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Topic 4.3
BronstedLowry theory
Acids and bases in water
Calculating pH
Buffer solutions
Acidalkali titrations
1. The BronstedLowry definition.
There are many definitions of acids and bases in existence, but the most useful one is the
BronstedLowry definition:
An acid is a substance which can behave as a proton donor. A proton can be represented
as a hydrogen ion, H+.…read more

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NH4+ == NH3 + H+
H2O == H+ + OH
HCO3 == H+ + CO32
Acids and bases thus come in pairs every acid can lose a proton to become a base and every
base can accept a proton to become an acid. Species related in this way are known as
conjugate acidbase pairs.…read more

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H2CO3 (CO2 + H2O) == HCO3 + H+
HSO3 == SO32 + H+
NH4+ == NH3 + H+
HCN == CN + H+
HCO3 == CO32 + H+
HPO42 == PO43 + H+
H2O == OH + H+
OH == O2 + H+
The strongest acids are at the top these have the weakest conjugate bases.
The strongest bases are at the bottom these have the weakest conjugate acids.
Note that some species (eg H2O, HSO3, HCO3 amongst others) appear in both columns.…read more

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Eg H3O+ + OH == H2O + H2O
Acid 1 Base 2 Base 1 Acid 2
Amphoteric substances can undergo acidbase reactions with themselves:
Eg H2O + H2O == H3O+ + OH
Acid 1 + Base 2 Base 1 Acid 2
Eg HCO3 + HCO3 == CO32 + CO2 + H2O
Acid 1 Base 2 Base 1 Acid 2
Any acidbase reaction which takes place in aqueous solution will reveal which of two acids is
the stronger:
Eg HCl(aq) + H2O(l) H3O+(aq) +Cl(aq) goes 95%…read more

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The expression [H3O+][OH] is known as the ionic product of water and has a value of 1.0 x
1014 mol2dm6 at 25oC. This value is a constant at a given temperature. The ionic product of
water is slightly higher at higher temperatures, suggesting that the dissociation is endothermic.
In pure water, the [H3O+] and [OH] concentrations are equal.
It follows that [H3O+][OH] = [H3O+]2 = Kw.
So [H3O+] = [OH] = Kw = 1.0 x 107 moldm3.…read more

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Acidic and alkaline solutions.
An acid can be regarded as a substance which reacts with water by forcing it to accept a proton
and behave as a base:
HA(aq) + H2O(l) == H3O+(aq) +A(aq)
The resulting solution contains an excess of H3O+ ions. Any solution which contains more H3O+
ions than OH ions is said to be acidic. The measure of the acidity of the solution is the H3O+
concentration. The greater the H3O+ concentration, the more acidic the solution.…read more

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The pH scale.
The acidity and alkalinity of a solution is often measured by its pH.
The pH of a solution is the negative logarithm to the base ten of the H3O+ concentration. pH =
If [H3O+] = 1.0M, the pH of the solution is 0.
If [H3O+] = 0.1M, the pH of the solution is 1.
If [H3O+] = 0.01M, the pH of the solution is 2.
In alkaline solutions, the OH concentration is known.…read more

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Acids can be classified as one of two types strong acids and weak acids. Each must be
considered separately:
1. Strong acids.
A strong acid is one which dissociates completely in water into H3O+ and A. A strong
acid must therefore be a better proton donor than H3O+. Examples of strong acids are HCl,
HNO3 and H2SO4.…read more

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Weak acids.
A weak acid is one which only dissociates partially in water. It reaches equilibrium as follows:
HA(aq) + H2O(l) == H3O+(aq) + A(aq)
Cx x x
A weak acid is one which is a worse proton donor than H3O+. Examples are CH3COOH
(ethanoic acid), NaHSO4 (sodium hydrogensulphate) and CO2 (carbon dioxide).…read more


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