P8
Ions in Aqueous Solution
Water is the most important solvent. The study of dissolution and precipitation
reactions leads to the development of our understanding of equilibrium reactions.
Precipitate
The solid formed from the reaction of two clear solutions.
Precipitation reaction


Equations describing
precipitation
reactions
The driving force behind a precipitation reaction is the
formation of an insoluble substance.
A precipitation reaction occurs if a cation from one ionic
solution combines with an anion from another ionic
solution to form an insoluble substance
Balanced chemical equations
BaCl2(aq) + Na2SO4(aq)
BaSO4(s) + 2NaCl(aq)
Ionic equations
Ba2+(aq) + 2Cl-(aq) + 2Na+(aq) + SO42-(aq)
2Na+(aq) + 2Cl-(aq)
Net ionic equations
Ba2+(aq) + SO42-(aq)
Soluble ionic
compounds





Insoluble ionic
compounds






Saturated solution
BaSO4(s) +
BaSO4(s)
All compounds of group I elements and the ammonium
All nitrate compounds
All acetate compounds
All chlorides, bromides and iodide (except silver and
lead)
All sulfates (except silver, lead, barium, strontium, and
calcium)
All carbonates (except group I and ammonium
compounds)
All sulfites (except group I and ammonium compounds)
All phosphates (except group I and ammonium
compounds)
All hydroxides (except group I, ammonium, barium,
calcium and strontium compounds)
All oxides (except group I, ammonium, barium, calcium
and strontium compounds)
All sulfides (except groups I and II and ammonium
compounds)
A solution in which no more of the particular solute can be
dissolved in a given quantity of the solvent at the specific
temperature.
Solubility


The solubility of a substance in a particular solvent is the
concentration of its saturated solution at the specified
temperature.
The maximum amount of the substance that will dissolve
in a given amount of the solvent.
Reversible reaction
A reaction that can go in either direction.
All precipitation/ dissolution reactions are reversible.
Reversible reactions frequently do not go to completion.
Equilibrium reaction



The reaction is reversible
There are no observable macro changes.
The rate of the forward reaction is equal to the rate of the
back reaction.
Measures of
concentration








Mass of solute per 100mL or 1 L of solvent.
Mass of solute per 100mL or 1 L of solution
Volume of liquid solute per 100mL or 1 L of solvent
Volume of liquid solute per 100mL or 1 L of solution
% (w/v)
% (v/v)
% (w/w)
Parts per million (ppm)
Apparatus for
measuring solutions




Measuring cylinders are used for approximate volumes
Pipettes are used for accurate fixed volumes.
Burettes are used for accurate variable volumes.
Volumetric flasks are used for accurate fixed volumes.
Dilution
The diluted solution and the volume taken of the original
solution both contain the same amount of solute.
C1V1 = C2V2
Where
Molarity
C1
C2
V1
V2
= concentration of the original solution
= concentration of the diluted solution
= volume of the original solution
= volume of the diluted solution
Number of moles of solute per litre of solution.
C = n
V
Where
C = concentration (molarity) of the solution
n = number of moles of solute
V = volume of the solution in litres
The units of molar concentration are written as mol L-1.
Pollution
Presence of harmful or undesirable substances in the
environment at concentrations significantly greater than those
in the natural (or unpolluted) environment.
Heavy metal
pollution


Heavy metals are the transition metals plus lead
(particularly mercury, lead, cadmium, chromium, zinc
and copper).
It is the ions or compounds of the metals that are
pollutants, the actual metals are insoluble in water and
quite harmless.
Thermal properties of
water


Water has a relatively high thermal conductivity
Water has a high heat capacity
Temperature

Measure of the degree of hotness or coldness of an object
or substance.
The hotter an object the higher its temperature.
Heat flows from an object with a high temperature to an
object with a low temperature.


Quantity of heat


Specific heat capacity


The amount of heat energy in a substance is proportional
to its mass.
The amount of heat energy contained in equal masses of
different substances depends on the nature of the
substances involved.
The amount of heat required to increase the temperature
of 1g of the substance through one Celsius degree (or one
Kelvin degree).
The specific heat capacity of water and aqueous solutions
is 4.18 Jg-1K-1
Calculating quantities
of heat
q = mCT
Where
Exothermic



Thermal pollution
q
m
C
T
= quantity of heat (J)
= mass (g)
= specific heat capacity (Jg-1K-1)
= change in temperature (oC)
Reactions that release energy
More energy is released in forming bonds in the products
than is needed to break bonds in the reactants.
H is negative
The discharge into a river or lake of quantities of hot water
large enough to increase significantly (2 to 5oC) the
temperature of the water body.