Indicators

Question 1

What does pH graph for strong acid and strong base look like

Answer 1

Straight lines at top and bottom quickly forming vertical line, equivalence point at 7

Question 2

What does pH graph look like for strong acid and weak base

Answer 2

Equivalence point lower than 7, less quickly goes to vertical

Question 3

pH graph for weak acid and strong base

Answer 3

Initial steep increase then slows due to production of sodium ethanoate (CB), horizontal line buffer zone, more slowly reaches vertical and equivalence point greater than 7

Question 4

pH graph of weak acid and weak base

Answer 4

Change of pH around equivalence point is gradual with very small vertical section, if dissociation constants for weak acid and weak base are approx equal then equivalence point around 7

Question 5

What is the equivalence point

Answer 5

Point in titration when amount of reactant added from burette is just enough to completely neutralise solution in flask

Question 6

How to choose indicator

Answer 6

Must change colour completely in the pH range of the near vertical part of pH curve

Question 7

How do indicators change colour

Answer 7

They are themselves weak acids/bases which change colour when they lose/gain protons

Question 8

How is weak acid indicator represented

Answer 8

Hln, ln shorthand for the rest of the molecule other than the ionisable hydrogen atom

Question 9

Equation to show weak acid indicator in water

Answer 9

Hln(un-ionised indicator) reversible arrow to H+ and ln- (indicator after losing proton)

Question 10

What determines pH of colour change of indicator

Answer 10

Will change colour approximately centred around its pKa