Boiling point Formula

Definition: Boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure, and this pressure in general is considered to be 1 atm (also known as standard pressure).

Physically, the boiling point is the temperature in which the molecules presents in a liquid had the kinetic energy enough to break the intermolecular attractive interaction of the liquid and escape to the vapor phase.

Formula: the boiling point can be measure with the enthalpy of vaporization, which is the defined as the quantity of heat consumed or released by a system for promoting the change of phase from liquid to gas. The enthalpy of vaporization is:

ΔH_vap = H_vapor − H_liquid and

Where, ΔH_vap is the changee of enthalpy of vaporization, H_vap is the enthalpy or heat of the gas state of a compound and H_liquid is the enthalpy of the liquid state of a compound.

Use: The boiling point is an important parameter for characterizing substance. It is a physical property that is different for every compound or substance, thus it can be used as an identification parameter. It is always useful and almost obligatory to know the boiling of substances before making any chemical reaction or synthesis. Particularly, the boiling point of solvents because it determines the temperature of the reactional system.

Example : The enthalpy of the 1 mol of liquid water is 40.66 kJ/mol and the heat of 1 mol of gas water is -8 kJ/mol. Calculate the change for the enthalpy of vaporization.

ΔH_vap = H_vapor − H_liquid = 40.66 kJ/mol - (-8 kJ/mol) = 48.66 kJ/mol

Considerations: The magnitude of the boiling point of the liquids is directly related to the chemical structure of the molecules. For example, alcohol, water and other liquids (with boiling points of 80 -100 C), which can form hydrogen bonds with oxygen and hydrogen atoms, present stronger intermolecular attractive interactions, it is because the hydrogen bonding keeps molecules bend. Consequently, the energy required to break these interactions and allows the molecules escapes to the vapor phase, is high. Contrary, molecules with zero hydrogen bonding interactions and another weak intermolecular interaction as dipole-dipole interaction (for example diethyl ether with a bp of 35 C), have low boiling points.