What is the relationship between molar heat of vaporization and molar heat of condensation?

• Define heat of vaporization.
• Define heat of condensation.
• Perform calculations involving heats of vaporization and condensation.

How much energy is available?

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Heat of Vaporization and Condensation

Energy is absorbed in the process of converting a liquid at its boiling point into a gas. As with the melting of a solid, the temperature of a boiling liquid remains constant and the input of energy goes into changing the state. The molar heat of vaporization  

When 1 mol of water at 100°C and 1 atm pressure is converted to 1 mol of water vapor at 100°C, 40.7 kJ of heat are absorbed from the surroundings. When 1 mol of water vapor at 100°C condenses to liquid water at 100°C, 40.7 kJ of heat are released into the surroundings.

Other substances have different values for their molar heats of fusion and vaporization and these are summarized in the Table below .

Substance	(kJ/mol)	(kJ/mol)
Ammonia (NH 3 )	5.65	23.4
Ethanol (C 2 H 5 OH)	4.60	43.5
Methanol (CH 3 OH)	3.16	35.3
Oxygen (O 2 )	0.44	6.82
Water (H 2 O)	6.01	40.7

Notice that for all substances, the heat of vaporization is substantially higher than the heat of fusion. Much more energy is required to change the state from a liquid to a gas than from a solid to a liquid. This is because of the large separation of the particles in the gas state. The values of the heats of fusion and vaporization are related to the strength of the intermolecular forces. All of the substances in the Table above , with the exception of oxygen, are capable of hydrogen bonding. Consequently, the heats of fusion and vaporization of oxygen are far lower than the others.

Sample Problem: Heat of Vaporization

What mass of methanol vapor condenses to a liquid as 20.0 kJ of heat are released?

Step 1: List the known quantities and plan the problem .

Known

Unknown

First the kJ of heat released in the condensation is multiplied by the conversion factor of 

Step 2: Solve .

Step 3: Think about your result.

Condensation is an exothermic process, so the enthalpy change is negative. Slightly more than one-half mole of methanol is condensed.

Summary

• Molar heats of condensation and vaporization are defined.
• Examples of calculations involving these parameters are illustrated.

Calculate problems 6-7 at the link below:

http://ths.sps.lane.edu/chemweb/unit4/problems/heatcalc/index.htm

Review

Questions

1. What is common to all the substance in the table except oxygen?
2. How much heat is needed to convert 2.7 moles of ethanol at its boiling point from liquid to vapor?
3. How many moles of water will condense from vapor to liquid if 45 Kj are removed?

• molar heat of condensation
  
  : The heat released by one mole of a substance as it is converted from a gas to a liquid.
• molar heat of vaporization
  
  : The heat absorbed by one mole of a substance as it is converted from a liquid to a gas.

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