Page 73 - Chemistry ICSE Class X

Study of Acids, Bases and Salts 61

Preparation of acids by the oxidation of nonmetals
Certain nonmetals, e.g., sulphur and phosphorus, when oxidised with conc.
nitric acid (HNO ), give sulphuric acid and phosphoric acid, respectively.
3
For example,
S(s) + 6HNO (aq) o H SO (aq) + 2H O(l) + 6NO (g)
3
2
2
2
4
sulphur nitric acid (conc.) sulphuric acid nitrogen dioxide
P(s) + 5HNO (aq) o H PO (aq) + H O(l) + 5NO (g)
4
3
2
2
3
phosphorus nitric acid (conc.) phosphoric acid nitrogen dioxide
General Properties of Acids
Some typical properties of acids are given below:
ƒ Taste. Acids and their solutions have sour taste.
ƒ Corrosive nature. Concentrated mineral acids, such as sulphuric acid
and nitric acid, attack human tissues, cloth, paper and metals. The
reaction in these cases is quite fast.
The naturally-occurring acids attack metals slowly producing toxic
compounds. It is because of this that citrus fruits, curd etc., are never
stored in the utensils made of copper, lead and zinc.
Citrus fruits
ƒ Ionisation/dissociation in solution. All acids in aqueous solution
+
(solution in water) release free hydronium ion (H O ).
3
Strong acids are completely dissociated in solutions.
HCl + H O o H O + + Cl –
3
2
hydrochloric acid water hydronium ion chloride ion Acid rain ŝƐ ĐĂƵƐĞĚ ďLJ ƚŚĞ ĚŝƐƐŽůƵƟŽŶ
2–
H SO 4 + 2H O o 2H O + + SO ŽĨ ŽdžŝĚĞƐ ŽĨ ƐƵůƉŚƵƌ ;^K ĂŶĚ ^K Ϳ
4
3
2
2
3
2
sulphuric acid water hydronium ion sulphate ion ĂŶĚ ŽdžŝĚĞƐ ŽĨ ŶŝƚƌŽŐĞŶ ;ŵĂŝŶůLJ EK Ϳ
2
ŝŶ ĂƚŵŽƐƉŚĞƌŝĐ ŵŽŝƐƚƵƌĞ Žƌ ƌĂŝŶ
Weak acids in solution dissociate to very small extent. ǁĂƚĞƌ͘
CH COOH + H O U H O + + CH COO – SO ;gͿ н , K;lͿ o , SO ;aqͿ
3
3
2
3
2
3
2
2
acetic acid water hydronium ion acetate ion SO ;gͿ н , K;lͿ o , SO ;aqͿ
4
2
3
2
H CO 3 + H O U H O + + HCO – ϮEK ;gͿ н , K;lͿ o ,EK ;aqͿ н
3
3
2
2
carbonic acid water hydronium ion bicarbonate ion 2 2 2 ,EK ;aqͿ
ƒ Action on indicators. Acids and their solutions turn 3
Š Blue litmus to red
Š Yellow methyl orange to red
Acids and their solutions have no effect on the colour of
phenolphthalein.
ƒ Reaction with metals. Acids react with active metals (the metals which
are above hydrogen in activity series) to produce the corresponding
salts and hydrogen gas is given out. Ŷ ŝŶĚŝĐĂƚŽƌ ŝƐ Ă ƐƵďƐƚĂŶĐĞ ǁŚŝĐŚ
Dilute sulphuric and hydrochloric acids react with metals like ŐŝǀĞƐ ĚŝīĞƌĞŶƚ ĐŽůŽƵƌ ŝŶ ƚŚĞ ĂĐŝĚŝĐ
magnesium and zinc to give the corresponding salt and hydrogen gas. ĂŶĚ ďĂƐŝĐ ŵĞĚŝĂ͘ >ŝƚŵƵƐ͕ ŵĞƚŚLJů
ŽƌĂŶŐĞ ĂŶĚ ƉŚĞŶŽůƉŚƚŚĂůĞŝŶ ĂƌĞ
Mg(s) + H SO (dil.) o MgSO (aq) + H (g) ƐŽŵĞ ĐŽŵŵŽŶůLJ ƵƐĞĚ ŝŶĚŝĐĂƚŽƌƐ͘
4
4
2
2
magnesium sulphuric acid magnesium sulphate hydrogen
Zn(s) + H SO (dil.) o ZnSO (aq) + H (g) /Ĩ Ă ƐŽůƵƟŽŶ ĐŚĂŶŐĞƐ ƚŚĞ ĐŽůŽƵƌ ŽĨ
4
2
2
4
zinc sulphuric acid zinc sulphate hydrogen blue ůŝƚŵƵƐ ƚŽ red͕ ŝƚ ŝƐ ĂŶ acid Žƌ ŝƚƐ
ƐŽůƵƟŽŶ͘
Mg(s) + 2HCl (dil.) o MgCl (aq) + H (g)
2
2
magnesium hydrochloric acid magnesium chloride hydrogen
dŚĞ ĂĐŝĚƐ ĨŽƵŶĚ ŝŶ ĨŽŽĚ ŝƚĞŵƐ ƌĞĂĐƚ
Zn(s) + 2HCl (dil.) o ZnCl (aq) + H (g) ǀĞƌLJ ƐůŽǁůLJ ǁŝƚŚ ŵĞƚĂůƐ͘
2
2
zinc hydrochloric acid zinc chloride hydrogen