Energy and Human Ambitions on a Finite Planet, 2021a

B.3 Chemical <strong>Energy</strong> 379
atoms is exactly 12 grams, that hydrogen is 1 g, <strong>and</strong> oxygen is 16 g. That
means one mole of water molecules (H 2 O) will be 18 g (16 + 1 + 1), one
mole of carbon dioxide (CO 2 )is44g(12 + 16 + 16), <strong>and</strong> one mole of
ethanol (C 2 H 6 O)is46g(12 + 12 + 1 + 1 + 1 + 1 + 1 + 1 + 16). We refer to
this figure as the molar mass, <strong>and</strong> st<strong>and</strong>ard periodic tables display the
molar masses for each element: the mass of one mole of the substance.
The unit is typically grams per mole, or g/mol.
Example B.2.2 How much mass of CO 2 will emerge from the burning
of 1 kg of ethanol? We start with the formula we worked out in
Example B.2.1:
C 2 H 6 O + 3O 2
→ 2CO 2 + 3H 2 O
This problem can be approached in Two equivalent ways: either figure
out how many moles of ethanol it takes to amount to 1 kg <strong>and</strong> then
scale the formula accordingly; or just work it out for one mole to get a
ratio <strong>and</strong> then apply to 1 kg. We’ll do it both ways.
Since ethanol has a molar mass of 46 g, one kilogram corresponds to
21.7 moles. So we could re-write the formula as:
21.7C 2 H 6 O + 65.2O 2 → 43.5CO 2 + 65.2H 2 O
where we have multiplied each prefactor (coefficient) by 21.7. CO 2 has
a molar mass of 44 g/mol, so 43.5 moles will come to 1.91 kg.
The other approach is to note that 2 moles of CO 2 are produced for
every one mole of ethanol combusted. So 88 g of CO 2 (44 g/mol)
results for every 46 g of ethanol supplied. This ratio is 1.91. So 1 kg of
ethanol input will make 1.91 kg of CO 2 out, as before.
B.3 Chemical <strong>Energy</strong>
Atoms (elements) can bond together to make molecules (compounds).
The bond—formed by outer electrons within the atoms—can be strong
or weak. It takes energy 15 to pull apart bonded atoms. It st<strong>and</strong>s to reason
that when two atoms form a new bond, energy is released—usually as
vibrations that we know as heat. In a typical reaction, some bonds are
broken <strong>and</strong> other new ones formed. If the balance is that the new bonds
are stronger than the broken bonds, energy will be released. Otherwise,
energy will have to be put into the reaction to allow it to happen.
15: Recall that energy is a measure of work,
or a force times a distance.
In the context of this book, chemical energy is typically associated with
combustion (burning) a substance in the presence of oxygen. This is true
for burning coal, oil, gas, biofuels, <strong>and</strong> firewood. In a chemistry class,
one learns to look up the energetic properties of various compounds in
tables, combining them according to the stoichiometric reaction formula
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.