Weak acids
Strong acids, Kw and strong bases
Acids and bases
Know the Bronsted-Lowry definition of an acid and a base and write
equations to show the dissociation of each in water
Write full and ionic equations for the reactions of acids with metals,
carbonates, bases and alkalis
Explain the difference between strong and weak in terms of
dissociation, and represent this with equations
Identify acidic and basic conjugate pairs
Know that pH is a measure of the H+ concentration in solution
Calculate pH using pH = -log10[H+(aq)] for strong monoprotic and diprotic
acids
CARE: the ‘-‘ means negative, not subtract
Calculate concentration using [H+(aq)] = inverse log –pH for strong
monoprotic and diprotic acids
Write equations for the partial dissociation of water
Write an expression for the equilibrium constant of water, giving units
Write an expression for the ionic product of water, Kw, stating units,
and know its value at 298K CARE: this changes with temperature
Calculate the pH of a strong base by rearranging the expression
for Kw to find the [H+(aq)] then using pH = -log10[H+(aq)]
Calculate the pH change of a solution when acid and alkali are
added to one another CARE: the volume of solution will change
Write equations for the partial dissociation of weak acids
Write an expression for the acid dissociation constant, Ka, giving units
Rearrange Ka to find any of the values in the expression
Know that in a weak acid, [H+(aq)] [A-(aq)] are equal i.e [H+(aq)]2
Calculate the pH of a weak acid using Ka to find the [H+(aq)] CARE:
remember to √ - see above, then using pH = -log10[H+(aq)]
Calculate the concentration of a weak acid by using [H+(aq)] =
inverse log –pH and substituting this into the expression for Ka
Know why pKa values may be given instead of Ka values
Convert Ka to pKa using pKa = -log10Ka
Convert pKa to Ka using Ka = inverse log –pKa
CARE: always use Ka in pH calculations
Exam ques
Revised
): S:
Covered
ACIDS, BASES AND pH
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A2 Chemistry – Unit 4
pH curves
Buffers
Be able to sketch pH curves for any combination of strong/weak acid
with strong/weak base CARE: read carefully which is being added so
the curve is drawn the correct way round
Understand what is meant by the equivalence point
Use the equivalence range to select a suitable indicator
Know the pH ranges in which methyl orange and phenolphthalein
operate, and the associate colour changes
Use a pH curve to carry out titration calculations
Interpret a pH curve with two equivalence points as a result of a
diprotic acid
Define what a buffer is
Describe how to make an acid and alkali buffer
Write equations to show the formation of an acid and alkali buffer,
explaining relative concentrations of species referring to partial or full
dissociation.
Explain how buffers resists changes in pH when small amounts of acid
or alkali are added, referring to the position of equilibrium
Write equations for the buffer action described above
Calculate the pH of buffers using the expression for Ka to find [H+(aq)]
then using pH = -log10[H+(aq)]
Calculate the pH of buffers after the addition of acid or alkali
(CARE: recalculate moles, volume and molarity)
Discuss some uses of buffers e.g. shampoo, washing powder, controlling
blood pH
Optional: Write equations for the buffer which keeps our blood pH
constant and explain the importance of this