Hess 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