Page 258 - Physical chemistry understanding our chemical world

PHASE EQUILIBRIA INVOLVING VAPOUR PRESSURE 225

the spark has been applied, so the explosions cease to be synchro-
nized. It is clearly undesirable for a cylinder to fire out of sequence, We experience knock-
ing (which we collo-
since the kinetic energy is supplied in a jerky, irreproducible man-
quially call ‘pinking’)
ner. The engine sounds dreadful, hence the word ‘knock’.
when explosions within
Modern petrol contains small amounts of additives to inhibit this
a car engine are not
knocking. ‘Leaded’ petrol, for example, contains the organometallic
synchronized.
compound lead tetraethyl, PbEt 4 . Although PbEt 4 is excellent at
stopping knocking, the lead by-products are toxic. In fact, most
Lead tetraethyl is the
EU countries now ban PbEt 4 .
most widely made
So-called ‘green’ petrol is a preferred alternative to leaded petrol:
organometallic com-
it contains about 3 per cent of the aromatic hydrocarbon benzene
pound in the world.
(C 6 H 6 , IV) as an additive, the benzene acting as a lead-free alter- It is toxic, and killed
native to PbEt 4 as an ‘anti-knocking’ compound. over 40 chemical work-
ers during its early
development.

(IV)

The PbEt 4 in petrol does not smell much because it is not volatile. By contrast,
benzene is much more volatile – almost as volatile as petrol. The vapour above ‘green’
petrol, therefore, contains quite a high proportion of benzene (as detected by its
cloying, sweet smell) as well as gaseous petrol. That is why green petrol has a
sweeter smell than petrol on its own.

Why do some brands of ‘green’ petrol smell different
from others?

Raoult’s law

The ‘petrol’ we buy comprises a mixture of naturally occurring
In the countries of
hydrocarbons, a principal component of which is octane; but the
North America, petrol is
mixture also contains a small amount of benzene. Some brands of
often called ‘gas’, which
petrol contain more benzene than others, both because of varia- is short for gasoline’.
tions in the conditions with which the crude oil is distilled into
fractions, and also variations in the reservoir from which the crude
oil is obtained. The proportion varies quite widely: the average is
presently about 3 per cent.
Petrol containing a lot of benzene smells more strongly of ben-
Raoult’s law is merely a
zene than petrol containing less of it. In fact, the intensity of
special form of Henry’s
the smell is in direct proportion to the amount of benzene in the
law.
petrol: at equilibrium, the pressure of vapour above a liquid mixture