Every spontaneous process leads to the formation of new products. Of all the processes known to us, some absorb energy while other result in evolution of energy. Hence, we always experience a change in enthalpy upon accomplishment of processes. This change in enthalpy can be due to the enthalpy of atomization, solution etc. Some common enthalpy changes are explained below:
Enthalpy of Atomization:
Enthalpy of atomization, ΔaH0, is the change in enthalpy when one mole of bonds is completely broken to obtain atoms in the gas phase. For example: atomization of methane molecule.
CH4 (g) → C (g) + 4H (g) ΔaH0= 1665.0 kJ mol-1
For diatomic molecules, enthalpy of atomization is equal to the enthalpy of bond dissociation. For example: atomization of dihydrogen molecule.
H2 (g) → 2H (g); ΔaH0= 435.0 kJ mol-1
Enthalpy of solution:
Enthalpy of solution, ΔsolH0 is the enthalpy change when one mol of a substance is completely dissolved in a solvent. For example:enthalpy of dissolution of ionic compound in water.
Enthalpy change during phase transition:
When a substance undergoes a phase transition, that is the phase of a substance changes from one form to another, some energy is released or absorbed. For example when ice melts to water, energy is required for melting. Common enthalpy change during phase transition includes:
Standard Enthalpy of vaporization:
Standard enthalpy of vaporization, ΔvapH0 is the amount of heat required to vaporize one mole of a liquid at constant temperature and under standard pressure (1bar).
Standard enthalpy of vaporization:
Standard enthalpy of vaporization, ΔvapH0is the amount of heat required to vaporize one mole of a liquid at constant temperature and under standard pressure (1bar).
Standard enthalpy of sublimation:
Standard enthalpy of sublimation,ΔsubH0is the change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1bar).
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