June 10, 2016
Explaining Solutions
By the mid 1800s nobody was able to
explain why ionic substances, acids and
bases were electrolytes and molecular
substances were non-electrolytes. In 1887
Svent Arrhenius proposed an explanation.
June 10, 2016
Arrhenius proposed that when an ionic
substance dissolves in water, it dissociates
into individual ions. For example:
+
NaCl(s) → Na (aq) + Cl¯(aq)
June 10, 2016
original solid substance
+
NaCl(s) → Na (aq) + Cl¯(aq)
arrow represents
dissolved substance
in water
represents the
individual ions in
solution
June 10, 2016
Dissolving NaCl in water creates a
solution that contains a vast number of
positively and negatively charged
entities (the cations and anions).
This explains why the solution
conducts electricity.
June 10, 2016
The solution will conduct
electricity because ions will move.
http://www.mchs.gsacrd.ab.ca/pages/chem20/media/solutionSalt.html
June 10, 2016
Molecular substances do not dissociate. The
molecules simply dissolve in water and disperse,
with no charged entities formed. For example:
C12H22O11(s) → C12H22O11(aq)
There are no charged entities in the solution.
There is nothing to carry an electrical current.
See memorization solubility rules for molecular
substances on page 201.
June 10, 2016
Solution does not conduct electricity
because there are no charged entities
in the solution to move.
June 10, 2016
Bases
Arrhenius proposed that a base is an ionic
substance that dissociates so that the anion
produced is hydroxide. Example:
Ba(OH)2(s) → Ba2+(aq) + 2OH¯(aq)
Basic solutions conduct electricity for the same
reason that ionic solutions do: charged entities
in solution that can move.
June 10, 2016
Acids
Acids begin as molecular substances, such as
HCl(g).
Arrhenius proposed that HCl(g) dissolves to form
HCl(aq), which will then ionize to produce H+(aq)
and Cl¯(aq).
HCl(g) → HCl(aq)
then
+
HCl(aq) → H (aq) + Cl¯(aq)
This is called ionization.
June 10, 2016
Dissociation / Dispersal / Ionization equations:
Is substance soluble
or insoluble?
Yes
No
Done
electrolyte or
non-electrolyte?
electrolyte
non-electrolyte
ionic (includes base)
or acid?
dispersal
equation
ionic
dissociation
equation
acid
ionization
equation
June 10, 2016
Write dissociation, dispersal or
ionization equations as appropriate:
EX: magnesium iodide
Soluble?
Electrolyte?
Ionic or acid?
MgI2(s) → Mg2+(aq) + 2I¯(aq)
June 10, 2016
EX: nitric acid
Soluble?
Electrolyte?
Ionic or acid?
HNO3(aq) → H+(aq) + NO3¯(aq)
June 10, 2016
EX: ethanol
Soluble?
Electrolyte?
C2H5OH(l) → C2H5OH(aq)
June 10, 2016
EX: silver chloride
Soluble?
Insoluble