Transcript Aqueous solutions

Aqueous solutions
Many ionic or covalent compounds are soluble in water;
others are insoluble.
When ionic substances dissolve in water, the ions break
away from the crystal and become surrounded by water
molecules. We indicate this by writing (aq) beside each
ion:
NaCl(s) → Na+(aq) + Cl-(aq)
Solutions containing dissolved ions will conduct
electricity — the greater the concentration of ions,
the greater the conductivity.
When a molecular substance dissolves in water, the
molecules move apart and become surrounded by
water molecules.
C6H12O6(s) → C6H12O6(aq)
Dissolving a molecular substance in water will not
increase the number of ions in solution.
Some molecular compounds react with water,
producing ions. In this case the solution formed will
conduct electricity to some extent.
Measuring conductivity
We can tell how many ions are in solution by how well it
conducts electricity.
This conductivity tester uses mains electricity and
different sizes of light bulbs to estimate conductivity.
The brighter the bulb the better the conductor.
If a 150 W bulb
glows brightly it
is an excellent
conductor.
This 60 W bulb is
only half lit. This
solution is only
partially ionised.
Distilled water
150 W
bulb
60 W
bulb
25 W
bulb
5 W bulb
Water is a molecular substance, but the fact that the 5 W
bulb does glow tells us that it contains a few ions.
2H2O
H3O+(aq) + OH–(aq)
Since water has a pH of 7 we know that it contains
10–7 mol L–1 H3O+ and 10–7 mol L–1 OH–.
Ethanol
Ethanol is a molecular
substance which is liquid at
room temperature.
It dissolves readily in
water.
This sample of ‘pure’
ethanol is conducting
electricity very slightly.
That’s probably because it
contains a trace of water.
If it was completely pure it
would not conduct
electricity at all.
A mixture of ethanol in
distilled water conducts
electricity in the same way
that distilled water itself
does – because of the
already present in the
distilled water.
Distilled water
Sodium chloride
Sodium chloride is an ionic compound which dissolves
easily in water.
The solution formed conducts
electricity well.
NaCl(s) → Na+(aq) + Cl–(aq)
Calcium sulfate
Calcium sulfate is an ionic compound which is not very
soluble in water.
However, the solution formed is a relatively good
conductor of electricity, so there are quite a few ions in
solution:
CaSO4(s)
Ca2+(aq) + SO42–(aq)
Lead carbonate
Lead carbonate appears to be insoluble,
but the 25 W bulb glows, indicating that even this
‘insoluble’ substance dissolves a little:
PbCO3(s)
Pb2+(aq) + CO32–(aq)
Hydrogen chloride/hydrochloric acid
Pure hydrogen
chloride, HCl, is
a gas at room
temperature. It
is a molecular
substance which
dissolves rapidly
in water.
The solution formed is known as hydrochloric acid. It is
an excellent conductor of electricity, which indicates
that although HCl(g) is molecular, HCl(aq) is ionic.
Hydrogen chloride reacts with water, producing H3O+:
HCl(g) + H2O(l)
→
H3O+(aq) + Cl–(aq)
The pH of a 0.1 mol L–1 solution of HCl is 1, which tells us
that [H3O+] is 0.1 mol L–1 and that therefore HCl is fully
ionised in water.
We say HCl is a strong acid because it is fully ionised.
Ethanoic acid
Ethanoic acid (acetic acid) is a colourless liquid with a
strong smell. It is a molecular compound and does not
conduct electricity at all. Even the 5 W bulb does not glow.
Ethanoic acid dissolves readily in water.
The solution is conducts electricity to a limited extent.
The 60 W bulb glows.
For ions to be present, the ethanoic acid must react with
water:
CH3COOH(aq) + H2O(l)
CH3COO–(aq) + H3O+(aq)
We know this is an equilibrium reaction because the poor
electrical conductivity tells us that there are not many
ions in solution.
Ethanoic acid is a weak acid because it is only partially
ionised in water.
Strong acid:
fully ionised
Weak acid:
partially
ionised
Sodium hydroxide
Sodium hydroxide is an ionic solid which dissolves rapidly
in water, forming a solution which conducts well.
NaOH(s) → Na+(aq) + OH–(aq)
We say that sodium hydroxide is a strong base because it is
fully ionised in water.
Ammonia
Ammonia, NH3, is a
gas with a
characteristic
smell.
It is a molecular
compound and is
very soluble in
water.
Ammonia solution conducts electricity to a limited extent.
Molecular NH3 reacts with water to form ions:
NH3(aq) + H2O(l)
NH4+(aq) + OH–(aq)
We know that this is an equilibrium because:
•
there are few ions in solution
•
the pH of a 0.1 mol L–1 solution is about 11, indicating
that [H3O+] = 10–11 and [OH–] = 10–3.
•
we can smell NH3 in an open bottle of ammonia
solution, which means there must be many molecules
of NH3 present.
We say ammonia is a weak base because it is only partially
ionised in water.
Sodium ethanoate
Sodium ethanoate (sodium
acetate) is the salt formed
when ethanoic acid reacts with
sodium hydroxide.
It is a white crystalline
substance which dissolves
readily in water.
Ethanoic acid is molecular, and sodium hydroxide is ionic,
but what kind of substance is sodium ethanoate?
The solution is a good conductor of
electricity: sodium ethanoate is an
ionic substance:
CH3COONa(s) →
CH3COO–(aq) + Na+(aq)