1
Meeting 19
Last time we saw how to measure “heats” of
reaction, Δ
H
rxn
using calorimetry.
For some reactions it is impossible to use
this technique.
Example:
2C(Gr)) + O
2
(g) → 2CO(g); Δ
H
= ?
The reaction cannot be stopped at CO, and
CO
2
(g) forms. What to do?
Calculate
Δ
H
from reactions whose
Δ
H
rxn
s
are
known
.
Use Hess’s Law of Constant Heat Summation
:
The heat of a reaction is constant, whether the
reaction is carried out directly or via a number
of steps
.
Δ
H
overall
=
Σ
Δ
H
’s of individual reactions
Reverse an equation, change the sign of
Δ
H.\
Multiply an equation’s coefficients, multiply Δ
H
by
the same coefficient for that reaction

This
** preview**
has intentionally

**sections.**

*blurred***to view the full version.**

*Sign up*