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Heat of Vaporization

1. Introduction
2. The Equation
3. The Enthalpy of Condensation
4. Definitions:
5. Heat of Vaporization
6. References
7. Inside Linkes
1. 7.1. Changing Liquids and Gases
2. 7.2. Enthalpy
8. Outside Links
9. Contributors

The Heat (or Enthalpy) of Vaporization is the quantity of heat that must be absorbed if a certain quantity of liquid is vaporized at a constant temperature. In a solution with both a vaporized and liquidized states, the kinetic energy of the vapor is higher than the kinetic energy of the liquid. Temperature follows kinetic energy, showing a lower temperature in the remaining liquid.

1. Introduction
2. The Equation
3. The Enthalpy of Condensation
4. Definitions:
5. Heat of Vaporization
6. References
7. Inside Linkes
1. 7.1. Changing Liquids and Gases
2. 7.2. Enthalpy
8. Outside Links
9. Contributors

Introduction

The Enthalpy of Vaporization is measured using constant temperature, meaning that something must be added to bring the lower temperature to the temperature of the solution prior to vaporization. To raise the temperature (and as a result, the kinetic energy), heat is added. And the Enthalpy of Vaporization measures the amount of heat added, because it is the quantity that must be absorbed for a liquid to remain at constant temperature during vaporization.

The ΔH_vap is expressed in kJ/mol.

The Equation

ΔH_vaporization = H_vapor - H_liquid

ΔH_vaporization corresponds to the change in enthalpy of vaporization. It is usually written as ΔH_vap.

H_vapor and H_liquid are the absolute enthalpies of the given gas state and liquid state of a compound or element. Because they are not in the process of changing, it is impossible to measure their values. Therefore, H_liquid and H_vapor simply exist to represent the gas and liquid phase enthalpies.

However, enthalpy is a function of state meaning that under a specific set of conditions, the change in enthalpy of the gas and liquid has a unique value. Absolute enthalpies cannot be measured, but by subtracting the absolute enthalpy of the vapor state and the absolute enthalpy of the liquid state, the given difference has a unique value that can be measured.

Because heat is added to the system to maintain the temperature, vaporization is an endothermic process. This means that the ΔH_vap is always positive.

The Enthalpy of Condensation

Condensation is the opposite of vaporization, and therefore ?H_condensation is also the opposite of ?H_vaporization. Because ?H_vap is an endothermic process, where heat is lost in a reaction and must be added into the system from the surroundings, ?H_condensation is an exothermic process, where heat is absorbed in a reaction and must be given off from the system into the surroundings.

ΔH_condensation = H_liquid - H_vapor = -ΔH_vap

Because ΔH_condensation, also written as ΔH_cond, is an exothermic process, its value is always negative. Moreover, ?ΔH_cond is equal in magnitude to ΔH_vap, so the only difference between the two values for one given compound or element is the positive or negative sign.

Example

2.055 liters of steam at 100°C was collected and stored in a cooler container. What was the amount of heat involved in this reaction? (The ΔH_vap of water = 44.0 kJ/mol.)

Solution

1. First, convert 100°C to Kelvin.

°C + 273.15 = K
100.0 + 273.15 = 373.15 K

2. Find the amount involved (in moles).

n_water = PV/RT = (1.0 atm)(2.055 L)/(0.08206 L atm mol^-1 K^-1)(373.15 K)
n_water = 0.0671 mol

3. Find the ΔH_cond.

ΔH_cond = -ΔH_vap, so
ΔH_cond = -44.0 kJ/mol

4. Using the ΔH_cond of water and the amount in moles, calculate the amount of heat involved in the reaction. To find kJ, multiply the ΔH_cond by the amount in moles involved.

(ΔH_cond)(n_water)
= (-44.0 kJ/mol)(0.0671 mol)
= -2.95 kJ

Definitions:

Vaporization- (or Evaporation) the transition of molecules from a liquid to a gaseous, or vapor, state; the molecules on a surface are usually the first to undergo a phase change.
Enthalpy- the amount of heat consumed or released in a system at constant pressure
Kinetic Energy- the energy of a moving object measured in Joules
Δ- change
Endothermic- when heat is added to a system from the surroundings, due to a decrease in heat in a reaction
Exothermic- when heat from a system is given off into its surroundings, due to the increase in heat in a reaction
Heat of Vaporization- the amount of heat required to evaporate a liquid
System and its Surroundings- the system is the area in which a reaction takes place and the surroundings is the area that interacts with the system
Condensation- the transition of molecules from a gaseous or vapor state to a liquid
State Function- (or Function of State) any property, such as temperature, pressure, enthalpy or mass, that has a unique value or number for the specific state of a system