Acids and Bases
• You should already know quite a lot about
acids and bases:
• Acids are corrosive chemicals with a
characteristic sour taste.
• They form solutions with a pH < 7
• More about pH later!
• All acids react in similar ways:
• Write balanced equations for the following:
a) Iron with dilute sulfuric acid
b) Lead carbonate with nitric acid
c) Zinc oxide with hydrochloric acid
d) Calcium hydroxide with nitric acid
e) Sodium hydrogen carbonate with sulfuric
acid
f) Potassium hydroxide with hydrochloric acid
(write an ionic equation!)
a) Fe(s) + H2SO4(aq)  FeSO4(aq) + H2(g)
b) PbCO3(s) + 2HNO3(aq)  Pb(NO3)2(aq) + CO2(g) + H2O(l)
c) ZnO(s) + 2HCl(aq)  ZnCl2(aq) + H2O(l)
d) Ca(OH)2 + 2HNO3(aq)  Ca(NO3)2(aq) + 2H2O(l)
e) 2NaHCO3(s) + H2SO4(aq)  Na2SO4(aq) + 2CO2(g) +2H2O(l)
f) OH-(aq) + H+(aq) H2O(l)
• When equal volumes of 2 mol dm-3 sulfuric
acid and 2 mol dm-3 aqueous sodium
hydroxide are mixed, how can you tell they
react?
a) A gas is evolved
b) The mixture becomes warm
c) The solution changes colour
d) A solid precipitate is formed
• When equal volumes of 2 mol dm-3 sulfuric
acid and 2 mol dm-3 aqueous sodium
hydroxide are mixed, how can you tell they
react?
a) A gas is evolved
b) The mixture becomes warm
c) The solution changes colour
d) A solid precipitate is formed
All neutralisation reactions are exothermic!
Definitions of acids and bases
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What’s the formula of sulfuric acid
H2SO4
Hydrochloric acid?
HCl
Nitric acid?
HNO3
Ethanoic acid?
CH3COOH
Carbonic acid?
H2CO3
• What do they all have in common?
• The most basic definition of an acid is the
Arrhenius definition.
• An acid is a substance that splits up in water
to give a hydrogen ion and an anion
• E.g.
• HCl(aq)  H+(aq) + Cl-(aq)
• The (aq) bit is important
• Acids do not show acidic properties in many nonaqueous solvents.
• We can emphasise the importance of water:
• HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq)
• H3O+ is known as a hydronium ion (or sometimes
a hydroxonium ion).
• We don’t usually bother showing this!
• Because of the importance of the hydrogen
ion in acidic reactions, we often omit the
spectator ions
• (the ions which don’t change during a
reaction)
• You did lots of examples of this at IGCSE
• E.g.
• Mg + 2HCl  MgCl2 + H2
• Write it as ions:
• Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2
• Remove any ions which are the same on both
sides:
• Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2
• Remove any ions which are the same on both
sides:
• Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2
• Leaving the ionic equation:
• Mg + 2H+  Mg2+ + H2
How do we define a base?
• A base is a substance which reacts with an
acid to form water.
• i.e. undergoes neutralisation!!!
• What’s the difference between an alkali and a
base?
• An alkali is soluble in water.
• This fits in much better with the Arrhenius
definition for an acid . . .
• An alkali is a substance which gives OH-(aq) ions
in water.
• E.g.
• NaOH(aq)  Na+(aq) + OH-(aq)
A puzzle:
• Name a common alkali gas
• Ammonia (there is only one)
• How does ammonia give hydroxide ions in
water?
• NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
• Carbonates and hydrogen carbonates act in a
similar way.
Bronsted – Lowry Acids
• This is just a different way of defining an acid.
• An acid is a substance that acts as a proton
donor.
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Why “proton” ?
A hydrogen atom has 1 proton and 1 electron
To make a H+ ion we take away the electron
So a H+ ion is JUST a single proton
• A base is a proton acceptor
• [ there is a slight technical difference between
the B-L definition and Arrhenius’ definition. In
the B-L definition we don’t need to have
H+(aq) present at any time – so we can talk
about acids and bases without needing an
aqueous solvent]
• If an acid can donate one proton it is
monoprotic
• If it can donate 2 protons it is diprotic
• 3 is triprotic
• Many is polyprotic
• A few species can either donate a proton or
accept a proton.
• These are known as amphiprotic
• E.g.
• H2SO4  HSO4-  SO42-
• E.g.
• H2SO4  HSO4-  SO42-
Accept a proton
• E.g.
• H2SO4  HSO4-  SO42-
• Donate a proton