Section 18.2 Lecture (part B) for Honors & Prep Chemistry

1. Bellwork‐ Equilibrium
The system described by the equa8on
H2O(l)  H2O(g) ΔH= 40.7 kJ
is at equilibrium.
a)How does the concentra8on of H2O gas
change?
b)Does condensa8on occur?
c) Does vaporiza8on occur?
d)What condi8ons must be met for equilibrium
to occur?

2. Bellwork
The system described by the equa8on
H2O(l)  H2O(g) ΔH= 40.7 kJ
is at equilibrium.
a)How does the concentra8on of H2O gas change?
it doesn’t, at equilibrium concentra8ons are constant
b)Does condensa8on occur?
YES
c) Does vaporiza8on occur?
YES, at the same rate that condensa8on occurs
d)What condi8ons must be met for equilibrium to
occur?Closed container, stable temperature, low Ea

3. A system is at equilibrium due to a
balancing of many factors.
Changing the condi8ons of a system at
equilibrium will “shiT” the equilibrium.
A “shiT” is when EITHER
more products are formed
(forward reac8on speeds up)
OR
more reactants are formed
(reverse reac8on speeds up)

4. Le Chatelier’s Principle
When an equilibrium system is
disturbed the equilibrium will shiT to
undo the disturbance.

5. Changing concentra8ons
H2(g) + Cl2(g) ↔ 2HCl(g)
Increasing H2 or Cl2 concentra8on will shiT
the equilibrium to the right (more
products) because this counteracts the
addi8on of reactant.
EXAMPLE‐ Increasing [HCl] will shiT the
equilibrium to the leT (towards reactants).
GETS RID OF extra HCl!
What about decreasing concentra8ons?

6. Le Chatelier’s Principle
Whatever is done to an equilibrium
system will be partly undone.

7. Changing pressure
N2O4(g) ↔ 2NO2 (g)
1 mole of gas ↔ 2 moles of gas
Increase pressure – shiTs leT
(less moles of gas will decrease pressure)
Decrease pressure – shiTs right
(more moles of gas will increase pressure)
UNDER PRESSURE‐ SHIFT TO LESS MOLES
Of gas!

8. Changing pressure only affects
equilibrium if products have a
different number of gas moles than
reactants.
H2O(l)  H2O(g)
0 gas moles 1 gas mole

9. Changing temperature
Write equa8on with energy as a product or
reactant.
Endothermic
Energy + A + B ↔ C + D
Increase temp – favors forward (shiT to the
right) because that uses up the added
energy
Decrease Temp?

10. Changing temperature
Exothermic
A + B ↔ C + D + Energy
Increase temp – favors reverse reac8on
and a shiT to the leT because that uses
up the added energy
Decrease temp? The system will do what
it can to get more energy.

11. A catalyst will not affect the
equilibrium position (a.k.a. shift
the equilibrium).
A catalyst will increase the reverse
reaction rate as much as it increases the
forward reaction rate.
The system will reach equilibrium faster,
but the final concentrations of reactants
and products are not changed by the
addition of a catalyst.

12. Changing concentra8on does not
affect pure liquids or solids.
Changing pressure only affects
gases.

13. 18.
2The equilibrium constant (Keq) is a ra8o
of product concentra8ons to reactant
concentra8ons at equilibrium.
For aA + bB  cC + dD
Keq = [C]c[D]d
[A]a [B]b