Page 77 - Chemistry ICSE Class X

Study of Acids, Bases and Salts 65

How are bases classified on the basis of strength
1P VJG DCUKU QH VJGKT UVTGPIVJ DCUGU CTG ENCUUKſGF KPVQ VJG HQNNQYKPI VYQ
categories:
ƒ Strong bases: Bases which are almost completely dissociated/ionised
in their solutions are called strong bases. Strong bases are also called ^ƚƌŽŶŐ ďĂƐĞƐ ;Žƌ ĂůŬĂůŝƐͿ ĂƌĞ ĂůƐŽ
ƐƚƌŽŶŐ ĞůĞĐƚƌŽůLJƚĞƐ͘
alkalis.
Š Some typical strong bases (or alkalis) are
y Potassium hydroxide (KOH)
y Sodium hydroxide (NaOH)
y Barium hydroxide [Ba(OH) ]
2
Š Solutions of strong bases (or alkalis) are highly conducting because
–
they contain high concentration of OH ions.
+
–
NaOH(aq) o Na (aq) + OH (aq)
2+
–
Ba(OH) (aq) o Ba (aq) + 2OH (aq)
2
ƒ Weak bases: Bases which are partially dissociated/ionised in their
aqueous solutions are called weak bases.
Š Some typical examples of weak bases are
y Ammonium hydroxide (NH OH), Calcium hydroxide
4
[Ca(OH) ], Magnesium hydroxide [Mg(OH) ] tĞĂŬ ďĂƐĞƐ ĂƌĞ ĂůƐŽ ǁĞĂŬ
2
2
y Solutions of weak bases are very poorly conducting because ĞůĞĐƚƌŽůLJƚĞƐ͘
–
they contain very low concentration of OH ions.
–
+
NH OH(aq) U NH (aq) + OH (aq)
4
4
–
2+
Mg(OH) (aq) U Mg (aq) + 2OH (aq)
2
How are bases classified on the basis of their acidity
–
The number of hydroxide ions (OH ) furnished by one molecule of the base
in its aqueous solution is called its acidity.
dŚĞ ďĂƐĞƐ ŚĂǀŝŶŐ ĂĐŝĚŝƚLJ ŽĨ
–
For an insoluble base, the acidity is equal to the number of OH ions ͻഩone (1) ĂƌĞ ĐĂůůĞĚ monoacid bases͘
present in its molecule. ͻഩtwo (2) ĂƌĞ ĐĂůůĞĚ diacid bases͘
ͻഩthree (3) ĂƌĞ ĐĂůůĞĚ triacid bases͘
The acidity of a base is always a whole number, e.g., 1, 2 and 3.
ƒ The bases which on dissociation/ionisation in solution produce only
–
one hydroxide ion (OH ) per molecule are called monoacid bases.
Š Monoacid bases dissociate/ionise in one step only.
–
+
NaOH(aq) o Na (aq) + OH (aq)
–
+
NH OH(aq) U NH (aq) + OH (aq)
4
4
Š Monoacid bases form only one normal salt. For example, NaOH
forms NaCl (sodium chloride) which is a normal salt. EĂK,͕ <K,͕ E, K, ĂƌĞ monoacid
ƒ The bases which on dissociation/ionisation in aqueous solutions bases͘ 4
–
produce two hydroxide ions (OH ) per molecule of the base are called ͻ
diacid bases. Ă;K,Ϳ ͕ DŐ;K,Ϳ ͕ Ƶ;K,Ϳ ͕ Wď;K,Ϳ
2
2
2
2
2+
–
Ca(OH) (aq) o Ca (aq) + 2OH (aq) ĂƌĞ diacid bases͘
2
Š Diacid bases dissociate/ionise in two steps.
–
+
Cu(OH) (aq) o Cu(OH) (aq) + OH (aq)
2
+
–
2+
Cu(OH) (aq) o Cu (aq) + OH (aq)
Š Diacid bases form one basic salt and one normal salt. For example,
the basic salt and the normal salt of lead hydroxide [Pb(OH) ] are,
2
Basic salt: Pb(OH)Cl
Normal salt: PbCl 2