bThe Bronsted-Lowry Theory of acids and bases/b
An acid is a uproton donor/u.
A base is a uproton acceptor/u.
Note: A proton is the same as a Hsup+/sup ion.
A umonobasic/u acid is able to donate one proton, e.g. HCl.
A ubibasic/u acid is able to donate two protons, e.g. Hsub2/subSOsub4/sub.
A utribasic/u acid is able to donate three protons, e.g. Hsub3/subPOsub4/sub.
bAcid-base reactions involve the transfer of protons/b
When hydrogen chloride gas dissolves in water to produce hydrochloric acid, the hydrogen chloride molecule gives a proton to a water molecule, forming a hydroxonium ion.
(H2O(l) + HCl_{(g)} ⟶ H_3O^+{(aq)} + Cl-{(aq)} )
This is usually written in its simplified form: (HCl ⟶ H+ + Cl^- )
When an acid reacts with a base, the hydroxonium ion reacts with hydroxide ion. A proton is transferred from a hydroxonium ion (the proton donor) to a hydroxide ion (the proton acceptor) to make water.
(H3O+{(aq)} + OH-{(aq)} ⟶ 2H2O(l) )
This is commonly written as (H+{(aq)} + OH-{(aq)} ⟶ H2O(l) ), but it is important to realise that whenever you talk about Hsup+/sup in the context of acids, you are actually referring to the hydroxonium ion Hsub3/subOsup+/sup.
bConjugate pairs/b
The general equation for any acid dissolving in water is:
(HA + H2O ⇌ H_3O^+ + A^- )
Consider the forwards reaction:
The HA is an acid because it is donating a proton to the water.
The water is a base because it is accepting a proton from the HA.
Consider the reverse reaction:
The Hsub3/subOsup+/sup is an acid because it is donating a proton to the Asup-/sup ion.
The Asup-/sup ion is a base because it is accepting a proton from the Hsub3/subOsup+/sup.
When the acid, HA, loses a proton, it forms a base, Asup-/sup. When the base, Asup-/sup, accepts a proton, it reforms the acid, HA. These are called uconjugate pairs/u.
If HA is considered as the acid, then Asup-/sup is its uconjugate base/u.
If water is considered as the base, then Hsub3/subOsup+/sup is its uconjugate acid/u.
An uamphoteric substance/u can act as either an acid or a base.