Na+
(aq) + Cl (aq)V.C.E Chemistry
Year 11 Revision Notes
Chemistry of Electrolytes
Write Ionic, Equations Showing Ionizations,
Dissociations and Associations
Differentiate between strong, weak and non-electrolytes
given:
Mechanism of Conductivity in Electrolytes
For
a substance to conduct electricity, it must contain particles that are
a)
charged
b)
free
to move
Such
substances can be grouped
These have mobile electrons which are able to move under
a potential difference e.g. metals
these have mobile ions which move under the influence of
a potential difference. Substances that behave in this manner are called
Electrolytes can be defined as a solution or a liquid
which conducts an electric current and is decomposed by the passage of charge.
e.g. fused salts, solutions of salts in water etc.
Solutions which contain a large number of ions are good
conductors, those with few ions are poor conductors.
An example of an electrolyte is CuCl2. If a
current is passed through a 1M solution of CuCl2, copper is deposited
on the cathode (negative electrode) and chlorine is evolved at the anode
(positive electrode).
Substances that have virtually complete ion producing
reactions with water are called Strong Electrolytes. Those substances that have
incomplete ion producing reactions are called Weak Electrolytes. Some pure
liquid substances show virtually no conductivity. We call such substances non
electrolytes.
-
examples
of strong electrolytes are KNO3 , HCl , NaOH
-
examples
of weak electrolytes are NH3 , CH3COOH , H2O
-
examples
of non electrolytes are benzene , acetone , toluene
However conductivity data are only a guide to the
strengths of the electrolytes. Unless the concentrations of solutions are the
same, precise comparisons of the strength of electrolytes cannot be made. For
example, 0.1M HCl is a strong electrolyte and a good conductor. 0.0001M HCl is a
strong electrolyte but a poor conductor because it produces fewer H+
and Cl- ions.
The solubility of salts must also be taken into account.
for example BaSO4 is only sparingly soluble. and so is a poor
conductor. However, the BaSO4 that is dissolved is completely
dissociated into ions, so BaSO4 must be regarded as a strong
electrolyte.
Electrolyte
solutions are formed in three main ways
i)
dissociation
ii)
ionisation
iii)
association
(opposite to dissociation)
Consider the compounds that have ions as well as
basic structural units, with few exceptions all produce solutions that
are strong electrolytes. This is because dissociation in an appropriate polar
solvent releases the ions from the lattices and allows them to move about the
solution with a solvation sphere of solvent molecules. Take for example sodium
chloride which consists of Na+ and Cl- ions held together by electrostatic
attraction to form a three dimensional structure. When sodium chloride is
dissolved in water the electrostatic attraction is reduced and ions become free
to move independently, this process is called dissociation. If, in solution, a
compound dissociates completely into ions, the compounds behaves as a strong
electrolyte. This complete dissociation can be represented by the ionic equation
NaCl (s) + aq
Na+
(aq) + Cl (aq)
The pressence of mobile Na+ (aq)
and Cl (aq) ions in the solution accounts for its
conductivity.
Another example is the dissociation of sodium hydroxide
in water
NaOH(s) + aq
Na+
(aq) + OH (aq)
As both NaCl and NaOH dissociate completely in water they
are called strong electrolytes
Molecular
liquids will be non electrolytes unless the molecules interact to form ions.
Molecules of
some
polar substances, for example
water, interact in this way. Molecules of different polar substances may also
react to form ions, Hydrogen chloride, (hydrochloric acid), for example, which
is a pure liquid is virtually a non electrolyte, becomes a strong electrolyte
when mixed with water, because of the reaction : (ionic equation)
HCl
(l) + (aq)
H+
(aq) + Cl (aq)
Such
a reaction, in which ions are formed from non ionic substances, is called
ionisation. Since the solution behaves as a strong electrolyte, the ionisation
must be virtually complete. Other substances that we describe as strong
acids behave in a similar fashion to HCl when dissolved in water.
On
the other hand, a 0.1 M acetic acid solution produced by dissolving acetic acid,
CH3COOH, in water behaves as a weak electrolyte. This can be
explained by assuming that acetic acid does not ionise completely in water. This
partial ionisation can be represented by the ionic equation.
