LEVEL THREE CHEMISTRY: AQUEOUS SYSTEMS
Solubility and equilibrium
Solubility constant and ionic product
Factors affecting solubility
Acids and bases
Buffers
Titration curves
Indicators
SOLUBILITY AND EQUILIBRIUM:
I can show knowledge from level 2 by defining the term ‘dynamic
equilibrium’.
I can show knowledge from level 2 by stating Le Chatelier’s
principle and giving examples of situations in which equilibrium
will shift.
I can show that I understand the relationship between
equilibrium and saturation by explaining verbally or with a
diagram what is happening to the precipitate formed in a
saturated solution.
I can show that I understand what is meant by ‘the solubility of
a substance’ by defining it in terms of dissolving and saturation
point.
SOLUBILITY CONSTANT AND IONIC PRODUCT:
I can show that I understand the solubility constant, KS, by:
Writing the KS formula for a given saturated solution and explaining why the solid reagent is omitted.
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Interpreting the value of KS to indicate the solubility of the solute.
I can show that I understand the relationship between solubility
and saturation point by:
Calculating the KS of a reaction given the solubility of the solute and the mole ratio of the products (reaction
equation).
Calculating the solubility of the solute given KS and the mole ratio of the products (reaction equation).
This may involve knowing that for [A][B] type
reactions, KS = s2 and for [A][B]2 type reactions,
KS = 4s3
I can show that I understand the ionic product, Q or IP, by:
Writing the Q/IP formula for a given solution.
Calculating the Q/IP of a reaction given the
concentrations of the reactants.
Determining, by comparing the Q/IP and the KS of a
solute, whether a precipitate will form.
FACTORS AFFECTING SOLUBILITY:
I can show that I know what is meant by the common ion effect
by defining it in terms of equilibrium and Le Chatelier’s principle
and explaining how it can affect solubility.
I can show that I understand the importance of the common ion
effect by recognising situations in which it will occur and
calculating the resulting solubility of a solute by taking the
common ion effect into account.
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I can explain the effect of complex ion formation on the
solubility of a solute.
I can identify situations in which a complex ion will form and
correctly calculate the resulting solubility of the solute by taking
the complex ion formation into account.
I can use the following equations: n = m
and n = cV to calculate
M
concentrations and the effects of dilution when necessary to
answer questions on solubility.
ACIDS AND BASIS:
I can show knowledge from level 2 by defining:
‘Acid’ and ‘base’ in terms of proton accepting and
donating.
Conjugate acid and conjugate base in terms of proton accepting and donating.
Strong and weak acid in terms of dissociation.
I can use the following equations to solve given problems:
pH = -log10 [H3O+]
[H3O+ ] = 10-pH
Kw = [H3O+][OH-] = 1x10-14
I can show that I understand weak acids and bases by:
Writing Ka and Kb expressions for the equilibria of weak acids and bases respectively.
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Identifying when it is necessary to use Ka and Kb values to calculate pH values and correctly using Ka and Kb to do so.
Converting between Ka and pKa using:
pKa = -log10 Ka
Ka = 10-pKa
Interpreting Ka and pKa values to find out the relative strength of an acid.
Converting between Ka and Kb using: Ka x Kb = Kw
I can show that I understand the difference between strong and
weak acids and bases by writing the species in a dissociation
equation in order of their concentrations in the solution.
I can show that I understand the implications of complete and
partial dissociation by relating the relative concentrations of the
species identified in the point above to the pH and conductivity
of the solution.
BUFFERS:
I can show that I understand what a buffer is by giving a concise
definition.
I can explain how a buffer works by explaining it in terms of
conjugate acids and bases and writing equations to show what
happens when an acid or base is added to a given buffer
solution.
I can show that I understand what a buffer is by stating whether
or not dilution changes the pH of a buffer.
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TRITRATION CURVES:
I can draw the characteristic shapes of the following titration
curves:
Strong base added to strong acid
Strong base added to weak acid
Strong acid added to weak base
I can therefore identify from a titration curve whether the acid
and base used are strong or weak.
I can show that I understand what a titration curve shows by:
Defining the term ‘buffer zone’ and indicating its position on a titration curve.
Discussing the buffer zone in terms of pH and
concentration of acidic and basic ions.
Defining the term ‘equivalence point’ and indicating its position on a titration curve.
Identifying which species are present at the half-way point and the equivalence point of a given titration curve.
Stating the relationship between pH and pKa at the
half-way point.
INDICATORS:
I can define the term ‘indicator’ and state the point at which an
indicator will change colour in terms of pH and pKa and the
concentrations of the acid and its conjugate base.
Given the pKa values of several indicators, I can identify and
discuss which indicator would be suitable from a given titration
curve.
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