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3.1.10 and 3.1.11 acid and base dissociation constants (HL)

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chem whatarethemechanismsofchemicalchange

Reactivity 3.1.10 - the strengths of weak acids and bases are described by their , , , or values
Reactivity 3.1.11 - for a conjugate acid-base pair, the relationship can be derived from the expressions for and
  • weak acids and bases do not dissociate fully, so the concentration of ions cannot be deduced from the initial concentrations
  • the ion concentrations rather depend on the extent of dissociation

more 2.3.1 dynamic equilibrium 😭 💀

considering the generic weak acid dissociating in water:

(given that the concentration of water is a constant)

  • is known as the acid dissociation constant
  • it will have a fixed value for a particular acid at a specific temperature
  • it is also known as the acid ionisation constant

as the value of depends on the position of the equilibrium of acid dissociation, it gives a direct measure of the strength of an acid.

the higher the value of at a particular temperature, the greater the dissociation, the stronger the acid

since is an equilibrium constant, its value does not depend on the concentration of the acid or on the presence of other ions

considering the dissociation of a generic base in water

  • is known as the base dissociation constant
  • it will have a fixed value for a particular base at a specified temperature

same principles apply to as

calculations involving and

the values of and allows for the comparison of strength of weak acids and bases

NOTE

  • the given concentration of an acid or base is its initial concentration
  • the or of a solution refers to the concentration of ions or ions at equilibrium
  • when the extent of dissociation is very small, it is appropriate to use the approximations:
Missing \end{align}\begin{align}

\ce{[acid]{\text{initial}}&\approx[acid]{\text{equilibrium}}} \

\ce{[base]{\text{initial}}&\approx[base]{\text{equilibrium}}}
\end{align}

and are logarithmic expressions of and

directly using and is a bit clumsy as there are negative exponents and span a wide range of values

the relationship between and is reciprocal for a conjugate acid-base pair

for a conjugate acid-base pair :

this shows the inverse relationship between the strength of acids and their conjugate bases.

  • the stronger the acid, the weaker the conjugate base
  • the stronger the base, the weaker the conjugate acid
challenge questions
  1. increasing the length of the carbon chain decreases the acid strength of the group but increases the basic strength of the group. can you suggest why this is so? hint: think about the electron density distributions in the molecules

increasing the length of the carbon chain causes less electron withdrawal from the bond as electrons are pushed towards the carbonyl atom

the basic strength of amines depends largely on the availability of the lone pair electrons on nitrogen. longer carbon chains push electrons towards more than shorter carbon chains, so longer carbon chains are stronger

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