Acid Base Equilibria

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  • Created on: 25-04-14 00:01
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Acid-Base Equilibrium ­ Answers
1. Acids were first defined in terms of their sour taste. When indicators such as litmus were discovered, an acid was
thought of as any substance which turned litmus red.
2. Arrhenius theory of acids and bases:
Acids are substances which produce hydrogen ions in solution.
Bases are substances which produce hydroxide ions in solution.
Neutralisation happens because hydrogen ions and hydroxide ions react to produce water.
3. Arrhenius theory is very limited because it requires the solutions to be aqueous. It only applies to substances that
produce Hydrogen ions (H+) or hydroxide ions (OH- ). Therefore it has some flaws including:
An acid is expected to be an acid in any solvent: However, HCL acts as an Arrhenius acid when dissolved in
water but when HCL is dissolved in benzene there is no dissociation. This is against Arrhenius theory
Arrhenius did not explain in his theory the behaviour of acids and bases in a non-aqueous solution
All salts should produce solutions that are neither acidic nor basic: However, if equal amounts of HCl
and ammonia react, the solution is slightly acidic.
4. Bronsted-Lowry theory of acids and bases:
An acid is a substance that gives a proton to a base
A base is a substance that accepts a proton from an acid
5. The American chemist Lewis extended the Bronsted-Lowry theory further. A base is a species that has a lone pair
of electrons, which it uses to form a covalent bond. An acid is a substance that can accept the pair of electrons
and form a bond.
6. The reaction of an acid with a base can be written as the chemical equation:
HA + B BH+ + A-
For many acids this is a reversible reaction.
7. The acid, HA, and the base, A-, derived from it by loss of a proton, are called an acid-conjugate base pair. The
base, B, and the acid, BH+, derived from it by acceptance of a proton, are also a base-conjugate acid pair. The
relationship between conjugate pairs is:
Acid ­ H+ conjugate base
Base + H+ conjugate acid
8. Acid Conjugate Base
HCO3- CO32-
H2SO4 HSO4-
CH3COOH CH3COO-
-
HSO4 SO42-
9. Base Conjugate Acid
OH- H2O
+
NH3 NH4
SO42- HSO4-
-
HCO3 H2CO3
10. A strong acid is an acid that is totally ionised in aqueous solution forming hydrated hydrogen ions, H3O+. For
example, hydrochloric acid, HCl (aq) is a strong acid:
HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)
11. A weak acid is an acid that is only very slightly ionised in aqueous solution. E.g. ethanoic acid is a weak acid:
CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq)
12. A strong base is totally ionised in aqueous solution forming hydroxide ions, OH-. For example, sodium hydroxide:
NaOH (aq) Na+ (aq) + OH- (aq)
13. A weak base is protonated to only a small degree in solution and so only forms a small proportion of hydroxide
ions, for example ammonia is a weak base:
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

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A weak acid is in equilibrium with its conjugate base in aqueous solution. The equilibrium constant is known in this
context as the acid dissociation constant Ka. The expression for this constant is:
[H O+][A-]
K a = 3 [HA] N.B. H2O is not included in this expression as its concentration in aqueous solutions is constant.
The equilibrium is often written in a shorthand version:
HA (aq) A- (aq) + H+ (aq)
Therefore, in the Ka expression, H3O+ can be written as H+
15.…read more

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Sulfuric acid is only a strong acid in its first ionisation. Its second ionisation is weak:
HSO4- H+ + SO42-
The second ionisation is suppressed by the H+ ions from the first ionisation. So a solution of sulfuric acid of
concentration 0.10 mol dm-3 has [H+] of just above 0.10 mol dm-3 and hence its pH is very slightly less than 1.00.
24. A strong base is totally ionised in aqueous solution. The pH can be worked out in one of two ways:
1.…read more

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The ammonium ions act as an acid, reacting reversibly with water to produce H3O+ ions, which make the solution
acidic. If you know the Ka value and conc. of the salt solution, you can then calculate the pH of the solution.
32. The anions in the salts of weak acids are their conjugate bases. For example, the ethanoate ion in sodium
ethanoate is the conjugate base of ethanoic acid, which is a weak acid.…read more

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The number of A- ions from the totally ionised salt is much greater than the few A- ions from the weak acid,
so it can be assumed that [A-] = [salt]
The ionisation of the weak acid is so suppressed that [HA] = [weak acid]
The equilibrium expression can, therefore, be simplified:
+
][A-] [H+][salt]
K a = [H[HA ] = [weak acid]
40. The pH of a buffer solution made by adding 50cm3 of 0.100 mol dm-3 ethanoic acid to 50 cm3 of 0.…read more

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Strong acid Strong base 1 7 3-11 Just < 13
Strong base Strong acid 13 7 11-3 Just > 1
Weak acid Strong base 3 9 7-11 Just < 13
Strong base Weak acid 13 9 11-7 Just > 3
Strong acid Weak base 1 5 3-7 Just < 11
Weak base Strong acid 11 5 7-3 Just > 1
49. The volume at the equivalence point has to be worked out by the usual titration method.…read more

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Titration curve of acid added to alkali Titration curve of alkali added to acid
53. When a strong base is added to a solution of a weak acid, the point halfway to the equivalence point is when half
the acid has been neutralised. At the half neutralisation point, [HA] = [A-]. Therefore at the point, Ka = [H+] and so
pKa = pH. The pH at the half-neutralisation point can be read off the graph.…read more

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