Chemical Thermodynamics — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryChemical Thermodynamics2 Marks
Question
Explain the enthalpy of solution of an ionic compound.
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Answer
An ionic compound dissolves in a polar solvent like water in two steps as follows :
Step-I : The ions are separated from the molecule involving crystal lattice enthalpy $\Delta_L H$. $MX _{( s )} \rightarrow M _{( g )}^{+}+ X _{( g )}^{-} \quad \Delta_{ L } H$ $\Delta_{ L } H$ is always positive.
Step-II : The gaseous ions are hydrated with water molecules involving hydration energy, $\Delta_{\text {hyd }} H$. $\begin{aligned} & M _{( g )}^{+}+ xH _2 O _{( l )} \rightarrow\left[ M \left( H _2 O \right)_{ x }\right]^{+} \\ & X _{( g )}^{-}+ yH _2 O _{(l)} \rightarrow\left[ X \left( H _2 O \right)_{ y }\right]^{-} \end{aligned}$ $\Delta_{\text {hyd }} H$ is always negative. The enthalpy change $\Delta$ solnH of solution is given by, $\Delta_{\text {soln }} H=\Delta_L H +\Delta_{\text {hyd }} H$ For example, consider enthalpy of solution of $NaCl ( s )$. $\Delta_{ L } H _{ NaCl }=790 kJ mol ^{-1}$ $\Delta_{\text {hyd }} H _{ NaCl }=-786 kJ mol ^{-1}$ Hence enthalpy change for solution of $NaCl ( s )$ is, $\begin{aligned} & \Delta_{\text {soln }} H =\Delta_L H _{ NaCl }+\Delta_{\text {hyd }} H _{ NaCl } \\ & =790+(-786) \\ & =+4 kJ mol ^{-1} \end{aligned}$ Therefore dissolution of $NaCl$ in water is an endothermic process.
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