rain' covers fog, snow and dew as well as rain but the IB more correctly uses the term acid deposition. Technically there is wet deposition due to acidic oxides dissolving and reacting in atmospheric water and dry deposition which covers acidic gases and particles but for this sub-topic the IB does not distinguish between these two categories of acid deposition.

Nature of science

Acid deposition is a good example of the risks and problems facing chemistry. Acid deposition, caused from the reactions of non-metal oxides with water, can be discussed from different perspectives. Through understanding the underlying theory, chemists are able to reduce the environmental impact of human activities.

Learning outcomes

After studying this topic students should be able to:

Understand:

Rain contains dissolved carbon dioxide so is naturally acidic with a pH of 5.6. The pH of acid deposition is below 5.6 (usually it is actually below 5.0).
Acid deposition occurs when oxides of nitrogen or sulfur dissolve in water to form nitric acid, HNO₃, nitrous acid, HNO₂, sulfuric acid, H₂SO₄ and sulfurous acid, H₂SO₃.
The sources of the oxides of sulfur and nitrogen together with the effects of acid deposition.

Apply their knowledge to:

Balance equations describing the combustion of sulfur and nitrogen to form their oxides and the subsequent reactions with water to form H₂SO₃, H₂SO₄, HNO₂ and HNO₃.
Distinguish between pre-combustion and post-combustion methods to reduce emissions of sulfur oxides.
Deduce the equations for the reactions of the acids in acid deposition with reactive metals and carbonates.

Clarification notes

There are no clarification notes given for this topic as the acids concerned have already been specified in the opposite column.

International-mindedness

Often the country causing the pollution is different to the country receiving the pollution. Acid deposition is a secondary pollutant that affects regions that may be far from the primary source. International cooperation. is required to solve this problem.

Teaching tips

In many ways this is quite an easy topic to teach at this level and one which you could get students to make presentations to give to the rest of the class (see the third reference below in 'other resources').

Stress the main causes of acid 'rain' (I still call it that!) and the fact that carbonic acid is a weak acid so acid rain must have a pH of less than 5.6. Students will need all the equations for the formation of the oxides of sulfur and nitrogen and their conversion to the four acids, H₂SO₃, H₂SO₄, HNO₃ and HNO₂.

For the effects of acid deposition talk about leaching out important nutrients from the soil, and the leaching of aluminium from rocks into the soil. The effect on aquatic life as the pH becomes lower, the increased risk of respiratory illnesses in humans and the destruction of limestone based buildings and statues with the relevant equations. Much of this information together with examples is given in the page on Topics 8 & 18 in the section on Incorporating International-mindedness, Theory of Knowledge and 'Utilization'.

The methods of reducing the sources of SO_x and NO_x include the use of catalytic converters, improved efficiency of engine design, 'scrubbing' and fluidized bed combustion etc. etc. You should also emphasise the need to reduce our dependence upon fossil fuels and the post-combustion practice of treating acid lakes with lime to increase the pH.

Study guide

Page 62

Questions

For ten 'quiz' multiple choice questions with the answers explained see MC test: Acid deposition.

For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Acid deposition questions.