Page 95 - Physical chemistry understanding our chemical world

62 INTRODUCING INTERACTIONS AND BONDS

substance undergoing the process. Most chemists prefer data to be
Strictly, this amount presented in the form of energy per mole. In practice, we measure
of energy is liber- accurately the amount of heat energy liberated or consumed by a
ated only when the
temperature remains known amount of steam while it condenses.
◦
at 100 C during the A physical chemist reading from a data book learns that 40.7kJ
−1
condensation process. mol of energy are liberated when 1 mol of water condenses and
Any changes in tem- will ‘translate’ this information to say that when 1 mol (18 g) of
perature need to be steam condenses to form liquid water, bonds form concurrently
considered separately. with the liberation of 40 700 J of energy.
As 40.7kJ mol −1 is the molar energy (the energy per mole), we
can readily calculate the energy necessary, whatever the amount of water involved.
In fact, every time the experiment is performed, the same amount of energy will be
liberated when 18 g condense.

Worked Example 2.3 How much energy is liberated when 128 g of water con-
denses?

Firstly, we calculate the amount of material n involved using
Note the way the units
of ‘g’ cancel,toleave n mass in grams
expressed in the units amount of material n = (2.8)
molar mass in grams per mole
of moles.
so, as 1 mol has a mass of 18 g mol −1
128 g
n = −1
18 g mol
n = 7.11 mol
−1
Secondly, the energy liberated per mole is 40.7kJ mol , so the overall amount of energy
given out is 40.7kJ mol −1 × 7.11 mol = 289 kJ.

SAQ 2.5 How much energy will be liberated when 21 g
Cappuccino coffee is of water condense?
named after Marco
d’Aviano, a ‘Capuchin’
A physical chemist will go one stage further, and say that this
monk who was recently −1
made a saint. He energy of 40.7kJ mol relates directly to processes occurring dur-
entered a looted Turk- ing the condensation process. In this case, the energy relates to the
ish army camp, and formation of hydrogen bonds.
found sacks of roasted As each water molecule forms two hydrogen bonds, so 1 mol of
coffee beans. He mixed water generates 2 mol of hydrogen bonds. The energy per hydrogen
it with milk and honey bond is therefore (40.7kJ mol −1 ÷ 2); so the energy of forming a
−1
to moderate its bit- hydrogen bond is 20.35 kJ mol .
ter flavour. In summary, the macroscopic changes in energy measured in an
experiment such as this are a direct reflection of microscopic energy
changes occurring on the molecular level. The milk of a cappuccino
coffee is warmed when steam passes through it because the steam