F325: Module 1: Indicators

Question 1

I) What type of chemical are indicators?

Answer 1

Weak acids of Weak bases

Question 2

I) What is the equilibrium equation for an indicator?

Answer 2

HIn = H+ + In-
All in aqueous solution
LHS = Colour 1
RHS = Colour 2

Question 3

I) How would you give a rough estimate of an indicator’s pH range?

Answer 3

The pH range over which an indicator changes colour is approximately equal to pKa +/- 1

Question 4

I) Name 2 common indicators and state their pKa values.

Answer 4

Phenolphthalein - pKa = 9.3

Methyl Orange - pKa = 3.7

Question 5

I) Which indicator out of PP or MO would be suitable for the following titrations:
Strong acid/Strong Base
Weak acid/Strong base
Strong acid/Weak base
Weak acid/Weak base

Answer 5

SA/WB = MO or PP since the endpoint is within both of the indicator’s ranges.

WA/SB = PP

SA/WB = MO

WA/WB = Neither since there is no endpoint (rapid pH change in any region of the titration curve)

Question 6

I) Define the enthalpy change of neutralisation.

Answer 6

The enthalpy change when an aqueous acid is neutralised by and aqueous base to form one mole of H2O(l) under standard conditions.

Question 7

I) What is the enthalpy change of neutralisation (roughly)?

Answer 7

-58kJmol^-1