Equilbrium Constant and EXTERNAL EFFECTS

1
Equilbrium Constant and EXTERNAL EFFECTS

• Formation of stalactites and stalagmites
• CaCO3(s) H2O(liq) CO2(g)fg Ca2(aq) 2
  HCO3-(aq)

2
EQUILIBRIUM

• Temperature, catalysts, and changes in
  concentration/ pressure affect equilibria.
• The outcome is governed by LE CHATELIERS
  PRINCIPLE
• ...if a system at equilibrium is disturbed, the
  system tends to shift its equilibrium position to
  counter the effect of the disturbance.

3
Equilibrium constant and Concentration

• Concentration changes
• no change in K
• only the position of equilibrium changes.

4
Butane-Isobutane Equilibrium
butane
isobutane
5
Butane Isobutane
butane
isobutane

• At equilibrium with iso 1.25 M and butane
  0.50 M. K 2.5.
• Add 1.50 M butane.
• When the system comes to equilibrium again, what
  are iso and butane?

6
Butane e Isobutane

• Solution
• Calculate Q immediately after adding more butane
  and compare with K.

Q is LESS THAN K. Therefore, the reaction will
shift to the ____________.
7
Butane e Isobutane

• Q is less than K, shifts right
• toward isobutane.
• Set up ICE table
• butane isobutane
• Initial
• Change
• Equilibrium

0.50 1.50
1.25
- X X
2.00 x 1.25 x
8
Butane e Isobutane
x 1.07 M At the new equilibrium position,
butane 0.93 M and isobutane 2.32 M.
Equilibrium has shifted toward isobutane.
9
Equilibrium Constant and Catalyst

• Add catalyst NO change in K
• A catalyst only affects the RATE it approach
  equilibrium.

Catalytic exhaust system
10
Pressure and EquilibriumN2O4(g) e 2 NO2(g)

• Increase P in the system by reducing the volume
  (at constant Temp).

e
11

• N2O4(g) e 2 NO2(g)
• Increase P in the system by reducing the volume.
• In gaseous system the equilibrium will shift to
  the side with fewer molecules (in order to reduce
  the P).
• Therefore, reaction shifts LEFT and P of NO2
  decreases and P of N2O4 increases.

12
Temperature Effects on Equilibrium
Figure 16.6
13
Temperature Effects on Equilibrium

• N2O4 (colorless) heat
• e 2 NO2 (brown) ?Ho 57.2
  kJ (endo)

Kc (273 K) 0.00077 Kc (298 K) 0.0059
14
Every T has a unique K

• Temperature change change in K
• Consider the fizz in a soft drink
• CO2(aq) HEAT e CO2(g) H2O(l)
• K P (CO2) / CO2
• Increase T. What happens to equilibrium position?
  To value of K?
• K increases as T goes up because P(CO2) increases
  and CO2 decreases.
• Decrease T. Now what?
• Equilibrium shifts left and K decreases.

15
NH3 Production

• N2(g) 3 H2(g) e 2 NH3(g) heat
• K 3.5 x 108 at 298 K

16
Le Chateliers Principle

• Change T - changes K
• causes change in P or concentrations at
  equilibrium
• Use a catalyst K not changed.
• Reaction comes more quickly to equilibrium.
• Add or take away reactant or product K does not
  change
• Reaction adjusts to new equilibrium position

17
Examples of Chemical Equilibria

• Phase changes such as H2O(s)
  H2O(liq)

e