Page 70 - Physical chemistry understanding our chemical world

2

Introducing interactions

and bonds

Introduction

We look first at deviations from the ideal-gas equation, caused by inter-particle
interactions. Having described induced dipoles (and hydrogen bonds) the interaction
strengths are quantified in terms of the van der Waals and virial equations of state.
Next, formal bonds are described, both covalent (with electrons shared between
participating atoms) and ionic (in which electrons are swapped to form charged ions;
these ions subsequently associate in response to electrostatic forces). Several under-
lying factors are expounded, such as ionization energy I and electron affinity E (ea) .
The energy changes occurring while forming these interactions are alluded to, but are
treated properly in Chapter 3.

2.1 Physical and molecular interactions

What is ‘dry ice’?

Deviations from the ideal-gas equation

We call solid carbon dioxide (CO 2 ) ‘dry ice’. To the eye, it looks
Substances sublime if
just like normal ice, although it sometimes appears to ‘smoke’;
they pass directly from
see below. Carbon dioxide is a gas at room temperature and only
asolid to form agas
solidifies (at atmospheric pressure) if the temperature drops to about without being a liquid
◦
−78 C or less, so we make dry ice by cooling gaseous CO 2 below
as an intermediate
its freezing temperature. We call it dry ice because, unlike normal phase; see Chapter 5.
ice made with water, warming it above its melting temperature
leaves no puddle of liquid, because the CO 2 converts directly to a gas. We say
it sublimes.
Gases become denser as we lower their temperature. If CO 2 was still a gas at
◦
3
−90 C, then its molar volume would be 15 200 cm . In fact, the molar volume of