Topic: Potential Energy Curve
Do Now: p.26
Spontaneous Processes
=physical or chemical change that occurs with no
outside intervention
Iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s)
H = -1625 kJ
• However, some energy may be supplied to get
process started = activation energy
Combustion
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
H = -891kJ
Can you think of others?
VIDEO
Activation Energy & Reaction
Activation Energy
• = Energy needed to initiate reaction
• Energy needed to overcome reaction
barrier
• difference between where reactants
start & top of hill
• Difference between reactants &
activated complex
Energy Diagram of a Reaction
Activated Complex =
intermediate
formed during conversion
from reactants to products
Reactants combine to form
an unstable complex
Potential Energy Curve: Endothermic
Products have
more P.E. than
reactants
Start low, end
high
Potential Energy Curve: Exothermic
Products have
less P.E. than
reactants
Start high, end
low
Have to label 6 energies on curve:
reactants & products
1.
2.
3.
4.
5.
6.
PE reactants
PE products
PE activated complex
Ea forward reaction
Ea reverse reaction
H
Enthalpy 
40
30
Ea
reverse rxn
Ea
forward rxn
20
10
PE
reactants
PE
activated
complex
PE
products
Time
What kind of reaction is represented?
Enthalpy 
40
30
20
10
H of
reaction
Enthalpy 
40
30
20
PE of
reactants
Ea
forward
rxn
Ea
reverse
rxn
PE of
activated
complex
10
P.E. of
products
Time
What kind of reaction is represented?
Enthalpy 
40
30
20
10
H of
reaction
Catalyst
(in the body = enzyme)
• Substance that increases rate of
reaction without itself being
consumed
– does not participate in reaction
• Lowers the activation energy for the
reaction
DOES IT AFFECT ΔH?
Catalysts do not affect ΔH