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Weak Acids & Bases (A2 Only)

Chemistry: AQA A Level, topic 2.5.4

Weak Acids and Bases

Not all acids and bases fully dissociate in solution.

Weak acids

Weak acids

  • Weak acids do not fully ionise in solution.
  • For a strong acid, like HCl, the equilibrium for the following equation lies very far to the right:
  • HCl ⇋ H+ + Cl-
  • For a weak acid (e.g. ethanoic acid), the equilibrium lies closer to the middle.
Weak bases

Weak bases

  • Weak bases behave similarly. They do not fully ionise in solution.
  • So, for weak acids and bases, the concentration of H+ or OH- ions is not just the concentration of the acid or base.
  • This makes it harder to calculate the concentrations in solution.
Ka

Ka

  • We define a new constant as Ka = [H+][A−][HA]\frac{[H^+][A^-]}{[HA]}
  • HA refers to the un-ionised acid molecule, and A- refers to the anion remainder after loss of a proton.
  • A large Ka value means that a lot of the acid ionises in solution.
  • A small Ka value means that very little acid ionises in solution.
pKa

pKa

  • Just like pH, we use pKa because Ka values vary a lot.
  • The definition of pKa is:
  • pKa = −log10(Ka)

Calculating the Concentration of Ions

We can use pKa along with pH to calculate the concentrations of varying ions in solution.

Converting pKa to Ka

Converting pKa to Ka

  • The definition of pKa is:
  • pKa = -log10(Ka)
  • This can be rearranged into:
  • Ka = 10-pKa
Calculating [H+] from Ka

Calculating [H+] from Ka

  • When you dissolve a weak monoprotic acid in solution, [H+] is always equal to [A-].
  • This means you can simplify Ka into:
  • Ka = [H+]2[HA]\frac{[H^+]^2}{[HA]}
  • But, [HA] is the concentration of the acid added ([HA]0, minus the concentration of the ionised acid), so we can simplify further to:
  • Ka = [H+]2[HA]0−[H+]\frac{[H^+]^2}{[HA]_0 - [H^+]}
  • If you know the values of Ka and [HA]0, you can calculate the concentration of H+ ions.
Calculating [HA] from Ka and pH

Calculating [HA] from Ka and pH

  • First, use the definition of pH to calculate the [H+].
  • Next, use [H+] = [A-] to simplify the equation for Ka.
  • Then, rearrange Ka to give [HA]:
  • [HA] = [H+]2Ka\frac{[H^+]^2}{K_a}

Test yourself on Weak Acids & Bases (A2 Only)

  • HCl is a strong acid, where would the equilibrium lie in the following equation?
    HCl ⇋ H+ + Cl-

  • What is the correct mathematical expression for pKa?

  • PKa = -log10(Ka).
    What is the rearranged equation for Ka = ?

  • For a weak monoprotic acid, what is an expression for [HA]?

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