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Explain enthalpy changes during phase transformations. |
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Answer» Solution :Standard enthalpy of fusion `(Delta_("fus") H^(Theta) )` (molar enthalpy of fusion) : The enthalpy change that accompanies melting of one mole of a solid substance in standard state is called standard enthalpy of fusion or molar enthalpy of fusion, `Delta_("fus") H^(Theta)` `H_(2) O_((s) ) to H_(2) O_((l)) , Delta_("fus")H^(Theta) = 6.00 "kj mol"^(-1)` Melting of a solid is endothermic, so all enthalpies of fusion are positive. Water requires heat for evaporation at constant temperature of its boiling point `T_(B)` and at constant pressure. Standard enthalpy of vaporization or molar enthalpy of vaporization `(Delta_("vap") H^( Theta) ) :` Amount of heat required to VAPORIZE one mole of a liquid at constant temperature and under standard pressure (1 bar) is called its standard enthalpy of vaporization or molar enthalpy of vaporization `Delta_("vap")H^( Theta)` `H_(2) O_((l)) to H_(2) O_((g)), Delta_("vap")H^(Theta) = +40.79 "kj mol"^(-1)` Standard enthalpy of sublimation `(Delta_("SUB") H^(Theta) ):` Standard enthalpy of sublimation is the change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1 bar). Sublimation is direct conversion of a solid into its vapour. `CO_(2(s)) to (CO_(2 (g)) Delta_("sub") H^( Theta)= 25.2 "kj mol"^(-1)` Solid `CO_(2)` or .dry ice. sublimes at 195 K with `Delta_("sub")H^( Theta) = 25.2 kj mol^(-1)`. Naphthalene sublimes slowly and for this `Delta_("sub") H^( Theta) = 73.0 "kj mol"^(-1)` The magnitude of the enthalpy change depends on the strength of the intermolecular interactions in the substance undergoing the phase transformations. The strong hydrogen bonds between water molecules hold them tightly in liquid phase. For an organic liquid, such as acetone, the intermolecular dipole-dipole interactions are significantly weaker. Thus, it requires less heat to vaporize 1 mol of acetone than it does to vaporize 1 mol of water. Standard Enthalpy CHANGES of Fusion and Vaporisation 
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