Barrons SAT Subject Test Chemistry, 13th Edition

Adding

yields

Simplification gives the net equation:

TIP

Know how to use Hess’s Law.

The principle underlying the preceding calculations is known as Hess’s Law
of Heat Summation. This principle states that, when a reaction can be expressed
as the algebraic sum of two or more other reactions, the heat of the reaction is the
algebraic sum of the heats of these reactions. This is based upon the First Law of
Thermodynamics, which, simply stated, says that the total energy of the universe
is constant and cannot be created or destroyed.
These laws allow calculations of ΔH’s that cannot be easily determined

experimentally. An example is the determination of the ΔH^0 f of CO from the ΔH^0 f

of CO 2.

The calculation for the above example is shown below using ΔH^0 that can
easily be found.

The equation wanted is

To get this, we reverse the second equation and add it to the first: