1. Gaseous Chemical Equilibrium Chapter 12

2. 2HI(g) H2(g) + I2(g)
If pure HI(g) is placed in a sealed container at 52 C, H2(g) and I2(g) are formed.
Then the reverse reaction can occur forming HI(g)
The partial pressure of HI(g) drops, slowing the forward reaction, all the while, the partial pressures of H2(g) and I2(g) increase; increasing the rate of the reverse reaction

3. Soon the forward and reverse rates become equal and a dynamic equilibrium is established.
At equilibrium appreciable amounts of all gases are present.
At equilibrium the partial pressures of all gases remain constant.

4. Note: P HI = -2 P H2 = -2 P I2 2HI(g) H2(g) + I2(g) Po(atm) 0.200
0.000
P(atm)
Peq(atm)
0.160
0.020
Note:
P HI = P H2 = -2 P I2

5. Equilibrium constant Kp
Depends only on temperature
Is independent of:
Original pressures
Total pressure
Volume

6. Equilibrium expression
Only gaseous and aqueous species allowed
Products numerator
Reactants denominator
Gases enter as pressures (atm)
Aqueous enter as concentrations [M]
Adding or removing a solid or liquid does NOT affect the position of the equilibrium

7. 2 HI H2 + I2
(g)
(g)
(g)
( )
( )
= Kp
Equilibrium expression
( )

8. Try:
Remember: solids and liquids do not appear in equilibrium expressions
The pressure of CO2 will be the same regardless of how much CaO or CaCO3 is present

9. Try:

10. 2HI(g)
H2(g) +
I2(g)
Po(atm)
0.200
0.000
P(atm)
Peq(atm)
0.160
0.020

11. For the reverse rxn
for

12. If Eqn 3 = Eqn 1+ Eqn 2 then K3 = K1 x K2

13. Applications of K If K is large If K is small Equilibrium moves
Product formation is favored
If K is small
Reactant formation is favored

14. Determining the direction of a reaction
Compare the actual pressure quotient, Q, to K.
If Q < K the forward reaction occurs to make Q larger
If Q > K the reverse reaction occurs to make Q smaller
If Q = K system is at equilibrium nothing happens

15. K = 0.016
Predict the direction of the reaction if HI = I2 = H2 = 0.10 atm
Q > K reaction shifts

16. What is the pressure of all the gases at equilibrium when 0
What is the pressure of all the gases at equilibrium when atm of pure HI is allowed to react?
2HI(g)
H2(g) +
I2(g)
K = 0.016 Po
0.100 atm
0.000 atm P
-2X +X
Peq X X

17. What are the partial pressure of N2O4 and NO2 when 0
What are the partial pressure of N2O4 and NO2 when atm of N2O4 is allowed to come to equilibrium?
N2O4(g)
2NO2(g)
K = 11 Po
0.100 atm
0.000 atm P
- X
+ 2X
Peq X 2X
Quadratic equation
X = 0.097; NO2 = 0.194atm; N2O4 = 0.003atm

18. Le Chatelier’s Principle
If a system at equilibrium is subjected to stress, the reaction occurs in a direction so as to partially relieve the stress.

19. 2HI(g) H2(g) + I2(g) If H2 is added Reaction shifts
P HI is greater than before disturbance
P I2 is less
P H2 is intermediate between original equilibrium value and that immediately after disturbance

20. 2HI(g) H2(g) + I2(g) If H2 is removed reaction shifts
Some HI decomposes to bring P H2 back part way to its equilibrium value
P I2 is increased
P HI decrease

21. Increasing the volume
Immediate effect is to lower the concentration of molecules
To counteract this, reaction occurs in which increases the number of molecules of gas

22. Expansion ( V; P) Contraction ( V; P) N2O4(g) 2NO2(g)
N2(g) + 3H2(g) NH3(g)
H2(g) + I2(g) HI(g)
No effect
No effect

23. Changes in temperature
Endothermic process is favored by increased Temperature
Temperature change is the only change that affects the value of K
If forward reaction is endothermic: K as T
If forward reaction is exothermic: K as T
van’t Hoff equation

24. H T
N2O4(g) NO2(g)
+58.2 kJ
N2(g) + 3H2(g) NH3(g)
- 924 kJ