(a) Explain the term chemical weathering of rocks. [2 marks] (b) Discuss four processes of
(a) Explain the term chemical weathering of rocks. [2 marks]
(b) Discuss four processes of chemical weathering of rocks [10 marks]
(c) State four ways in which soil temperature is important for crop growth (4 marks)
Explanation
(a) Explanation of chemical weathering of rocks:
Chemical weathering is the decomposition of rocks by chemical agents formed through the reaction of water with atmospheric gases such as air (oxygen and carbon dioxide) As some minerals in the rocks are dissolved and others change into new chemical products therefore disintegration of rocks occurs.
(b)Discussion of chemical weathering of rocks (Any four)
Hydration
Occurs when water combines with or binds to some minerals Intact water may bind to silicates, oxides of iron
Hydrated compounds are soft and easily fragmented It does not usually affect the chemical composition of the hydrated product
Example of hydration reaction are
Fe203 + 3H20 → . Fe203.3H20
Haematite Limonite/Hydrated Haematite (Yellow)
(Red)
OR
CaSO4 + 2H20 → CaSO4 .2H20
(Calcium Sulphate) (Gypsum)
OR CuS04 + 5H20 → CuS04. 5H20
(Anhydrous Copper Sulphate)Carbonation (Hydrated Copper II sulphate )/Blue stone/
Carbonation
This is brought about by the combination of carbonate (C02-3) or bicarbonte (HCO-3) ions with rock minerals
The carbon dioxide released to the atmosphere from various sources combines with water/rainwater to form carbonic acid
C02 + H20 →← H2C03
The carbonic acid attacks the minerals and C02 in the rock.
In limestone or marbles the calcite present is dissolved.
CaCO3 + H2CO3 → Ca(HCO3)2 →
Oxidation/Reduction
Atmospheric oxygen and free oxygen in rain water carry out this reaction
Minerals containing iron (Ferrous form), manganese and sulphur are the ones most frequently affected by this reaction
When they are exposed to air and water, the ferrous ion is oxidized to the ferric Fe2 + state.
The change in valency from Fe ++ to Fe + destabilizes the molecule and leads to disintegration
4Fe C03 + 02→ 2Fe203 + 4C02
Iron (II) Oxygen Iron Oxide Carbon dioxide
carbonate
Hydrolysis
A decomposition reaction in which silicate minerals are broken down Water molecule is split into H+ and OR ions
H20 + H+ → HO-
The H+ replaces the cation from the mineral structure while the mineral is released for plant uptake
Example:
CaSi03 + 2H20 → H2Si03 + Ca(OH)2
Calcium silicate water