Page 94 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 61

absorption of the energy from the cheese ‘overcomes’ the interactions – i.e. breaks
them – and enables the CFC to convert from its liquid form to form a gas:

CFC (l) + energy −−→ CFC (g) (2.6)

The fridge pump circulates the CFC, so the hotter (gaseous) CFC is removed from
the fridge interior and replaced with cooler CFC (liquid). We increase the pressure of
the gaseous CFC with a pump. The higher pressure causes the CFC to condense back
to a liquid. The heat is removed from the fridge through a so-called heat exchanger.
Incidentally, this emission of heat also explains why the rear of a
fridge is generally warm – the heat emitted is the energy liberated Forming bonds and
when the cheese cooled. interactions liberates
In summary, interactions form with the liberation of energy, but energy; breaking bonds
adding an equal or greater amount of energy to the system can and interactions re-
break the interactions. Stated another way, forming bonds and inter- quires the addition
actions liberates energy; breaking bonds and interactions requires of energy.
the addition of energy.
A similar mechanism operates at the heart of an air-conditioning mechanism in a
car or office.

Why does steam warm up a cappuccino coffee?

Forming a bond releases energy: introducing calorimetry

To make a cappuccino coffee, pass high-pressure steam through a cup of cold milk
to make it hot, then pour coffee through the milk froth. The necessary steam comes
from a kettle or boiler.
A kettle or boiler heats water to its boiling point to effect the process:

(2.7)
H 2 O (l) + heat energy −−→ H 2 O (g)
Water is a liquid at room temperature because cohesive forces bind together the
molecules; the bonds in this case are hydrogen bonds – see p. 44. To effect the phase
transition, liquid → gas, we overcome the hydrogen bonds, which explains why we
must put energy into liquid water to generate gaseous steam. Stated another way,
steam is a high-energy form of water.
Much of the steam condenses as it passes through the cool milk. This condensation
occurs in tandem with forming the hydrogen bonds responsible for
the water being a liquid. These bonds form concurrently with the The word ‘calorimetry’
liberation of energy. This energy transfers to the milk, explaining comes from the Latin
why its temperature increases. calor,which means
Calorimetry is the measurement of energy changes accompany- heat.Weget theword
ing chemical or physical changes. We usually want to know how ‘calorie’ from the same
much energy is liberated or consumed per unit mass or mole of root.