Liquid-Vapor Equilibrium (Intermolecular Forces and Liquids and Solids) Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

Evaporation and Condensation Greatest Order Least Focus on Evaporation and Condensation Write an equation for the equilibrium between liquid water and its vapor: Is evaporation an endothermic or exothermic process? Evaporation Condensation 11.8

Before Evaporation At Equilibrium 11.8

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H2O (l) H2O (g) Rate of condensation evaporation = Dynamic Equilibrium 11.8

Factors affecting the vapor pressure SURFACE AREA increasing surface area only means equilibrium is reached faster TEMPERATURE

What is the name of this diagram? Why is it important? T2 > T1 What is the name of this diagram? Why is it important? 11.8

Vapor Pressure Versus Temperature Patterns? Why? 11.8

Enthalpy of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point. Enthalpy of vaporization : physical process (phase change) as ______________________: chemical reaction 11.8

Vapor Pressure Versus Temperature The relationship between vapor pressure and temperature can be expressed mathematically – NOT req’d by IB ln P = - DHvap RT + C Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K•mol) C = integration constant Vapor Pressure Versus Temperature 11.8

Why are there plateaus on this graph? Heating Curve Why are there plateaus on this graph? 11.8

The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. 11.8

What are the three primary intermolecular forces? List in order of increasing strength.

Enthalpy of vaporization Intermolecular Forces The boiling point and enthalpy of vaporization are influenced by intermolecular forces… Compound Enthalpy of vaporization in kJ mol-1 Boiling Point in K Intermolecular Forces methane CH4 9.0 109 methoxy CH3OCH3 27.2 248 ethanol CH3CH2OH 38.6 352

State the relationship between intermolecular forces and enthalpy of vaporization. State the relationship between intermolecular forces and boiling point.

A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water 11.9

Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water 11.9