Acids and Bases

Question 1

Define a Bronstead-Lowry acid and base.

Answer 1

Bronstead-Lowry acid: donates a proton.

Bronstead-Lowry base: accepts a proton.

Question 2

Identify the chemical species that function as the Bronstead acid, the Bronstead base, the conjugate acid and conjugate base from the molecular equation of an acid-base reaction.

Answer 2

Conjugate acid: on product side of equation; proton lost.

Conjugate base: on product side; proton gained.

Question 3

Recognise conjugate acid-base pairs.

Answer 3

Practise.

Question 4

Define Kw, Ka, Kb (and pKa and pKb) and calculate them from pH, pOH and solution concentrations.

Answer 4

Kw = autoionisation constant of water.
LOOK at CHEAT SHEET for equations.

Ka = acid ionisation constant (measure of acid strength); conjugate base of strong acid is weak.
LOOK at CHEAT SHEET for equations.

Kb = base ionisation constant (measure of base strength); conjugate acid of strong base is weak.
LOOK at CHEAT SHEET for equations.

Question 5

Provide the value of Kw and hence calculate the molar concentrations of H3O+ and OH- in pure water at 25 C.

Answer 5

Kw = 10e-14 at 25 C.

Question 6

Define a polyprotic acid.

Answer 6

Polyprotic acid = acid that can donate more than one proton.

Question 7

Describe an amphoteric substance.

Answer 7

Amphoteric substance = substance that can act as an acid or base by donating or receiving protons.

Question 8

Predict the relative strength of an acid or base given pH or pOH and Ka or Kb (and pKa and pKb).

Answer 8

{pKa is a negative log of the Ka constant.}

Refer to cheat sheet.

Question 9

Relate pH, pOH, Ka, Kw or Kb and equilibrium and initial concentrations for strong or weak acids/bases.

Answer 9

Practise.

Question 10

Construct acid-base titration curves (pH vs volume) from experimental data and identify equivelance and half/equivelance points.

Answer 10

Titration curve = pH on y-axis and volume of base added (mL) on x-axis.

Equivelance point is when pH = 7; equal number of moles of acid and base in solution at this point.

pH is around about the same as pKa at half-equivelance point.

Question 11

Identify strong/weak acid, strong/weak base titrations from titration curves.

Answer 11

Weak acid and strong base characteristics in titration curve:
- titration curve begins at bottom of graph and rises.
- pH rises slowly at first.
- sudden change in pH.
- pH slowly converges to value of strong base.
- stoichiometric endpoint ‘S’ occurs on the basic side of
7 pH.

Strong acid and weak base characteristics in titration curve:

• titration curve begins at the top of the graph and
  decreases.
• pH drops slowly at first.
• sudden change in pH.
• pH slowly converges to value of strong acid.
• stoichiometric point occurs on the acidic side of 7.

Question 12

Calculate the pH of a solution at a given point of a strong/weak acid and strong/weak base titration.

Answer 12

Henderson-Hasselbach Equation in equation sheets.

Question 13

Understand the role of buffers, their composition and their effective range.

Answer 13

Role: to maintain a constant concentration of a species in a solution; resist changes to pH upon addition of small amounts of acid or base.

Composition:
Acid buffer = mixture of weak acid and its salt, mixture of weak acid and its conjugate base.
Base buffer = mixture of weak base and its salt, mixture of weak base and its conjugate acid.

Effective range:
Acid buffer = pH below 7.
Base buffer = pH above 7.

Question 14

Calculate unknown quantities for a buffer solution using the Henderson-Hasselbach equation.

Answer 14

Refer to equations sheet.