Question
Enthalpy of sublimation of a substance is equal to
A.
enthalpy of fusion + enthalpy of vapourisation
B.
enthalpy of fusion
C.
enthalpy of vapourisation
D.
twice the enthalpy of vapourisation.
Answer :
enthalpy of fusion + enthalpy of vapourisation
Solution :
$$\eqalign{
& {\rm{Solid}}\buildrel {{\rm{Sublimation}}} \over
\longrightarrow {\rm{Vapour}} \cr
& {\rm{This}}\,\,{\rm{process}}\,\,{\rm{occurs}}\,\,{\rm{in}}\,\,{\rm{two}}\,\,{\rm{steps,}} \cr
& {\rm{Step 1:}}\,\,{\rm{Solid}}\buildrel {{\rm{Fusion}}} \over
\longrightarrow {\rm{liquid}} \cr
& {\rm{Step 2: Liquid}}\buildrel {{\rm{Vaporisation}}} \over
\longrightarrow {\rm{Vapour}} \cr} $$
Thus, Enthalpy of sublimation = Enthalpy of fusion + Enthalpy of vaporisation.