Hess’s Law
Determine the enthalpy change of a reaction that is
the sum of two or three reactions with known enthalpy
Changes.
Hess’s Law

the heat evolved or
absorbed in a
chemical process is
the same whether
the process takes
place in one or in
several steps.
Combustion of methane


∆H1 shows the
magnitude of the
single step
reaction.
∆H2 and ∆H3 when
combined are
equivalent to ∆H1,
demonstrating
Hess’s Law.
Most reactions occur in more than one
step.
 We focus on the OVERALL reaction
based on the balanced chemical
equation, but there could be many
steps involved to reach the final
outcome.

Rule #1

If you reverse an equation the ∆H must
switch signs.

Na (s) + ½ Cl2(g) NaCl (s) ∆H = -411 kJ

NaCl (s)  Na (s) + ½ Cl2(g) ∆H = 411 kJ
Rule #2

If you need to multiply or divide any part of
the equation, the whole equation must follow
the same operation, including ∆H.

Na (s) + ½ Cl2(g) NaCl (s) ∆H = -411 kJ

2 Na (s) + Cl2(g) 2 NaCl (s) ∆H = -822 kJ
Practice
A + B  AB
∆H1 = 20 kJ
 AB + B  AB2 ∆H2 = 50 kJ


What is the ∆H for the overall reaction
A + 2 B  AB2 ∆H3 = ?
∆H3 = 70 kJ
Practice:
Practice: