Different liquids have widely differing vapor pressures at the same temperature.

Question

Different  liquids  have  widely  differing  vapor  pressures  at  the  same  temperature. Discuss  this range  of  vapor  pressures  in  terms  of  entropy  and  enthalpy,  recalling  that  entropy  changes  arechanges  in  order  (or  disorder),  while  enthalpy  changes  result  principally  from changes  in bonding. Different  liquids  have  widely  differing  vapor  pressures  at  the  same  temperature. Discuss  this range  of  vapor  pressures  in  terms  of  entropy  and  enthalpy,  recalling  that  entropy  changes  arechanges  in  order  (or  disorder),  while  enthalpy  changes  result  principally  from changes  in bonding.

Explanation / Answer

Entropy effect : when he disorder is very high in a system, the attractive forces that tend to bind the molecules together are very low, implying the ease with which a molecule in liquid state can go into vapor state. Similarly, when the order is high in a system, the molecules tend to stay together, reducing the tendency to move out into vapor state. Hence, lower entropy implies lower tendency to move into vapor state. Higher entropy implies higher vapor pressure.

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Enthalpy : If average enthalpy of a system of molecules is lower than the enthalpy of a single molecules, there exist intermolecular and intramolecular forces between molecules that bind them together, reducing the vapor pressure lower limit. If it is the reverse, then the upper limit of vapor pressure range would be higher.

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