Question
$$\eqalign{
& HA + O{H^ - } \to {H_2}O + {A^ - } + {q_1}\,\,kJ \cr
& {H^ + } + O{H^ - } \to {H_2}O + {q_2}\,\,kJ \cr} $$
The enthalpy of dissociation of $$HA$$ is
A.
$$\left( {{q_2} - {q_1}} \right)$$
B.
$$\left( {{q_1} - {q_2}} \right)$$
C.
$$\left( {{q_1} + {q_2}} \right)$$
D.
$$ - \left( {{q_1} + {q_2}} \right)$$
Answer :
$$\left( {{q_1} - {q_2}} \right)$$
Solution :
$${q_2}$$ is heat of neutralization of strong acid with strong base and $${q_1}$$ is that of weak acid $$HA$$ with strong base. The difference gives the heat of dissociation of $$HA$$ i.e.,
$$\eqalign{
& \Delta H = \Delta {H_2} - \Delta {H_1} \cr
& \,\,\,\,\,\,\,\,\,\,\, = \left( { - {q_2}} \right) - \left( { - {q_1}} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\, = {q_1} - {q_2} \cr} $$