Question

Explain the concept of relative strength of acids.

Answer 1

Strength of acids is determined by the extent to which they ionize in aqueous solutions. Strong acids yield high amounts of hydronium ions when ionized in water, whereas weak acids give small amounts of hydronium ions. The relative strength is then determined by measuring their equilibrium constants in aqueous solutions.

Complete answer:
Acids are hydrogen-containing substances that are capable of donating a proton (hydronium ions) to another substance.
When immersed in water, acids ionize to give conjugate acid (hydronium ion) and a conjugate base (anion) of the acid
Acid strength is the measure of the ability of an acid to lose its proton in the form of a hydronium ion.
Strong acids easily dissociate in aqueous solutions to give hydronium ions, whereas weak acids do not completely dissociate to yield hydronium ions.
This strength depends on the bond strength between the hydrogen and the anion of the acid. Looking at periodic table trends, if the anion is larger, the bond strength is weaker, implying that the acid is stronger (dissociates easily).
The relative strength of acids is determined by measuring their equilibrium constants in aqueous solutions.
In solutions of the same concentrations, stronger acids ionize to a great extent and hence, yield higher concentrations of hydronium ions compared to weaker acids.
The equilibrium constant for an acid is called the acid-ionization constant, Ka.
Considering the reaction of an acid HA:
$HA(aq)+{{H}_{2}}O(l)\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+{{A}^{-}}(aq)$
For this reaction, the equilibrium equation will be written using the formula:
${{K}_{c}}=\dfrac{({{[C]}^{c}}\times {{[D]}^{d}})}{({{[A]}^{a}}\times {{[B]}^{b}})}$
for reaction: $aA(aq)+bB(aq)\rightleftharpoons cC(aq)+dD(aq)$
Therefore, the equilibrium equation for the acid HA ionization will be:
${{K}_{a}}=\dfrac{[{{H}_{3}}{{O}^{+}}][{{A}^{-}}]}{[HA]}$
Despite water being a reactant, it is also a solvent, which is why it is not included in the equation for equilibrium.
The larger the Ka. value of an acid, the larger concentration of hydronium and anions are found.
Therefore, a stronger acid has a larger ionization constant ( ${{K}_{a}}$ ) compared to that of a weaker acid.
As the strength of acids increases, the ionization constant also increases.

Additional Information:
Depending on the Ka values, the strength of a few acids can be arranged:
$C{{H}_{3}}C{{O}_{2}}H < HN{{O}_{2}} < HS{{O}_{4}}^{-}$
$C{{H}_{3}}C{{O}_{2}}H(aq)+{{H}_{2}}O(aq)\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+C{{H}_{3}}C{{O}_{2}}^{-}(aq)$
$HN{{O}_{2}}(aq)+{{H}_{2}}O(l)\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+N{{O}_{2}}^{-}(aq)$
$HS{{O}_{4}}^{-}(aq)+{{H}_{2}}O(aq)\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+S{{O}_{4}}^{2-}(aq)$
${{K}_{a}}=1.8\times {{10}^{-5}}$
${{K}_{a}}=4.6\times {{10}^{-4}}$
${{K}_{a}}=1.2\times {{10}^{-2}}$

Note:
Remember the definitions of acids, strength of acids and the formula for equilibrium. The word ‘relative’ implies that you need to compare the new value (hydronium concentration) to the original value (concentration of acid).