L6 - Acid Deposition

objectives

Understandings

• Rain is naturally acidic because of dissolved CO2 and has a pH of 5.6. Acid deposition has a pH below 5.6.

• Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3.

• Sources of the oxides of sulfur and nitrogen and the effects of acid deposition should be covered.

Applications and skills

• Balancing the equations that describe the combustion of sulfur and nitrogen to their oxides and the subsequent formation of H2SO3, H2SO4, HNO2 and HNO3.

• Distinction between the pre-combustion and post-combustion methods of reducing sulfur oxides emissions.

• Deduction of acid deposition equations for acid deposition with reactive metals and carbonates.

Why is Rain Acidic?

The atmosphere contains the soluble gas carbon dioxide, which dissolves in the rain giving a solution with pH 5.6.

CO₂ + H₂O ⇋ H₂CO₃

The carbonic acid formed is weakly acidic by dissociation:

H₂CO₃ ⇋ HCO₃^- + H⁺

The term "acid depostion" refers to precipitation, rain, hail, snow etc. with a pH lower than 5.6. It is caused by dissolved gases in the water making up the precipitation. These gases react with water forming acids.

Acidic Pollution

The English chemist Robert Angus Smith is generally credited with coining the phrase 'acid rain' in 1872 to describe the increased acidity of the rain in British industrial centers (such as Manchester), which was apparently caused by the unbridled excesses of the early Industrial Revolution.

Typical pH values for rain in the continental United States now range from 4 to 4.5, with values as low as 2.0 reported for areas such as Los Angeles. Even “normal” rain is now as acidic as tomato juice or black coffee.These are mostly oxides of sulfur and nitrogen that are present in the air due to natural or anthropogenic (human) activity. All of these non-metal oxides dissolve in water forming acids.

The nitrogen oxides are described by the general formula NOx.

The sulfur oxides are either sulfur(IV) oxide or sulfur(VI) oxide.

Nitrogen oxides

Natural Sources of Nitrogen Oxides (NO_x)

Nitrogen is present in the air at a percentage of about 80%. Nitrogen is a very stable gas and reacts only with difficulty. However, the temperatures attained during electrical discharges cause nitrogen to combine with the oxygen in the air producing nitrogen monoxide, NO.

N₂ + O₂ → 2NO

Nitrogen(II) oxide (nitrogen monoxide) is a very reactive gas that combines directly with oxygen in the air forming nitrogen(IV) oxide (nitrogen dioxide):

2NO + O₂ → 2NO₂

Nitrogen dioxide is a red brown gas which dissolves easily in water forming an acidic solution:

2NO₂ + H₂O → HNO₂ + HNO₃

This is a disproportionation reaction, with the nitrogen in oxidation state +4 being simultaneously oxidised to nitrogen(V) and reduced to nitrogen(III).

The nitric(III) acid formed in the equation above may also react further with oxygen in the air to produce more nitric(V) acid.

2HNO₂ + O₂ → 2HNO₃

Ultimately then, most of the nitrogen oxides in the atmosphere end up as nitric acid, a very strong acid.

Human Activity Creating NO_x

The exact same chemical reaction as above creates nitric acid but this time the source of the high temperatures in the exhausts of motor car engines.

Effects of NO_x

The nitric acid produced in the oxidation of nitrogen react with building materials such as dolomite or limestone.

Nitrogen oxides also produce acids when inhaled and cause bronchitis and pneumonia.

Nitric acid is an oxidising acid and can react with unreactive metals such as copper and lead:

8HNO₃ + 3Cu --> 3Cu(NO₃)₂ + 2NO + 4H₂O

Notice that one of the products is nitrogen monoxide, NO, which then goes on to react with oxygen in the air as outlined above.

Nitric acid is a strong acid that attacks carbonate based rocks such as dolomite (magnesium carbonate) and limestone (calcium carbonate):

2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂

Sulphur Dioxide

Sulphur Dioxide - Natural Sources

Sulphur Dioxide - Human Activity

Fossil Fuels can contain small Sulphur impurities. These impurities can combust in a combustion engine to make the acidic gas Sulphur Dioxide:

• S + O₂ → SO₂

Effects of Sulphur Dioxide

Sulfur dioxide dissolves readily in water forming sulfuric(IV) acid, which is then oxidised in the presence of oxygen (in the air) to sulfuric acid:

2SO₂ + H₂O → H₂SO₃

2H₂SO₃ + O₂ → 2H₂SO₄

Sulfuric acid is a strong acid which attacks buildings and metals such as iron. If it enters the living environment it decreases the pH levels and kills plants and aquatic animals.

H₂SO₄ + Fe → FeSO₄ + H₂

Sulfuric acid also attacks the naturally occurring carbonate ions in rocks such as dolomite (magnesium carbonate) and limestone and marble (calcium carbonate).

H₂SO₄ + MgCO₃ → MgSO₄ + H₂O + CO₂

Effects of Acid Deposition

• Acid deposition can react with limestone/marble based buildings, causing erosion, corrosion and decay.
• A pH of 4 or lower is fatal for virtually all fish, most invertebrate animals, and many microorganisms. As a result of acid rain, the pH of some lakes in Europe and the United States has dropped below 4.
• Trees and many other plants are sensitive to the presence of aluminum and other metals in groundwater. Under normal circumstances, aluminum hydroxide [Al(OH)₃], which is present in some soils, is insoluble. At lower pH values, however, Al(OH)₃ dissolves via the following reaction:

Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O

• The result is increased levels of Al³⁺ ions in groundwater.
• Al³⁺ ions are toxic to plants and high concentrations can affect plant growth. Acid rain can also weaken the leaves and roots of plants so much that the plants are unable to withstand other stresses.
• The combination of the two effects can cause significant damage to established forests, such as the Black Forest in Germany and the forests of the northeastern United States and Canada and other countries.

reducing atmospheric pollutants

Nitrogen Oxides

Vehicle Exhausts now have to be fitted with a catalytic converter.

Catalytic converters are honeycomb structures that contain Rhodium and Palladium.

The Rhodium and Palladium Catalyses the following reactions as the Effuse leaves the exhaust:

• CO is converted to CO₂
• NOx are converted to N₂

This process is characterised by the following equation:

2NO(g) + 2CO(g) —> N₂(g) + 2CO₂(g)

• Unburnt hydrocarbons converted to CO₂ and H₂O - as an example:

C₈H₁₈(g) + 12½O₂(g) —> 8CO₂(g) + 9H₂O(l)

Sulphur Dioxide - Pre-Combustion Method

This is usually carried out using hydrogen gas during the refining process (hydrodesufurisation or hydrotreatment).

The hydrogen gas reacts with the sulfur in the crude oil producing hydrogen sulfide gas that can be separated and lead to sulfur recovery units where it is oxidised to sulfur. This produces vast amounts of sulfur that can be used in the rubber industry or simply stored.

Sulphur Dioxide - Post Combustion Method

This method is called Flue gas Desulphurisation outside of the IB

The gases formed by combustion of fossil fuels are passed through alkaline 'scrubbers' that remove any acidic gases. These scrubbers contain calcium oxide and water spray that combines with the sulfur dioxide making calcium sulfite dihydrate, CaSO₃.2H₂O

CaO + SO₂ → CaSO₃

Removing SO₂ from the gases prevents its conversion to acid rain. Scrubbing systems are now commonly used to minimize the environmental effects of large-scale fossil fuel combustion.