Page 83 - Physical chemistry understanding our chemical world

50 INTRODUCING INTERACTIONS AND BONDS

and boiling points depend directly on the strength of intermolecular bonds, the overall
strength of the London forces varies as the molecule becomes larger.

Aside

When is a dispersion force sufficiently strong that we can safely call it a hydrogen bond?
Hydrogen bonds are much stronger than London dispersion forces for two princi-
pal reasons:

(1) The induced dipole is permanent, so the bond is permanent.
(2) The molecule incorporates a formal H–X covalent bond in which X is a
relatively electronegative element (see p. 42).

We call an interaction ‘a hydrogen bond’ when it fulfils both criteria.

2.2 Quantifying the interactions
and their influence

How does mist form?

Condensation and the critical state

Why is it that no dew forms if the air pressure is low, however cool the air tempe-
rature?
To understand this question, we must first appreciate how molecules come closer
together when applying a pressure. The Irish physical chemist Thomas Andrews
(1813–1885) was one of the first to study the behaviour of gases as they liquefy:
most of his data refer to CO 2 . In his most famous experiments, he observed liquid CO 2
at constant pressure, while gradually raising its temperature. He readily discerned a
clear meniscus between condensed and gaseous phases in his tube at low temperatures,
◦
but the boundary between the phases vanished at temperatures of about 31 C. Above
this temperature, no amount of pressure could bring about liquefaction of the gas.
Andrews suggested that each gas has a certain ‘critical’ tem-
The ‘critical temper- perature, above which condensation is impossible, implying that
ature’ T (critical) is that no liquid will form by changes in pressure alone. He called this
temperature above temperature the ‘critical temperature’ T (critical) .
which it is impossible Figure 2.8 shows a Boyle’s-law plot of pressure p (as y) against
to liquefy a gas. volume V (as x) for carbon dioxide. The figure is drawn as a
function of temperature. Each line on the graph represents data
obtained at a single, constant temperature, and helps explain why we call each line
◦
an isotherm. The uppermost isotherm represents data collected at 31.5 C. Its shape is
essentially straightforward, although it clearly shows distortion. The middle trace (at