5.111 Lecture Summary #19 Friday, October 24, 2014
Reading for Today: Sections 10.9-10.13 (Sections 9.9-9.13 in 4th ed.)
Reading for Lecture #20: Sections 9.8 - 9.13 (8.8-8.13 same in 4th ed.) on solubility;
Sections 11.1 – 11.2, 11.4-11.6 (10.1-10.2, 10.4-10.6 in 4th ed.) on acids and bases.
Topics: I. External effects on K (Le Châtelier’s Principle) continued
II. Temperature dependence of K
III. Applications of Le Le Châtelier’s Principle
IV. Sig figs for logs
I. EXTERNAL EFFECTS ON K (LE CHÂTELIER’S PRINCIPLE) CONTINUED
A. CHANGING THE VOLUME OF A GASEOUS SYSTEM
A decrease in the volume of a gaseous system FDXVHVDQLQWKHWRWDO
pressure.
I '($/*$6/$:ZKHUH3LVSUHVVXUH9LV9ROXPe,
T is temSHUDWXUHQLV5LVWKHLGHDOJDVFRQVWDQW
Le Châtelier's principle predicts that the system would respond, if possible, in such a
way as to reduce the total pressure.
Example 2P2 (g) P4 (g)
expand compress
i) A decrease in volume shifts the reaction to the right (toward product).
This change occurs because for every 2 molecules of P2 consumed only 1 molecule of P4 is
formed.
A shift to the right reduces the total pressure, partially compensating for the external
stress of the volume change.
1
, Now consider in terms of Q and K.
Suppose the volume is decreased by a factor of 2 at constant temperature.
This change will increase the partial pressure of P2 by 2 and of P4 by 2, initially.
PP
4 2 1
Q = 2
= =
(PP ) 22 2
2
Q decreases by a factor of 2, and Q < K. ∆ *LV
Reaction proceeds in forward direction (toward products) until Q=K again.
ii) An increase in volume shifts the reaction to the left (toward reactants).
For every one molecule of P4 that is consumHGPolecules of P2 are formed.
Thus a shift of the reaction toward reactants, increases the total pressure.
B. ADDING INERT GAS
Case 1) For 2P2 (g) P4, what happens if an inert gas is added to the container
increasing the total pressure at constant temperature?
$QVZHU
Why?
Q depends on the partial pressure of P2 and P4 gases and the partial pressures do not
change here.
Review Partial Pressure: The partial pressure is the pressure that each gas would exert if
it alone were present in the container.
1 atm 1 atm 2 atm
O2 N2 mixture of O2 and N2
PA = nA RT
V
Ptot = P A + PB + P C + etc = (nA + nB + nC + etc) RT = ntot RT
V V
2
Reading for Today: Sections 10.9-10.13 (Sections 9.9-9.13 in 4th ed.)
Reading for Lecture #20: Sections 9.8 - 9.13 (8.8-8.13 same in 4th ed.) on solubility;
Sections 11.1 – 11.2, 11.4-11.6 (10.1-10.2, 10.4-10.6 in 4th ed.) on acids and bases.
Topics: I. External effects on K (Le Châtelier’s Principle) continued
II. Temperature dependence of K
III. Applications of Le Le Châtelier’s Principle
IV. Sig figs for logs
I. EXTERNAL EFFECTS ON K (LE CHÂTELIER’S PRINCIPLE) CONTINUED
A. CHANGING THE VOLUME OF A GASEOUS SYSTEM
A decrease in the volume of a gaseous system FDXVHVDQLQWKHWRWDO
pressure.
I '($/*$6/$:ZKHUH3LVSUHVVXUH9LV9ROXPe,
T is temSHUDWXUHQLV5LVWKHLGHDOJDVFRQVWDQW
Le Châtelier's principle predicts that the system would respond, if possible, in such a
way as to reduce the total pressure.
Example 2P2 (g) P4 (g)
expand compress
i) A decrease in volume shifts the reaction to the right (toward product).
This change occurs because for every 2 molecules of P2 consumed only 1 molecule of P4 is
formed.
A shift to the right reduces the total pressure, partially compensating for the external
stress of the volume change.
1
, Now consider in terms of Q and K.
Suppose the volume is decreased by a factor of 2 at constant temperature.
This change will increase the partial pressure of P2 by 2 and of P4 by 2, initially.
PP
4 2 1
Q = 2
= =
(PP ) 22 2
2
Q decreases by a factor of 2, and Q < K. ∆ *LV
Reaction proceeds in forward direction (toward products) until Q=K again.
ii) An increase in volume shifts the reaction to the left (toward reactants).
For every one molecule of P4 that is consumHGPolecules of P2 are formed.
Thus a shift of the reaction toward reactants, increases the total pressure.
B. ADDING INERT GAS
Case 1) For 2P2 (g) P4, what happens if an inert gas is added to the container
increasing the total pressure at constant temperature?
$QVZHU
Why?
Q depends on the partial pressure of P2 and P4 gases and the partial pressures do not
change here.
Review Partial Pressure: The partial pressure is the pressure that each gas would exert if
it alone were present in the container.
1 atm 1 atm 2 atm
O2 N2 mixture of O2 and N2
PA = nA RT
V
Ptot = P A + PB + P C + etc = (nA + nB + nC + etc) RT = ntot RT
V V
2
