Page 267 - Chemistry ICSE Class X

Study of Compounds–Sulphuric Acid 253
Š It oxidises metals to the corresponding sulphates.
heat
Cu(s) + 2H SO (l) o CuSO + 2H O(l) + SO (g)
2
2
2
4
4
copper conc.
Š It oxidises certain salts to the corresponding nonmetal. For example,
y With hydrogen bromide, conc. H SO gives bromine.
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2
2HBr + H SO o Br (g) + 2H O(l) + SO (g)
2
2
4
2
2
(conc.) bromine
(reddish brown fumes)
y With hydrogen sulphide, conc. sulphuric acid gives sulphur.
H S(g) + H SO o S(s) + 2H O(l) + SO (g)
4
2
2
2
2
hydrogen sulphide sulphur
How is dilute sulphuric acid prepared
Dilute sulphuric acid is prepared by slowly adding concentrated acid to
water. While preparing dilute sulphuric acid, the concentrated acid is dŚĞ ĞůĞĐƚƌŽůLJƚĞ ƵƐĞĚ ŝŶ ƚŚĞ ůĞĂĚ
slowly added to water and not water to conc. sulphuric acid. This is because ƐƚŽƌĂŐĞ ĐĞůů ;Žƌ LJŽƵƌ ĐĂƌ ďĂƩĞƌLJͿ ŝƐ
ĚŝůƵƚĞ ƐƵůƉŚƵƌŝĐ ĂĐŝĚ ;ϰϬй ƐƵůƉŚƵƌŝĐ
when water and conc. H SO are mixed, a large amount of heat is liberated. ĂĐŝĚͿ͘ dŚŝƐ ĂĐŝĚ ƐŽůƵƟŽŶ ŝƐ ĐŽŵŵŽŶůLJ
4
2
The heat produced is so large that the diluted acid starts almost boiling very ĐĂůůĞĚ ďĂƩĞƌLJ ĂĐŝĚ͘
violently. If this hot acid falls on our body, it would cause severe burns on
it.
What are the properties of dilute sulphuric acid
Commonly used dilute sulphuric acid is about 10% H SO and 90% water.
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4
However, dilute sulphuric acid solutions of different concentrations can
also be prepared. Dilute sulphuric acid is written as H SO (aq).
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2
Some common reactions of dilute sulphuric acid are given below:
ƒ Action on litmus. Sulphuric acid turns blue litmus red.
ƒ Acidic character. Sulphuric acid is a strong dibasic acid. In aqueous
solutions, it dissociates in two steps as follows:
–
+
H SO 4 + H O(l) o H (aq) + HSO (aq)
2
4
2
sulphuric acid bisulphate ion
2–
+
HSO 4 – + H O(l) U H (aq) + SO (aq)
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4
bisulphate ion sulphate ion
Therefore, sulphuric acid forms two sets of salts — normal sulphates
(such as sodium sulphate, Na SO ) and acid sulphates (such as sodium
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2
hydrogensulphate or sodium bisulphate, NaHSO ).
4
ƒ 4GCEVKQP YKVJ CNMCNKU Sulphuric acid reacts with alkalis to form
hydrogensulphates and sulphates. With NaOH(aq), the reactions are
NaOH(aq) + H SO (aq) o NaHSO (aq) + H O(l)
4
2
4
2
sodium hydroxide sulphuric acid sodium water
(dilute) hydrogensulphate
(acid salt)
2NaOH(aq) + H SO (aq) o Na SO (aq) + 2H O(l)
2
4
2
2
4
sodium hydroxide sodium sulphate water
(normal salt)
ƒ 4GCEVKQP YKVJ ECTDQPCVGU CPF J[FTQIGPECTDQPCVGU Dilute sulphuric
acid decomposes carbonates and hydrogencarbonates (or bicarbonates)
liberating carbon dioxide gas.
With sodium hydrogencarbonate and sodium carbonate, the reactions are,
2NaHCO (aq) + H SO (aq) o Na SO (aq) + H O(l) + 2CO (g)
2
2
4
2
4
2
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sodium hydrogen sulphuric acid (dil.) sodium sulphate water carbon
carbonate dioxide