Title: Lesson 3 Hess’s Law and Enthalpy of
Formation and Combustion
Learning Objectives:
– Define Hess’s Law
– Using calculations show that enthalpies for formation of products are the
same regardless of the route taken
– Calculate change in enthalpy of reactions using enthalpy of formation or
combustion data
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
How much energy, in joules, is required to increase the
temperature of 2.0 g of aluminium from 25 to 30 °C?
(Specific heat of Al = 0.90 J g–1 K–1).
A.
B.
C.
D.
0.36
4.5
9.0
54
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Calculations linked to Enthalpy Cycles
(Hess’s Law)

Enthalpy of Formation
Enthalpy of Combustion
Bond Enthalpy

You must know the difference between these!


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Hess’ Law

The enthalpy change of a reaction is independent of the pathway of that reaction
 i.e. All that matters is the start and finish points
The first law of thermodynamics relates to the conservation of energy. It is sometimes
expressed in the following form: Energy cannot be created or destroyed, it can
only change form.
A+B
∆H1
C+D
∆H2
∆H3
E+F
A+B
∆H2
E+F
∆H1 = ∆H2 + ∆H3

∆H1
C+D
∆H4
∆H3
G+H
∆H1 = ∆H2 - ∆H3 + ∆H4
Note: add when going ‘with’ an arrow, subtract when going against an arrow.
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Why is this useful

It is not always possible to directly measure the enthalpy
change we want.




It may be an endothermic reaction that needs a constant heat
supply
It may be that the reaction doesn’t ‘stop’ where you need it to
It may be that the reaction is simply too slow
Hess cycles allow us to measure enthalpy changes
indirectly
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EXAMPLE OF CYCLE 1
HESS’S LAW
THE TOTAL ENTHALPY CHANGE OF A REACTION
IS INDEPENDENT OF THE REACTION ROUTE.
REACTANTS
H1
HR
Alternative route
PRODUCTS
H2
INTERMEDIATES
By Hess’s Law: HR = H1 + H2
Direct measurement of
ΔHR may not be possible
because :
1. Reaction incomplete
2. Other reactions occur
3. Reaction too slow
Intermediates = elements if ΔHf known,
oxides if ΔHC known,
or gaseous atoms if E[X-Y] (Bond Enthalpy) known
EXAMPLE OF CYCLE 2
HESS’S LAW
We can consider situations where the conversions are the other way round. In this
case the intermediate will convert to both reactants and products. (Look at the
direction of the arrows!)
HR
REACTANTS
H1
PRODUCTS
Alternative route
H2
INTERMEDIATES
Therefore By Hess’s Law:
Pathways:
I  R = H1
I  P = H2
HR = -H1 + H2
Intermediates = elements if ΔHf known,
oxides if ΔHC known,
or gaseous atoms if E[X-Y] (Bond Enthalpy) known
EXAMPLE OF CYCLE 3
HESS’S LAW
We can consider situations where the conversions are the other way round. In this
case the reactants and products will both convert to intermediates. (Look at
the direction of the arrows!)
HR
REACTANTS
H1
PRODUCTS
Alternative route
H2
INTERMEDIATES
Therefore By Hess’s Law:
Pathways:
R  I = H1
P  I = H2
HR = H1 + -H2
Intermediates = elements if ΔHf known,
oxides if ΔHC known,
or gaseous atoms if E[X-Y] (Bond Enthalpy) known
STANDARD ENTHALPY CHANGE OF FORMATION, H f
The heat change when ONE MOLE of a substance is
P
C
FORMED from its ELEMENTS in their standard states
at 298K and 100kPa
NB
Hof [ELEMENT in its standard state] = ZERO
ELEMENTS
Q
STANDARD ENTHALPY CHANGE OF COMBUSTION, HC
The heat produced when ONE MOLE of a
P
substance is burned in excess oxygen measured
C
at 298K and 100kPa
Q
The average heat needed when ONE MOLE of
P
covalent bonds are broken, measured in the
C
gaseous state at 298K and 100kPa
GAS ATOMS
BOND ENTHALPY or BOND ENERGY E[X-Y]
OXIDES
Q
7.4 HESS’S LAW
Hess’s Law states that the total enthalpy change is
independent of the route taken
2NO2(g)
ΔHr = -66.4kJmol-1
N2(g) +2O2(g)
Route 1
+114.4kJ
-180.8kJ
Route 2
2NO(g) + O2(g)
This is a
thermochemical
cycle
Route 2
ΔHr = +114.4 + (-180.8) = -66.4kJmol-1
The total enthalpy change for route 1 is the same as
for route 2
http://www.youtube.com/watch?v=y
01ePN0Hr-Y
Enthalpy Cycles and
Hess’s Law
Think of a hike to your nice wood cabin in the French Alps.
The wood cabin is your destination – but you can either walk through
the forest or over the mountains. Either way you will still end up at
the cabin!
Calculate the potential energy of each
climber taking route 1 and route 2
+87KJ
+102KJ
-193KJ
+125KJ
-163KJ
+52KJ
-147KJ
-269KJ
+7KJ
Regardless of the route the climber and the miner
took they ended up having the same amount of
potential energy!!
-137KJ
Using Hess’s Law

Although carbon and hydrogen do not combine directly to form propane,
C3H8, the enthalpy change for the reaction is:

This can be calculated from the enthalpy of combustion data of the
elements and the compound

The steps in the cycle may be hypothetical and may refer to reactions that
do not take place. The only requirement is that the individual chemical
reactions in the sequence must balance.
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Solutions
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Complete the Hess’s Law Worksheet
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