Historical Development of Acid/Base Theories - slider-chemistry-12

Year 12 Chemistry
 He classified all
chemicals into three
categories – acids, bases
and salts
 He believed that all acids
contained oxygen and it
was this that gave them
their sour taste
 Flaw: not all acids
contain oxygen and metal
oxides form bases
 Showed that all acids do
not contain oxygen
 Proposed that acids are
hydrogen containing
materials following the
discovery of HCl
 Flaw: not all substances
that contain hydrogen are
 Acids dissociate in water forming H+
as one product
 Bases dissociate in water forming OHas one product
 Neutralisation involves the reaction of
H+ and OH- forming a salt in water
 Flaws:
 theories only apply to aqueous
 Some substances such as NH3 are
bases and do not contain OH Relative strengths not addressed
 Amphoteric substances not addressed
 An acid is a proton (H+) donor
 A base is a proton acceptor
 Examples:
 HCl + H2O <===> H3O+ + Cl¯
 NH3 + H2O <===> NH4+ + OH¯
Identify the acids/bases
Any other acids/bases?
 Amphiprotic substances are those that can act as bases
and acids
 Water is an obvious example
Notice in the previous slide that water reacts with both
acids and bases.
Bisulfate is another amphiprotic substance. Construct
chemical equations to show this property.
 Acid 1 + Base 2  Base 1 + Acid 2
Conjugate pair 1
Conjugate pair 2
 What does this mean?
 An acid reacts and forms a conjugate base which can
also accept a proton
 A base reacts and forms a conjugate acid which can
donate a proton
 Conjugate pairs differ only by one H+
 Example:
 HNO3 + H2O <===> H3O+ + NO3¯
Here, nitric acid and the nitrate ion are
conjugates and water and the hydronium ion
are also conjugates
 Predicting Equilibrium
 The direction of acid-base equilibria is away
from the stronger acid base side and towards the
weaker acid base side
 The stronger the acid, the weaker its conjugate base
 The stronger the base, the weaker its conjugate acid
 Reactions that proceed to a large extent:
 A strong acid will force the equilibrium in the opposite
direction (in this case, forward or right)
 HCl + H2O <===> H3O+ + Cl¯
 Reactions that proceed to a small extent:
 If the weaker of the two acids and the weaker of the two
bases are reactants (appear on the left side of the
equation), the reaction is said to proceed to only a small
 NH3 + H2O <===> NH4+ + OH¯
 Identify the conjugate acid base pairs in each reaction.

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