while N
is not. The normal boiling points are 77 K for N 2
and 81 K for CO, so the impact 2
of the dipole is not very great in this case. The difference between F
and HCl, which have 2
very similar molar masses, is more dramatic
. The boiling point of polar HCl is over 100 K
higher than that of nonpolar F
even though the molar mass of HCl is slightly less. 2
The effect of hydrogen bonding can be seen by comparing the boiling points of the
hydrogen compounds of the 4A, 5A, 6A, and 7A elements (Figure 7.17). As expected, the boiling points decrease with decreasing molar mass (weaker dispersion forces). This
behavior continues for the 4A elements through the lightest member (CH
), which has the 4
lowest boiling point of the gr
oup. However, the lightest member of each of the other
groups has the highest boiling point in its series because of hydrogen bonding. Thus, based on the boiling points of H
Te, H 2
Se and H 2
S, water should be a gas at room conditions, 2
with a boiling point of around 200 K (-70
oC). Instead, water is a liquid at room conditions
and a solid at 200 K. This is due solely to the effects of H-bonding. Similar conclusions can be drawn for HF and NH
. 3
We referred to Figure 7.15 as a plot of vapor pressure versus temperature, but it can
also be viewed as a plot of boiling point versus pressure. The boiling points of CCland 4
H^2
O at 1 atm are 77
oC and 100
oC, respectively. The relative boiling points indicate the
same relative intermolecular forces as did th
e vapor pressures: the forces are stronger in
water than in carbon tetrachloride. Both have
moderate intermolecular interactions as both
are liquids with low vapor pressures at ro
om temperature. Those forces are strong
dispersion forces in CCl
, and even stronger hydrogen bonds in water. 4
Evaporation requires energy because molecules
in the gas phase are at a much greater
potential energy than those in the liquid, so liquids cool as they evaporate.
† The amount of
energy required to convert a liquid to a vapor at a given temperature is referred to as the heat of vaporization
, Δ
Hvap
, at that temperature. Just as
a solid cannot be warmed above
its melting point, a liquid cannot be heated
above its boiling point. Thus, a vigorously
boiling pot of water is at the same temperatur
e as a slowly boiling one. All of the heat that
is added at the boiling point is used to eva
porate the water not raise the temperature.
When the liquid is gone, the additional heat
increases the temperature of the gas.
HO^2HS^2HSe^2HTe^2HF NH^3CH4HClHBrHIPH3AsH3SbH3SiH4GeH4SnH4306090120(^400300200) Boiling Point (K)^100
Molecular mass
Figure 7.17 Boiling points of the Group 4A - 7A hydrides Boiling points of the hydrogen compounds of the Group 4A (green), 5A (red), 6A (black) and 7A (blue) elements
† Warm blooded animals use evaporation to cool themselves by using
body heat to evaporate a liquid. In
the case of humans, it is the
evaporation of perspiration. In dogs, it is the evaporation of saliva from the tongue.
Gases become increasingly difficult to li
quefy as their thermal energy increases. In
fact,
there is a temperature, called the
critical temperature
, beyond which the liquid
cannot exist. The pressure required to liquefy
a substance at its critical temperature is
called the
critical pressure
. The critical temperature and critical pressure define the
critical point
. Beyond the critical point, the substance behaves like a gas, but its density is
Chapter 7 States of Matter and Changes in State© byNorthCarolinaStateUniversity