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Topic 4.3

ACIDS AND BASES

BronstedLowry theory
Acids and bases in water
Calculating pH
Buffer solutions
Acidalkali titrations
Indicators




















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Topic 4.3

ACIDS AND BASES

BronstedLowry theory
Acids and bases in water
Calculating pH
Buffer solutions
Acidalkali titrations
Indicators

BRONSTEDLOWRY ACIDS AND BASES

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…

Page 3

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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…

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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…

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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 +…

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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…

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2. 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+…

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3. 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 =
log10[H3O+].

If [H3O+] = 1.0M, the pH of the solution is 0.
If [H3O+]…

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STRONG AND WEAK ACIDS

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…

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2. 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)…

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