Download 8. Acids and bases

8. Acids and bases
8.1 Theories of acids and bases
• Sour-tasting substances (acids) have
been known for thousands of years.
• Lavoisiers early theory: Acid= a
compound of oxygen and a nonmetal.
Arrhenius ionic theory
• Acid = a substance that produces hydrogen
ions (H+) in aqueous solution
• Base = a substance that produces hydroxide
ions (OH-) in aqueous solution
e.g
HCl(aq)
→ H+(aq) + Cl-(aq)
NH3(g) + H2O(l) → NH4+ (aq) + OH-(aq)
Bronstedt-Lowry theory
• An acid is a molecule or ion that can donate a
proton (H+) = proton donor
e.g HCl (g) + H2O(l)
H3O+(aq) + Cl- (aq)
• A base is a molecule or ion that can accept a proton
= proton acceptor
NH3 (g) + H2O (l)
NH4+ (aq) + OH- (aq)
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Substances such as water, which can act both as an
acids and as a bases depending on the species they
are reacting with are termed amphiprotic (amphoteric).
• Conjugate acid-base pairs differs by just one proton.
• A strong acid (HCl) has a weak conjugate base (Cl-)
• A weak acid (H2O) has a strong conjugate base.
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Monoprotic acids: have a single proton to donate
e.g HCl, HNO3, CH3COOH
Diprotic acids: have two protons to donate
e.g H2CO3, H2SO4, H2SO3
Triprotic acid: H3PO4
Lewis theory
• Acid: an electron pair acceptor
• Base: an electron pair donor
• A dative (coordinate) covalent bond is formed.
• Note that this definition does not involve protons or
hydrogen!
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A positively charged metal ion can acts as a
Lewis acid, and form a complex with a ligand
that acts as a base.
4 NH3 + Zn2+ → [Zn(NH3)4]2+
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Lewis bonding is found in the (coloured)
complex ions formed by the transition metals.
6 H2O + Fe3+ → [Fe(H2O)6]3+
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Typical ligands: H2O, CN-, NH3
8.2 Properties of acids and bases
1) Neutralization= acid + base ⇒ salt + water
Acid-base titration
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In a titration a solution of known concentration
(=standard solution) is added in small measured
quantities, from a burette, to a fixed volume of
another solution, measured with pipette.
The addition of the solution is continued until the
indicator that is present changes colour.
At the end-point (=eqvivalence point) the two
substances are present in stoichiometric (equal)
quantities.
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A 50,0 cm3 sample of concentrated sulfuric acid was
diluted to 1,00 dm3. A sample of the diluted sulfuric
acid was analyzed by titrating with aqueous sodium
hydroxide. In the titration, 25,00 cm3 of 1,00 mol dm-3
aqueous sodium hydroxide required 20,0 cm3 of the
diluted sulfuric acid for neutralization. Determine the
concentration of the original concentrated sulfuric
acid solution.
2) Metal oxide + acid ⇒salt + water
3) Acid + carbonate/hydrogencarbonate ⇒
salt + water + carbon dioxide
4) Acid + metal ⇒ salt + hydrogen
5) Acid/base + indicator
6) Ammonium salts ⇒ ammonia
8.3.1 Strong acids and bases
• Strong acids and bases are completely
dissociated into their ions in aqueous solution:
HCl (aq) → H+(aq) + Cl- (aq)
• Monoprotic acids: 1 mol of acid gives 1 mol of
hydrogen ions
• Diprotic acids: 1 mol of acid gives 2 mol of
hydrogen ions
8.3.1 Weak acids and bases
• Weak acids and bases are only slightly
dissociated into their ions in aqueous solution.
e.g.
CH3COOH(aq)
CH3COO-(aq) + H+(aq)
Acidic strength vs concentration
8.3.3 Distinguishing between strong and weak
acids and bases
1. Electrical conductivity:
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When solutions of the same concentration are
compared at the same temperature, strong acids and
bases have a high amount of ions present in their
solutions and therefore a much higher electrical
conductivity than weak acids and bases.
2. Rate of reaction:
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Strong acids can be distinguished from weak acids
of the same concenentration by comparing the
reactions with for example magnesium or calcium
carbonate.
The strong acid will react more rapidly, leading to
quicker evolution of gas.
3. pH:
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The higher the H+ concentration, the lower the pH
value.
The pH value can be measured using a pH meter
or a universal indicator.
8.4 The pH-scale
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For every billion of water molecules
only two are ionized:
pH = power of Hydrogen
Acid rain
• Rain is naturally acidic, because the water
molecules react with the CO2 in the air and
form the weak acid H2CO3.
• pH for rain containing this weak acid is max
5.6.
• Acid rain is therefore:
precipitation (rain, snow)
with pH lower than 5.6.