Question

Explain why the following are done, both of which are related to acid-base chemistry. a. Power plants burning coal with high sulfur content use scrubbers to help eliminate sulfur emissions. b. A gardener mixes lime (CaO) into the soil of his garden.

Short answer

In acid-base chemistry, acids donate protons and bases accept them. a) Power plants burning high-sulfur coal release SO2 gas, which contributes to air pollution and acid rain. Scrubbers use limestone to neutralize acidic SO2 gas through the reaction: SO2 (g) + CaCO3 (s) → CaSO4 (s) + CO2 (g). b) Gardeners mix lime (CaO) into soil to neutralize soil acidity and improve plant growth. The main reaction involved is: CaO + H2O → Ca(OH)2; Ca(OH)2 + 2H+ → Ca2+ + 2H2O. Lime reacts with water, forming Ca(OH)2, which then reacts with excess H+ ions in the soil to produce calcium ions and water, effectively neutralizing soil acidity and raising the soil pH.

Step-by-step solution

Review of Acid-Base Chemistry

Acid-base chemistry involves the study of the reactions of acidic and basic substances. In general, acids are substances that can donate a proton (H+) while bases are substances that can accept a proton. When an acid reacts with a base, a neutralization reaction occurs, resulting in the formation of a salt and water.

Sulfur Content in Coal

Coal contains varying amounts of sulfur, which, when burned in power plants, reacts with oxygen to form sulfur dioxide (SO2) gas. This gas is a major contributor to air pollution, and when it combines with water vapor in the air, it forms sulfuric acid (H2SO4), which is one of the main components of acid rain. Acid rain is harmful to the environment and can damage buildings, natural habitats, and human health.

Scrubbers in Power Plants

To reduce sulfur emissions, power plants use scrubbers to remove SO2 from the exhaust gases. These scrubbers work by spraying a slurry of limestone (CaCO3) or other basic materials into the exhaust stream. The chemical reaction that occurs is: SO2 (g) + CaCO3 (s) → CaSO4 (s) + CO2 (g) Sulfur dioxide reacts with calcium carbonate to form calcium sulfate (a solid), which can be easily removed, and carbon dioxide, which is released into the atmosphere. This process helps to neutralize the acidic SO2 gas, thus preventing it from forming acid rain.

Lime in Gardening

Soil acidity can negatively affect plant growth, as it influences nutrient availability and can be toxic to plants. Typically, soils with a pH below 6.0 are considered acidic, and gardeners will need to add a basic substance to neutralize the excess acidity and increase the soil pH to a more suitable level for plant growth. Lime (CaO), also known as quicklime, is commonly used for this purpose.

Lime Neutralizing Soil Acidity

When lime is mixed into the soil, it reacts with the acidic components, such as H+ ions and acidic compounds like aluminum sulfate, to form neutral or weakly basic products. The primary reaction involved is: CaO + H2O → Ca(OH)2 Ca(OH)2 + 2H+ → Ca2+ + 2H2O Calcium oxide reacts with water, forming calcium hydroxide (Ca(OH)2), which then reacts with the excess H+ ions in the soil to produce calcium ions and water, effectively neutralizing the soil acidity. By raising the soil pH, lime improves the overall soil conditions and promotes better plant growth.