AQA A Level Chemistry复习笔记5.6.2 pKₐ

• The range of values of Ka is very large and for weak acids, the values themselves are very small numbers

Table of Ka values

• For this reason it is easier to work with another term called pKa
• The pKa  is the negative log of the Ka value, so the concept is analogous to converting [H+] into pH values

pKa = -logKa

• Looking at the pKa values for the same acids:

Table of pKa values

• The range of pKa values for most weak acids lies between 3 and 7

Answer

Step 1: Write down the equation for the partial dissociation of ethanoic acid

CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)

Step 2: Write down the equilibrium expression to find Ka

Step 3: Simplify the expression

The ratio of H+ to CH3COO- is 1:1

The concentration of H+ and CH3COO- is, therefore, the same

The equilibrium expression can be simplified to:

Step 4: Substitute the values into the expression to find Ka

= 1.74 x 10-5

Step 5: Determine the units of Ka

= mol dm-3

The value of Ka is therefore 1.74 x 10-5 mol dm-3

Step 6: Find pKa

pKa = - log10Ka

= - log10 (1.74 x 10-5)

= 4.76

How are Ka values found?

• If we neutralise a weak acid with a strong base such sodium hydroxide the equation for the reaction is:

HA (aq) + NaOH (aq) → NaA (aq) + H2O (l)

HA (aq) + OH- (aq) → A- (aq) + H2O (l)

• When half of the acid has been neutralised the concentration of [HA] is equal to the concentration of [A-]
• Since the acid is poorly dissociated it is assumed all the A- comes from the product rather than HA dissociating
• This simplifies the expression to

• In practice, to find Ka the pH of a weak acid is measured as it is neutralised by a strong base and a graph is plotted for pH versus volume of base added
• This type of graph is known as a pH titration curve
• A vertical tie-line is drawn to the curve at half the volume of base needed for neutralisation
• At this point  pKa = pH, so by reading off the pH value from the y-axis, you find the Ka of the acid

Finding the Ka of a weak acid from a pH titration curve