Chemistry - Unit 3 Topic 1: Dissociation Constants and Acid-Base Indicators

Acid ionisation at equilibrium equation

HA(aq)(acid) + H2O(l) -->

Ka=

Ka > 1

strong acid

Ka < 1

weak acid

Strong acid position of ionisation equilibrium

far to the right

Weak acid position of ionisation equilibrium

far to the left

Steps to calculate Ka

1. Write out a chemical equation to visualise what is occurring

2. Write the equation for Ka

3. The first thing we can calculate is the H+ concentration from the Ph value provided

4. Use an ICE table to determine the unknown concentration

5. Substitute this value into the equation for Ka

Base ionisation at equilibrium equation

B(aq)(base) + H2O(l) -->

Kb=

Kb > 1

strong base

Kb < 1

weak base

Steps to calculate Kb

1. Write out the chemical equation for the dissociation

2. Determine the expression for Kb

3. Use pH to calculate [OH-]

4. Calculate Kb using the molar ratio from the ionisation equation, using an ICE table

Ka x Kb

Kw

pKa=

-log10Ka

pKb=

-log10Kb

Indicators

change colour in acidic/basic solutions, so they can identify the pH of a solution

Weak acid (Hln) indicator

HIn(aq)(colour A) + H2O(l) -->

Weak base indicator

B(colour A) + H2O -->

Indicators change colours when

pKa = pH

When equilibrium lies exactly in the centre

[In-] = [HIn]