Page 186 - Physical chemistry understanding our chemical world

THE EFFECT OF PRESSURE ON THERMODYNAMIC VARIABLES 153

disparity in air speed leads to a difference in pressure. In effect, a partial vacuum forms
above the paper, which ‘sucks’ the paper upwards. This is known as the Bernoulli effect.
A similar effect enables an aeroplane to fly: the curve on a plane’s wing is carefully
designed such that the pressure above the wing is less than that below. The air flows over
the upper face of the wing with an increased speed, leading to a decrease in pressure.
Because the upward thrust on the underside of the wing is great (because of the induced
vacuum), it counterbalances the downward force due to gravity, allowing the plane to
stay airborne.

How does a laboratory water pump work?

Gibbs function of pressure change

The water pump is another example of the Bernoulli effect, and is These highly oversim-
an everyday piece of equipment in most laboratories, for example plified explanations
being used during B¨ uchner filtration. It comprises a piece of rubber ignore the effects of
tubing to connect the flask to be evacuated to a pump. Inside the turbulent flow, and the
pump, a rapid flow of water past one end of a small aperture inside formation of vortices.
the head decreases the pressure of the adjacent gas, so the pressure
inside the pump soon decreases.
Gas passes from the flask to the pump where the pressure is The minimum pres-
sure achievable with
lower. The change in Gibbs function associated with these pressure
a water pump equals
changes is given by
the vapour pressure of
water, and has a value

p (final)
G = RT ln (4.39) of about 28 mmHg.
p (initial)
G is negative for the
where the G term represents the change in G per mole of gas. We
physical process of gas
will say here that gas enters the pump at pressure p (final) from a flask
moving from higher to
initially at pressure p (initial) . Accordingly, since p (final) <p (initial) lower pressure.
and the term in brackets is clearly less than one, the logarithm term
is negative. G is thus negative, showing that gas movement from
Since Equation (4.39)
a higher pressure p (initial) to a lower pressure p (final) is spontaneous.
relies on a ratio of pres-
It should be clear from Equation (4.39) that gas movement in
sures, we say that a
the opposite direction, from low pressure (p (final) ) to high (p (initial) )
gas moves from ‘higher’
would cause G to be positive, thereby explaining why the process
to ‘lower’, rather than
of gas going from low pressure to high never occurs naturally. ‘high’ to ‘low’.
Stated another way, compression can only occur if energy is put
into the system; so, compression involves work, which explains Compression involves
why pumping up a car tyre is difficult, yet the tyre will deflate of
work.
its own accord if punctured.