Theories of Acids and Bases - slider-dpchemistry-11

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 here?
HCl + H2O  H3O+ + Cl-
NH3 + H2SO4  NH4+ + HSO4-
HBr + NH2+  NH32+ + Br-
Identify the B-L acids and bases in each of these reactions.
 Amphiprotic substances are those that can act as bases
and acids. They can donate or accept protons.
 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.
Lewis Theory
 An acid is an electron pair receptor
 A base is an electron pair donor
Note that BF3 would not be
an acid under the B-L
Theory as there is no H+ to
A cobalt metal complex has 6 dative bonds
formed by the donation of e- pairs from the
ammonia molecules (Lewis bases)
Which of these substances are Lewis bases? Lewis acids?
Water reacts with carbon dioxide to form carbonic acid. Write out
the structures and show how the electrons are transferred, thereby
identifying the Lewis acid and Lewis base.
 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¯
Identify the conjugate pairs in this reaction
Here, nitric acid and the nitrate ion are conjugates
and water and the hydronium ion are also conjugates
Strong acids
Weak acids
completely dissociate in water
Partially dissociate in water
HA + H2O  H3O+ + A-
HA + H2O  H3O+ + A-
In general, these reactions are
reversible, but for a strong acid the
equilibrium is far right. So,
There is an equilibrium established
with weak acids, which means that
there is less H3O+ ions in solution.
HA + H2O  H3O+ + A-
CH3COOH + H2O  H3O+ + CH3COO-
Examples: HCl, HNO3, H2SO4
Examples: CH3COOH, H2CO3
What do you think is the difference in electrical conductivity?
Strong bases
Weak bases
completely dissociate in water
Partially dissociate in water
NaOH  Na+ + OH-
Again, as with acids, the
equilibrium is far right. So,
1 mol NaOH  1 mol OHExamples: group I hydroxides,
NH3 + H2O  NH4+ + OHOnly about 1% of ammonia
dissociates into hydroxide ions.
1 mol NH3  << 1mol OH-
Examples: NH3, other amines
Again, the electrical conductivity is greater for strong bases
 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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