CSEC Chemistry Paper 2 (May 14, 2020)
Acids, Bases & Salts
Salts at last.
Salt is a common term used to describe common table salt, sodium chloride (NaCl)
A salt however, is a compound formed when some or all of the H+ ions in the acid are replaced by a metal or ammonium ion.
If one is dealing with a monobasic or monoprotic acid (one that gives one “unit” of H+ ion per “unit” of acid) then all the H+ ion will always be replaced and one ends up with a normal salt. E.g. of monobasic acid is HCl.
HCl(aq) —> H+(aq) + Cl-(aq)
1 unit of acid 1 unit of H+
With a dibasic acid (one that gives two units of H+ per unit of acid) like sulphuric acid (H2SO4(aq)), then not all the H+ ions are always replaced. When only some of the H+ ions are replaced in a case like this, an acid salt is formed. (Not an abnormal salt)
The normal salt example: H2SO4 (aq) + 2NaOH(aq) —> Na2SO4 (aq) + H2O(l)
Acid normal salt
Note all the H+ from the acid was replaced by the sodium ion, in forming the salt.
Now take the not so normal example (acid salt formation):
H2SO4 (aq) + NaOH(aq) —> NaHSO4 (aq) + H2O(l)
Acid acid salt
Sodium hydrogen sulphate
In like manner, there can be two acid salts formed when phosphuric acid (H3PO4 (aq) -it is tribasic) reacts with aqueous sodium hydroxide. Of course, if the three units of H+ ions are replaced, then you would have the normal salt.
Practice writing the three equations hinted at in the last paragraph. Name the salt and acid salts.
A popular question asked as it relates to salts is to differentiate between a hydrated and an anhydrous salt.
Past Paper Question for 2 marks:
Differentiate between a hydrated salt and an anhydrous salt. (Break down the words and figure it out.)
Contributed by: Kemil Walford