Physical 2 Section 1 - Unit 21: Acid Base Equilibria

Question 1

Equation for pH (1 mark)

Answer 1

pH = -log [H+]

Question 2

Explain what is meant by a buffer solution (2 marks)

Answer 2

• Solution which resists change in pH /maintains pH

- Despite the addition of (small amounts of) acid/base

Question 3

If an acid is added to a buffer solution, what happens to the moles of reactants in buffer solution?

Answer 3

• Moles of acid increases

- Moles of salt decreases

Question 4

Explain how a mixture of solution can act as a buffer when small amounts of acid or alkali are added (3 marks)

Answer 4

• When acid added, there is an increase in concentration of H+ so equilibrium moves to left
• When alkali is added, OH- ions react with H+ ions so equilibrium moves to right to replace H+ ions
• Concentration of H+ remains constant

Question 5

State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement (1 mark)

Answer 5

• Over time / after storage, meter does not give accurate readings

Question 6

Describe how you would obtain the pH curve for a titration (5 marks

Answer 6

• Measure pH (of the acid)
• Add alkali in known small portions
• Stir mixture
• Measure pH (after each addition)
• Repeat until alkali in excess
• Add in smaller increments near endpoint

Question 7

Using the concept of electronegativity, justify why the acid strengths of ethanedioic acid and ethanoic acid are different (6 marks)

Answer 7

Difference in structure of two acids

• Acids are in form of RCOOH
• But in ethanoic acid, there is a R=CH₃
• Whilst in ethanedioic acid, there’s a R=COOH

Inductive effect

• The unionised COOH group contains 2 very electronegative oxygen atoms
• So there is a negative inductive (electron withdrawing) effect
• The CH₃ group has a positive inductive (electron pushing) effect

How the polarity of OH affects acid strength

• The O-H bond in the ethanedioic acid becomes more polarised
• And so there’s more dissociation into H+ ions
• Ethanedioic acid is therefore a stronger acid than ethanoic acid

Question 8

Explain what happens to the pH of water, according to Le Chatelier’s principle, when temperature is increased (3 marks)

Answer 8

• Dissociation of water is endothermic
• So equilibrium shifts to the right to decrease temperature again
• So higher [H+] produced and lower pH

Question 9

What is pKa used to measure (1 mark)

Answer 9

• The strength of an acid

Question 10

State the equation for pKa (1 mark)

Answer 10

pKa = -log [Ka]