[21.3] neutralisation

Question 1

describe the key features of a typical pH titration curve

Answer 1

• excess of acid - pH increases slowly as basic solution is added
• vertical section - pH increases rapidly on addition of a very small volume of base. acid and base concentrations similar
• equivalence point - centre of vertical section of pH titration curve
• excess of base - pH increases slowly as basic solution is added

Question 2

what is the equivalence point?

Answer 2

when the volume of one solution exactly reacts with the volume of the other solution according the stoichiometry of the reaction

Question 3

describe how the pH titration curves would look different in different titrations with acid added

Answer 3

• strong acid - strong base
• strong acid - weak base
• weak acid - strong base
• weak acid - weak base

Question 4

what is the end point of a titration?

Answer 4

when the indicator contains equal concentrations of [HA] and [A-] so the colour will be inbetween the two extreme colours

Question 5

what is the neutral point of a reaction?

Answer 5

when the pH is 7

Question 6

why should you avoid saying ‘neutral point’?

Answer 6

not all salts have a pH of 7

Question 7

what are indicators?

Answer 7

weak acids which have a different colour to their conjugate base

Question 8

what would indicators look like in low and high pH and why?

Answer 8

HIn ⇌ H⁺ + In⁻

• low pH: equilibrium pushed left = colour 1
• high pH: equilibrium pushed right = colour 2

Question 9

colours and pH range of methyl orange

Answer 9

• weak acid, HA = red
• conjugate base, A- = yellow
• end point = orange
• pH range of colour change = 3.2-4.4

Question 10

colours and pH range of phenolphthalein

Answer 10

• weak acid, HA = colourless
• conjugate base, A- = purple
• end point = pink?
• pH range of colour change = 8.2-10.0

Question 11

how sensitive is the end point?

Answer 11

• different indicators have different Ka values and change colour over different pH ranges
• at end point, Ka = [H+] and pKa = pH
• most indicators change colour over a range of about two pH units

Question 12

how do you choose which indicator to use?

Answer 12

• indicator colour change must coindice with the vertical section of the pH titration curve
• ideally, the end point and equivalence point would coincide

Question 13

why is there no indicator suitable for a weak acid-weak base titration?

Answer 13

• there is no vertical section
• even at its steepest, the pH requires several cm³ to pass through a typical pH indicator range of 2 pH units