Question

The negative potential energy of a particle at liquid surface is less than that of a molecule in bulk.Then why is it said that they have extra energy than molecules in bulk?

Answer 1

Potential energy is the energy by the virtue if its position.The molecules in bulk are bounded by particles in all directions and hence more molecules are exerting force on the molecules in bulk.But in surface,there are no molecules in upper direction of the molecules and hence their potential energy will be less than that of molecules in bulk.

Since potential energy of molecules in bulk are more,their binding energy will also be more than that of surface molecules.

(Binding energy is the minimum energy required to disassemble a system of molecules.)

Since binding energy is less for surface molecules,they are more free to move than that of bulk molecules.So that kinetic energy of surface molecules are more than that of bulk molecules.

This kinetic energy of surface molecules will be greater than that of the potential energy of bulk molecules

Hence,surface mole are said to have more energy.