Everyday acid and base reactions

Calcium carbonate and rocks.Limestone is also largely composed of calcium carbonate.

Bath Stone (Greater Oolite) is a limestone that was laid down in the Jurassic period, c180 – 140 million years ago.

It is composed of many tiny ooliths, egg stones, formed by chemical precipitation from warm tropical seas coating small fragments of shell in layers of lime

Chalk

Chalk is almost pure calcium carbonate, made up of the microscopic shells of plankton.

Reaction of calcium carbonate with acid.• Calcium carbonate reacts with acid to form a

salt, carbon dioxide and water.• CaCO3 + 2HCl → CaCl2 + CO2 + H2O• The effervescence produced is a standard

test for a carbonate.

Effect of rain water on limestone.• Rain water is weakly acidic, cpH 5.6,

due to carbon dioxide dissolving in it to form a weak acid, carbonic acid.

• CO2 + H2O → H2CO3• Carbonic acid can react with

limestone.• CaCO3 + 2H2CO3

↓• Ca(HCO3)2 + CO2 + H2O

Weathering

Eroding “clints and grykes” in rock pavements.

Underground acidic waters hollow out

caves.

Stalagtites and stalagmites.Limestone is permeable, mineral-rich water percolates through the rock.On evaporation calcium bicarbonate breaks down and calcium carbonate is deposited.

Ca(HCO3)2 → CaCO3+H2O + CO2

Forming stalagtites and stalagmites.

Acid Rain

Oxides of sulphur and nitrogen dissolve to produce acid rain, which increases the weathering of limestone.

• Eg; SO2 +½O2 → SO3

• SO3 + H2O → H2SO4

• Sulphuric acid will then react with limestone;

• H2SO4 + CaCO3

↓ • CaSO4 + H2O +CO2

• Calcium sulphate is a softer material, with less compact crystals.

• Hence surfaces are eroded.

Reactions of calcium carbonate.

Calcium carbonate decomposes when heated…

… forming calcium oxide.

CaCO3 → CaO + CO2

Manufacture of lime

Limestone was traditionally burnt in small lime kilns.

Reactions of calcium oxide (“quick lime”)

Calcium hydroxide reacts vigorously with water to give

calcium hydroxide.

CaO + H2O →Ca(OH)2The resulting solution has a

pH of 9 - 11

• During the Black Death corpses were buried with quicklime.

• By reacting with moisture from the bodies it prevent the spread of the disease.

Reactions of calcium hydroxide

Ca(OH)2 neutralises acids.

Ca(OH)2 + 2HCl→ CaCl2 + 2H2O

It is commonly known as “slaked lime” and is used by farmers to lower soil acidity. It is also used in some anti acids.

Reaction of limewater with carbon dioxide.

When carbon dioxide is passed through limewater

it turns milky due to the formation of

insoluble calcium carbonate.

Ca(OH)2 + CO2→ CaCO3 + H2O

This is the standard test for carbon dioxide.

Expired air; 0.4 % CO2

Inspired air: 0.004% CO2

Reaction of limewaterIf excess carbon dioxide is passed through lime water the carbonate will disappear…

… forming a colourless solution of bicarbonate.

CaCO3 + H2O + CO2 → Ca(HCO3)2

Hard waterIn limestone regions water contains relatively high amounts of calcium (and magnesium) bicarbonate.

Calcium ions react with detergents to make a scum rather than a lather.

Waters high in bicarbonates are therefore said to be hard.

Calcium bicarbonate is unstable, decomposing on heating to produce insoluble calcium carbonate.

Ca(HCO3)2 → CaCO3+H2O + CO2

Depositing lime scale on the elements of kettles.

Reactions of magnesium hydroxide

Mg(OH)2 neutralises acids.

Mg(OH)2 + 2HCl→ MgCl2 + 2H2O

Treating indigestion.

HCl is secreted in the stomach to aid

digestion.

Excess acid causes

indigestion.

“Milk of Magnesia” contains

magnesium hydroxide so neutralises

excess acid.