Question

Why is there a change in enthalpy when water is a liquid in the products (as opposed to a gas)?

a.	Because phase changes require or release heat.

b.	Because there is a chemical change associated with phase changes, which changes the enthalpy.

c.	Because liquid water is less stable than gaseous water.

d.	This is false – the enthalpy does not depend on the phases of the reactants and the products.

Answer 1

Correct answer (a)

When there is phase change, there is change in intermolecular force of attraction. Therefore, when there is phase change some amount of energy is needed or produced but there is no change in chemical structure of the molecules. For example, when liquid water is getting converted into gaseous water vapor then energy is needed because liquid has stronger force of attraction than gaseous vapor. But if gaseous vapor is converted into liquid water then energy will be produced as liquid water has stronger force of attraction between the molecules.