Chemistry_Grade_10-12 (1).pdf

3 the concentrations of hydronium and hydroxyl ions

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3. The concentrations of hydronium and hydroxyl ions in a typical sample of seawater are 10 8 mol.dm 3 and 10 6 mol.dm 3 respectively. (a) Is the seawater acidic or basic? (b) What is the pH of the seawater? (c) Give a possible explanation for the pH of the seawater. (IEB Paper 2, 2002) 20.5.4 Acid rain The acidity of rainwater comes from the natural presence of three substances (CO 2 , NO, and SO 2 ) in the lowest layer of the atmosphere. These gases are able to dissolve in water and therefore make rain more acidic than it would otherwise be. Of these gases, carbon dioxide (CO 2 ) has the highest concentration and therefore contributes the most to the natural acidity of rainwater. We will look at each of these gases in turn. Definition: Acid rain Acid rain refers to the deposition of acidic components in rain, snow and dew. Acid rain occurs when sulfur 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. 384
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CHAPTER 20. THE HYDROSPHERE - GRADE 10 20.5 1. Carbon dioxide Carbon dioxide reacts with water in the atmosphere to form carbonic acid (H 2 CO 3 ). CO 2 + H 2 O 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 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 ). 3 NO 2 ( g ) + H 2 O 2 HNO 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 H + + NO 3 3. Sulfur dioxide Sulfur dioxide in the atmosphere first reacts with oxygen to form sulfur trioxide, before reacting with water to form sulfuric acid . 2 SO 2 + O 2 2 SO 3 SO 3 + H 2 O H 2 SO 4 Sulfuric acid dissociates in a similar way to the previous reactions. H 2 SO 4 HSO 4 + H + Although these reactions do take place naturally, human activities can greatly increase the con- centration 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 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, in- creased 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
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