Hess's law states that the total enthalpy change for a reaction is independent of the pathway taken to get to the final products. It can be used to calculate enthalpy changes that cannot be measured directly through experimentation. The process involves writing balanced equations for each step of the reaction pathway and using standard enthalpy of formation values of reactants and products to determine the enthalpy change. An example is provided where the enthalpy change for the reaction SO2(g) + 2H2S(g) is calculated to be -234.6 kJ/mol by considering it as occurring in two steps and applying Hess's law.

1. 7.4 HESS’S LAW
OBJECTIVES:
To define Hess’s Law
To use Hess’s Law to find enthalpy changes that we cannot
measure directly
STARTER:
On your piece of paper write
down 1 fact you’ve learnt so
far.
Make it into a paper
aeroplane & fly it to someone
else
KEY WORDS:
ENTHALPY
HESS’S LAW
COMBUSTION
FORMATION

2. Hess’s Law states that the total enthalpy change is
independent of the route taken
2NO2(g)
2NO(g) + O2(g)
N2(g) +2O2(g)
ΔHr = -66.4kJmol-1
Route 1
Route 2
-180.8kJ
+114.4kJ
Route 2
ΔHr = +114.4 + (-180.8) = -66.4kJmol-1
The total enthalpy change for route 1 is the same as
for route 2
7.4 HESS’S LAW
http://www.youtube.com/watch?v=y
01ePN0Hr-Y
This is a
thermo-
chemical
cycle

3. Enthalpy changes of formation are useful for working out
enthalpy changes you can’t find directly by doing an
experiment.
You need to know the ΔHθ
f for all the reactants and
products that are compounds.
The ΔHθ
f for elements is zero – the element is being
formed from the element so there’s no change in enthalpy.
7.4 HESS’S LAW
http://www.youtube.com/watch?v=c8Adft3M8mg

4. SO2(g) + 2H2S(g)
3S(s) + 2H2(g) + O2(g)
3S(s) + 2H2O(l)
ΔHθ
r
Route 1
ΔHθ
f(reactants)
ΔHθ
f(products)
REACTANTS PRODUCTS
ELEMENTS
Step 1: Write the balanced equation for the reaction.
This will be ΔHθ
r
Step 2: Under the equation write a list of the elements
present. This must be balanced
7.4 HESS’S LAW

5. SO2(g) + 2H2S(g)
3S(s) + 2H2(g) + O2(g)
3S(s) + 2H2O(l)
ΔHθ
r
Route 1
ΔHθ
f(reactants)
ΔHθ
f(products)
REACTANTS PRODUCTS
ELEMENTS
Step 3:
ΔHθ
f[SO2(g)] = -297 kJmol-1
ΔHθ
f[H2S(g)] = -20.2 kJmol-1
ΔHθ
f[H2O(l)] = -286kJmol-1
Using Hess’ Law; Route 1 = Route 2
7.4 HESS’S LAW
ΔHθ
f values give
the enthalpy
change going from
the element to the
compound

6. SO2(g) + 2H2S(g)
3S(s) + 2H2(g) + O2(g)
3S(s) + 2H2O(l)
ΔHθ
r
Route 1
ΔHθ
f(SO2) +
2 x ΔHθ
f(H2S)
REACTANTS PRODUCTS
ELEMENTS
Step 3:
ΔHθ
f[SO2(g)] = -297 kJmol-1
ΔHθ
f[H2S(g)] = -20.2 kJmol-1
ΔHθ
f[H2O(l)] = -286kJmol-1
Using Hess’ Law; Route 1 = Route 2
7.4 HESS’S LAW
ΔHθ
f values give
the enthalpy
change going from
the element to the
compound
3 x ΔHθ
f(S) +
2 x ΔHθ
f(H2O)

7. SO2(g) + 2H2S(g)
3S(s) + 2H2(g) + O2(g)
3S(s) + 2H2O(l)
ΔHθ
r
Route 1
-297 +
(2 x -20.2)
REACTANTS PRODUCTS
ELEMENTS
Step 3:
ΔHθ
f[SO2(g)] = -297 kJmol-1
ΔHθ
f[H2S(g)] = -20.2 kJmol-1
ΔHθ
f[H2O(l)] = -286kJmol-1
Using Hess’ Law; Route 1 = Route 2
7.4 HESS’S LAW
ΔHθ
f(s) is zero
because its an
element
(3 x 0) +
(2 x -286)

8. Step 4:
ΔHθ
f(SO2) + ΔHθ
f(H2S) + ΔHθ
r = 3ΔHθ
f(S) + 2ΔHθ
f(H20)
-297 + (2 x -20.2) + ΔHθ
r = (3 x 0) + (2 x -286)
ΔHθ
r = (3 x 0) + (2 x -286) – (-297 + (2 x 20.2))
= -234.6 kJmol-1
SO2(g) + 2H2S(g)
3S(s) + 2H2(g) + O2(g)
3S(s) + 2H2O(l)
ΔHθ
r
Route 1
-297 +
(2 x -20.2)
7.4 HESS’S LAW
(3 x 0) +
(2 x -286)

9. TASKS:
1. Complete the ‘Enthalpy of Formation’ worksheet
2. Complete the Hess’s Law cut and stick exercise
Terry’s Cutting tip: Keep the left column and top row
in one piece
3. Attempt the summary questions on page 124
7.4 HESS’S LAW

10. I CAN…I AM…
How low can you go?? Write what you can do and what
grade this is  show some proof you can do this!
I CAN… I AM…
Define Hess’s Law C
Calculate the enthalpy change for a reaction if
the routes and values have been given
B
Calculate the enthalpy change for a reaction
by creating the route and using enthalpy of
formation values from a data table
A