CH3COOH
(l) + (aq)
H+
(aq) + CH3COO (aq)
The
long arrow pointing to the left is used to show that very few of the acetic acid
molecules ionise to form hydrogen and acetate ions H+(aq) and CH3COO
(aq)
The
weak electrolyte water can be formed by the association of H+ (aq)
and OH (aq). The H+(aq)
ions are present in acidic solutions and OH(aq)
ions are present in alkaline solutions. As a result, any reaction between such
solutions produces water as can be seen in the following ionic equation
H+
(aq) + OH (aq)
H2O(l)
A
number of other weak electrolytes may be produced by the association of ions.
Association
reactions can also result in the precipitation of a solid, e.g mix a solution of
AgNO3 with a solution of NaCl
AgNO3(aq)
+ NaCl(aq)
NaNO3(aq)
+ AgCl(s)
In
solution we have
Ag+
(aq) + NO3 (aq) + Na+ (aq)
+ Cl (aq)
AgCl2
(s) + Na+ (aq) + NO3 (aq)
As
Na+(aq) and NO3-(aq) ions remain unchanged, and as their presence so
not affect the course of the reaction they are called spectator ions. If these
ions are omitted from the equation for the reaction, we are left with only those
ions that associate, in this case, Ag+(aq) and Cl-
(aq). An ionic equation for precipitation includes only those ions
that associate and the resultant solid product
Ag+(aq)
+ Cl(aq)
AgCl
(s)
It
appears from this that ions in solution behave independently from one another. A
solution of NaCl may be regarded as simply a source of Na+ ions and
Cl- ions. The Na+ ions in NaCl have the same properties as
the sodium ions in any sodium salt, the same is true for the Cl- ion.
So it can be said that ions behave independently in solution
The
charge of ions in an electrolyte is a simple whole number multiple of the
fundamental charge on an electron. The charge is generally the same as the
electrovalency of the element or ion. Positively charged ions (electron
deficient) are called CATIONS, while negatively charged ions (excess electrons)
are called ANIONS.
When
a potential difference is applied to an electrolyte solution the cations are
attracted to the negatively charged cathode, and the anions are attracted to the
positively charged anode.
Electrons
are released from the cathode to the cations and are absorbed at the anode from
the anions. So there is a nett flow of electrons through the electrolyte
solution. As a result the anions and cations are decomposed at the electrodes.
The greater the concentration of free ions, the more electrons can be conducted
by the solution. The greater the surface area of the electrodes the more
electrons can be transferred
Its
is possible to differentiate between strong, weak and non electrolyes by three
methods
|
Strong
Electrolytes |
Weak
Electrolytes |
Non
Electrolytes |
|
HCl
solution |
Pure
water |
CH3CH2OH |
|
H2SO4
solution |
Concentrated
H2SO4 |
|
|
HNO3
solution |
NH3
solution |
CH3COCH3 |
|
NaOH
solution |
CH3COOH
solution |
|
|
KOH
solution |
H2CO3
solution |
C6H12O6 |
|
NaCl
solution |
H2SO3
solution |
|
|
Na2CO3
solution |
Citric
Acid solution |
|
|
CuSO4
solution |
|
|
|
NH4Cl
solution |
|
|
|
In
general, solutions of |
In
general solutions of |
|
|
i)
strong acids |
i)
weak acids |
|
|
ii)
strong alkalis |
ii)
weak alkalis |
|
|
iii)
salts |
|
|
|
|
|
|
obtained by experimental procedures
To
differentiate between strong, weak and non electrolytes given an equation, two
things must be considered
a)
the
reactant(s)
When
considering the reactant(s), remember
i)
solutions
of strong acids, alkalis and salts will produce strong electrolytes
ii)
solutions
of weak acids and bases as well as water and concentrated H2SO4
will produce weak electrolytes
iii)
non
polar molecules will not produce ions and so they are non electrolytes
If
a reactant(s) fits one of the above categories then the solution can be
immediately identified as a strong, weak or non electrolyte.
b)
the
arrow(s) in the equation
when
considering the arrow(s) in the equation the following points should be
remembered
i)
if
there is only one large arrow pointing to the right than this indicates that the
dissociation or ionisation goes to completion and so produces a strong
electrolyte
ii)
if
there are two arrow, the ionisation reaction does not go to completion and so a
weak electrolyte is produced