Page 163 - Chemistry ICSE Class X

Electrolysis 149

Strong electrolytes
The substances which give highly conducting solutions when dissolved in
water are called strong electrolytes.
ƒ Strong electrolytes when dissolved in water are almost completely
dissociated into ions. So, the solutions of strong electrolytes contain
only ions.
ƒ The compounds such as NaCl, HCl, NaOH and H SO , etc. are some
2
4
typical strong electrolytes. Fig. 6.1 Dissociation of an ionic
compound into ions when dissolved in
ƒ Sodium chloride when dissolved in water dissociates into sodium water
+
–
(Na ) and chloride (Cl ) ions.
–
+
–
+
Na Cl (s) + H O(l) o Na (aq) + Cl (aq)
2
sodium chloride water sodium ion chloride ion
ƒ Highly polar covalent compounds, e.g., hydrogen chloride, undergo
ionisation to a large extent and behave like a strong electrolyte. In
dilute solution, it is ionised almost completely.
–
+
HCl(g) + H O(l) o H O (aq) + Cl (aq)
3
2
hydrogen chloride water hydronium ion chloride ion

Weak electrolytes
dŚĞ ĐůĂƐƐŝĮĐĂƟŽŶ ŽĨ ĞůĞĐƚƌŽůLJƚĞƐ ŝŶ
The substances which give poorly-conducting solutions in water are ƚĞƌŵƐ ŽĨ ƐƚƌŽŶŐ ĂŶĚ ǁĞĂŬ ĞůĞĐƚƌŽůLJƚĞƐ
termed as weak electrolytes. For example, aqueous solutions of ammonia ŝƐ ďĂƐĞĚ ŽŶ ƚŚĞ ĐŽŶĚƵĐƟŶŐ ďĞŚĂǀŝŽƵƌ
[NH OH(aq)], acetic acid [CH COOH(aq)], etc. give poorly-conducting ŽĨ ƚŚĞŝƌ ƐŽůƵƟŽŶƐ ŝŶ ǁĂƚĞƌ͘ ^Ž͕ ŝĨ ƚŚĞ
4
3
solutions. ƐŽůǀĞŶƚ ŝƐ ĐŚĂŶŐĞĚ͕ ƚŚĞ ĐŽŶĚƵĐƟŶŐ
ďĞŚĂǀŝŽƵƌ ŽĨ ƚŚĞ ƐŽůƵƟŽŶ ĂůƐŽ
The compounds such as Ammonia (NH ), Acetic acid (CH COOH), ŐĞƚ ĐŚĂŶŐĞĚ͘ dŚŝƐ ŵĞĂŶƐ ƚŚĂƚ ĂŶ
3
3
Carbonic acid (H CO ), Hydrogencyanide (HCN), Mercury(II) chloride ĞůĞĐƚƌŽůLJƚĞ ǁŚŝĐŚ ŝƐ ƐƚƌŽŶŐ ŝŶ ǁĂƚĞƌ
3
2
(HgCl ) and Zinc chloride (ZnCl ), etc. are typical weak electrolytes. Weak ŵĂLJ ďĞĐŽŵĞ ǁĞĂŬ ŝŶ ƐŽŵĞ ŽƚŚĞƌ
2
2
electrolytes in solution, ionise to a very small extent. ƐŽůǀĞŶƚ͘
Weak electrolytes give poorly-conducting solutions because they ionise
to a very small extent. Acetic acid (CH COOH) when dissolved in water
3
+
–
ionise partially into acetate ion (CH COO ) and hydronium ion (H O ).
3
3
–
+
CH COOH(aq) + H O(l) U CH COO (aq) + H O (aq) Ŷ ŝĚĞĂů ĐůĂƐƐŝĮĐĂƟŽŶ ŽĨ ĞůĞĐƚƌŽůLJƚĞƐ
ƐŚŽƵůĚ ŶŽƚ ĚĞƉĞŶĚ ƵƉŽŶ ƚŚĞ ŶĂƚƵƌĞ
3
3
3
2
acetic acid water acetate ion hydronium ion ŽĨ ƚŚĞ ƐŽůǀĞŶƚ͘ /ƚ ƐŚŽƵůĚ ďĞ ƐŝŵƉůLJ
In the solution of acetic acid in water, only a part of acetic acid gets ďĂƐĞĚ ŽŶ ƚŚĞ ƐƚƌƵĐƚƵƌĞ ŽĨ ƚŚĞ
ionised. The rest is present in the molecular form. Thus, there exists an ĞůĞĐƚƌŽůLJƚĞ͘ KŶĞ ƐƵĐŚ ĐůĂƐƐŝĮĐĂƟŽŶ
equilibrium between unionised acetic acid molecules (CH COOH) and the ĚŝǀŝĚĞƐ ĞůĞĐƚƌŽůLJƚĞƐ ŝŶƚŽ ƚŚĞ ĨŽůůŽǁŝŶŐ
3
–
+
ions (CH COO and H O ) produced in the solution. ĐůĂƐƐĞƐ͗
3
3
ͻഩdƌƵĞ ĞůĞĐƚƌŽůLJƚĞƐ
Thus, the solutions of weak electrolytes contain both the molecules ͻഩWŽƚĞŶƟĂů ĞůĞĐƚƌŽůLJƚĞƐ͘
and ions. zŽƵ ǁŝůů ůĞĂƌŶ ĂďŽƵƚ ƚŚŝƐ ĐůĂƐƐŝĮĐĂƟŽŶ
ŝŶ ŚŝŐŚĞƌ ĐůĂƐƐĞƐ͘
Table 6.1 Differences between strong and weak electrolytes
Property Strong electrolyte Weak electrolyte
1. Conductivity Good conductor of Poor conductor of
electricity electricity
2. Molecular state Completely dissociated Partially ionised into ions
into constituent ions when dissolved in water,
when dissolved in water. (e.g., ammonia, acetic
acid)