Page 153 - Physical chemistry understanding our chemical world

120 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

We are only allowed (1) We wish to go from the left-hand side of the reaction to the
to make a choice of right-hand side. We can either follow the arrow labelled
route like this because H r , or we pass via the elements (along the bottom line)
enthalpy is a state and thence back up to the ethanol.
function.
(2) If we go along an arrow in the same direction as the arrow
is pointing, then we use the value of H as it is written.
(3) If we have to go along an arrow, but in the opposite direction to the
direction in which it points, then we multiply the value of H by ‘−1’.

In the example here, to go from the left-hand side to the right-hand side via the
elements, we need to go along two arrows H f(1) and H f(3) in the opposite directions
to the arrows, so we multiply the respective values of H and multiply each by −1.
We then go along the arrow H f(2) , but this time we move in the same direction as
the arrow, so we leave the sign of the enthalpy unaltered.
And then we tie the threads together and say:
Note there are three
arrows, so there are H r = (−1 × H f(1) ) + (−1 × H f(3) ) + H f(2)
three H terms within
H r . Inserting values into this equation:

−1
−1
−1
H r = (−1 ×−52 kJ mol ) + (−1 ×−286 kJ mol ) + (−235 kJ mol )
−1
H r = 52 kJ mol −1 + 286 kJ mol −1 + (−235 kJ mol )
so
H r = 103 kJ mol −1

We obtained this value of H r knowing the other enthalpies in the cycle, and
remembering that enthalpy is a state function. Experimentally, the value of H r =
−1
99 kJ mol , so this indirect measurement with Hess’s law provides relatively good
data.
Sometimes, these cycles are considerably harder than the example here. In such
cases, it is usual to write out a cycle for each reaction, and then use the results from
each cycle to compile another, bigger cycle.

How does an ‘anti-smoking pipe’ work?

Hess’s Law Cycles with Enthalpies of Combustion

Smoking causes severe damage to the heart, lungs and respiratory system. The tobacco
in a cigarette or cigar is a naturally occurring substance, and principally comprises
the elements carbon, oxygen, hydrogen and nitrogen.
Unfortunately, because the tobacco is contained within the bowl of a pipe or a paper
wrapper, complete combustion is rare, meaning that the oxidation is incomplete. One