Intermolecular Forces

Changes of State


Changes of State

The three states of matter include

In general, matter in one state can be changed into either of the other two states.

Such transformations are called "phase changes"

Energy Changes Accompanying Changes of State

Each change of state is accompanied by a change in the energy of the system

The melting process for a solid is also referred to as fusion

The heat needed for the vaporization of a liquid is called the heat of vaporization (DHvap)

Less energy is needed to allow molecules to move past each other than to separate them totally

Vaporization requires the input of heat energy.

Heating Curves

The heating of ice at -25 C to +125 C at constant pressure (1 atm) will exhibit the following characteristics

Note: greater energy is needed to vaporize water than to melt it

Heating ice, water and water vapor

In the region of the curve where we are not undergoing a phase transition, we are simply changing the temperature of one particular phase of water (either solid, liquid or gas) as a function of heat input

In the regions of the curve where we are undergoing a phase transition, the heat energy input is not raising the temperature of the sample, rather it is being used to disrupt the intermolecular forces


Calculate the enthalpy change for converting 2 moles of ice at -25C to +125C.


Critical Temperature and Pressure

Gases can be liquified by either decreasing the temperature or increasing the pressure

The highest temperature at which a substance can exist as a liquid is called its critical temperature

The critical pressure is the pressure required to bring about condensation at the critical temperature

For example, oxygen has a critical temperature of 154.4 K. It cannot be liquefied until the temperature is reduced to this point. At this temperature, the pressure needed to liquefy oxygen is 49.7 atm.


1996 Michael Blaber