Warm Up
2-5-16
1. Is combustion a reversible reaction?
2. Write the equilibrium constant for the
reaction
Fe3O4 (s) + 4 H2 (g)
3 Fe(s) + 4 H2O(g)
3. When a reversible reaction reaches
equilibrium, the forward reaction rate is
______ as the reverse reaction rate
Agenda
Homework
-Turn in posters
-Notes Unit 7-2
-Lab Equilibrium
Finish lab equilibrium
UNIT 7-2
LE CHATELIER’S PRINCIPLE
LE CHATELIER’S PRINCIPLE
When system at
equilibrium is
disturbed, the system
adjusts in a way to
reduce the change
and eliminate it
(restoring balance)
All Stressed Out
• Stress is anything that causes a change in
equilibrium that throws it off balance
Causes of Stress
• Changes in concentration
– Adding or taking away the reactants or
products will cause the system to shift
Causes of Stress
• Changes in Temperature
– Depends on whether the forward reaction
is endothermic or exothermic
– Exothermic- heat is a product
CaO + H2O  Ca(OH)2 + HEAT
– Endothermic- heat is a reactant
2Fe2O3 + 3C + HEAT  4Fe + 3CO2
Which direction will it shift if heat is added?
Causes of Stress
• Changes in Pressure
– Not a cause of stress for reactions in
solutions
– Only gases are affected
Example: N2(g) + 3H2(g) ⇌ 2NH3(g)
4 moles ⇌ 2 moles
Decrease
Pressure
Increase
Pressure
Causes of Stress
• Changes in Pressure
– Increases in pressure will shift the reaction
to the side with less total molecules
– Decreases in pressure will shift reactions to
the side with more total molecules
Check for Understanding
2H2 (g) + O2 (g)
2H2O (g)
1. Which way will it shift if you add H2 to the
system?
2. Which way will it shift if you decrease H2O?
3. Which way will it shift if you decrease the
pressure?
4. Which way will it shift if you decrease
temperature?
Lab Equilibrium/Paper Folding
1. Each group will need to cut out about 70-80
squares (1x1 inch)
2. Person #1 fold the paper so that the shaded
side is showing
3. Person #2 fold the paper so that the
unshaded side is showing
4. DO NOT RESTART after 30 seconds, just
keep folding
5. Stop every 30 seconds to count your pieces
of paper
Assignments
-Notes Summary
-Lab equilibrium