# Enthalpy change and change in state

Enthalpy change and change in state:
Phase change- when the physical state of a substance changes. While the phase changes occur,
the temperature stays the same until the phase change is complete.

Changes in state involve changes in potential energy of a system only. The temperature
of the system undergoing the state change remains constant during a state change.
Because energy is absorbed or released as heat, however, the temperature of the
surroundings often changes.

Changes of state involve breaking the intermolecular forces between molecules which is
much weaker than chemical bonds

Less energy is released/absorbed than a chemical reaction

Enthalpy change of state between liquid and gas are greater than the changes of state
between liquid and solids. This is because bonds between substances are completely
broken from a liquid to a gas. Remember: solids have strong bonds, liquids are loosely
bonded, and the formation of gases is when the bonds are completely broken.
ΔHvap- the enthalpy change for the state change of one mole of substance from a liquid to a
gas.
ΔHcond- the enthalpy change for the state change of one mole of substance from a gas to a
liquid
ΔHmelt- the enthalpy change for the state change of one mole of substance form a solid to a
liquid. Also referred to as the molar heat of fusion.
ΔHfreeze - the enthalpy change for the state change of one mole of substance from a liquid to a
solid. Also known as the molar heat of solidification.
Sublimation- the transfer from solid to a gas without entering the liquid phase. Ex: iodine
For the same substance...
ΔHmelt= - ΔHfreeze
ΔHvap= - ΔHcond
Example:
H2O(s) →H2O(l) ΔHmelt= 6.02KJ
H2O(l) →H2O(s) ΔHfreeze= -6.02KJ