Page 177 - Physical chemistry understanding our chemical world

144 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

SAQ 4.4 1 mol of oxygen is warmed from 300 K to 350 K. Calculate the
associated rise in entropy S if C p(O 2 ) /JK −1 mol −1 = 25.8 + 1.2 × 10 −2 T/K.

4.3 Introducing the Gibbs function

Why is burning hydrogen gas in air (to form liquid
water) a spontaneous reaction?

Reaction spontaneity in a system

Equation (4.19) describes the reaction occurring when hydrogen
The ‘twin’ subscript gas is burnt in air:
of ‘l and g’ arises
because the reaction
O 2(g) + 2H 2(g) −−→ 2H 2 O (l and g) (4.19)
in Equation (4.19) is so
exothermic that most
We notice straightaway how the number of moles decreases from
of the water product three to two during the reaction, so a consideration of the system
will be steam.
alone suggests a non-spontaneous reaction. There may also be a
concurrent phase change from gas to liquid during the reaction,
which confirms our original diagnosis: we expect S to be negative, and so we
predict a non-spontaneous reaction.
But after a moment’s reflection, we remember that one of the simplest tests for
hydrogen gas generation in a test tube is to place a lighted splint nearby, and hear the
‘pop’ sound of an explosion, i.e. the reaction in Equation (4.19) occurs spontaneously.
The ‘system’ in this example comprises the volume within which chemicals com-
bine. The ‘surroundings’ are the volume of air around the reaction vessel or flame;
because of the explosive nature of reaction, we expect this volume to be huge. The
surrounding air absorbs the energy liberated during the reaction; in this example, the
energy is manifested as heat and sound. For example, the entropy of the air increases
as it warms up. In fact, S (surroundings) is sufficiently large and positive that the value
of S (total) is positive despite the value of S (system) being negative. So we can now
explain why reactions such as that in Equation (4.19) are spontaneous, although at
first sight we might predict otherwise.
But, as chemists, we usually want to make quantitative predictions, which
are clearly impossible here unless we can precisely determine the magnitude of
S (surroundings) , i.e. quantify the influence of the surroundings on the reaction, which
is usually not a trivial problem.

How does a reflux condenser work?

Quantifying the changes in a system: the Gibbs function

All preparative chemists are familiar with the familiar Liebig condenser, which we
position on top of a refluxing flask to prevent the flask boiling dry. The evaporating