Unit 8 worksheets 5 Acid deposition
1. Carbon dioxide is naturally present in rain water as carbonic acid.
(a) Explain why pure rain water containing dissolved carbon dioxide cannot have a pH lower than 5.6
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(b) How much more acidic than rain water with a pH of 5.6 is a sample of acid rain with a pH of 4.6 ?
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2. (a) Nitrogen oxide, NO, is produced in car engines. Give the equations to show how it eventually
forms nitric acid, HNO3 and nitrous acid, HNO2 in the atmosphere.
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(b) Sulfur dioxide, SO2, is formed when S-containing fossils fuels are combusted. Give the equations to
show how it eventually forms sulfuric acid, H2SO4, and sulphurous acid, H2SO3, in the atmosphere.
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3. Describe and explain how acid rain affects the growth of trees.
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4. The image above shows a statue of a lion in Leeds, U.K. that has been badly affected by acid
deposition.
Give the ionic equation for the reaction of acid with the carbonate ions in the statue.
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5. Explain how adding either calcium hydroxide or calcium oxide can counter the effects of acidification
of lakes.
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Answers
1. (a) Carbonic acid is a weak acid so is only very slightly dissociated
H2CO3(aq) ⇄ H+(aq) + HCO3–(aq)
so that, even in a saturated solution, the concentration of hydrogen ions can never give a pH lower than
5.6.
(b) Ten times more acidic (as the pH has decreased by one unit)
2. (a) 2NO(g) + O2(g) → 2NO2
2NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)
(nitric acid can also be formed from: 4NO2(g) + O2(g) + 2H2O(l) → 4HNO3(aq) )
(b) SO2(g) + H2O(l) → H2SO3(aq)
2SO2(g) + O2(g) → 2SO3(g) then SO3(g) + H2O(l) → H2SO4(aq)
3. Tree growth is stunted with thinning of the tops and loss and yellowing of leaves. This is due to the
leaching of important nutrients, such as Ca2+, Mg2+ and K+ from the soil. The loss of Mg2+ causes a
reduction of chlorophyll which lowers the ability of the tree to photosynthesise. The leaching of
Al3+ from rocks into the soil affects the ability of the roots of the tree to take up sufficient water and
nutrients to survive.
4. CO32–(s) + 2H+(aq) → CO2(g) + H2O(l)
5. Calcium hydroxide and calcium oxide are both strong bases and can neutralise the acid.
Ca(OH)2(aq) + 2H+(aq) → Ca2+(aq) + 2H2O(l)
CaO(s) + 2H+(aq) → Ca2+(aq) + H2O(l)
This increases the amount of calcium ions in the lake water and also helps to precipitate aluminium ions
from the water.