Unit 13 LP02PS - Le Chatelier Princ v06.ppt

PPT

1. 1
Types of Equilibrium and
Le Chateliers Principle
Mr. Shields Regents Chemistry U13 L02

2. 2
Equilibrium reactions can be categorized as falling into 1 of 3
types:
1)Phase Equilibrium
2)Solution Equilibrium
3)Chemical Equilibrium
Chemical Equilibrium reactions can be further categorized
as either
1) Homogeneous or
2) Heterogeneous

3. 3
Types of Equilibrium
A homogeneous equilibriumis one in which the states of
Matter for all reactants and products are the same
Example: H
2(g) + I
2(g) 2HI (g)
A heterogeneous equilibriumis one in which the states of
Matter varybetween reactants and products
Example: C (s) + O
2(g) CO
2(g)

4. 4
Phase Equilibrium
Phase equilibrium exists between two phases of matter
(S, L, G) usually in a closed vessel. They are easily recognizable:
1) H
2O(s) ↔H
2O(l) ice –water equilibrium (def. of MP)
2) H
2O (l) ↔H
2O(g) Water –Vapor equilibrium
3) I
2(s) ↔I
2(g) Iodine sublimation
1 above occurs at 0 deg
2 & 3 above must be
Sealed systems! Why?
DYNAMIC EQUILIBRIUM

5. 5
Solution Equilibrium
Besides The dynamic equilibrium of a liquid vapor (or
Gas) and a liquid, solution equilibriums are created …
-When additional solid is added to an already
saturated solution
For a solid in a saturated
solutionat equilibrium:
The rate at which solids
re-crystallize from sol’n &
the rate at which Crystals
dissolve must be equal
(Saturated sol’n)

6. 6
Solution Equilibrium
In our example using NaCl we would write the equilibrium as:
NaCL (s) + H
20 ↔Na
+
(aq) + Cl
-
(aq)
If the solid is molecular (for example sugar) then the
Following equation would define the equilibrium condition:
C
12H
23O
11(s) + H
20 ↔C
12H
23O
11(aq)
Notice there are No IONSformed for molecularcompounds.

7. 7
Solution Equilibrium
For GASESdissolved in Liquidsin a SEALEDsystem
The rate at which the gas leavesthe liquid and the
Rate at which the gas re-dissolvesmust be equal to
be in equilibrium.
O
2(aq) ↔O
2(g) Why a sealed system?
But this equilibrium can be affected by shifts in Tand/or P
-if T increases the equilibrium shifts to the right 
and
-If P increases the equilibrium shifts to the left 
WHY ?

8. 8
Le Chatelier’s Principle
This principle describes how a system at
equilibrium reacts to changes (stresses) in
system conditions.
Le Chatelier’s principle:
If a change is imposed on a system at equilibrium, then the
system will shift the directionof the reaction to reduce
the change in conditions.
In other words, When a stressis imposed
on a system the system will respond to
counteractthe stress
Henri Le Chatlier
1850-1936)

9. 9
Le Chatelier’s Principle
For example: consider the rxn … A + B ↔C + D + Heat
If the above reaction proceeds to the right, we producemore
C, D and heat.
If the reaction proceeds to the leftwe consume heat, and
Produce more A and B
If we consume heat what does Le Chatelier’s principle predict?
Le Chatelier’s Principle says if we remove heat the system will
Try to return to equilibrium conditions by producing heatto
Replacethe heat consumed. The Rxn will be Driven to the right!

10. 10
Factors affecting Equilibrium
When considering Le Chatelier’s principle there are Several
Factorsthat will affect equilibrium:
1) Adding or Removing HEAT
2) Increasing or decreasing PRESSURE
3) Changing the CONCENTRATION …
By adding, decreasing OR totally removing one or
one or more of the reactants/products

11. 11
Head to Tail Method
We can use Le Chatelier’s principle to predicthow Chemical
Reactions will respondto external forces.
It sounds like this might be difficult to do but there’s an
Easy way to determine how an equilibrium will react to stress.
It’s called the “Head to Tail” methodand its used to
determine the Direction of the shiftas a stress is applied
To a system in equilibrium.
There are 3 simple rules …
1
2
3

12. 12
Head to Tail Method
Draw the 1
st
ARROW next to the component that is
changing. Draw it upwards if it is increasing and downward
If it is decreasing
Lets say we increase the heatin a system in which there
Is an equilibriumchemical reaction occurring
A + B ↔C + D + Heat
Draw a second arrow from the 1
st
arrow going across
the TOPof “↔”. Each arrow must go from “Head to Tail”
A + B ↔C + D + Heat
A + B ↔C + D + Heat
Rule 1:
Rule 2:

13. 13
Head to Tail Method
The directionof the second arrow indicates the
Direction the equilibrium reaction shiftswhen the stressis
Applied.
A + B ↔C + D + Heat
Rxn Shifts in this direction
Of course we could also have predicted this using Le
Chatelier’s principle. If we add stressto the system by
increasingheat the system responds by trying to opposethe
Stress. Reaction moves to the left because that direction
Consumesheat.
Rule 3:

14. 14
Problem:
In the following reaction the concentration of C is
decreased. Which way will the equilibrium reaction shift?
A + B ↔C + D + Heat
Ans:
A + B ↔C + D + Heat
Reaction shifts towards the rightin
Response to the stress (decreasing concentration of C)
to try to increasethe concentration of C back to it’s
Equilibrium value

15. 15
We’ve discussed how to determine the affects of
Changing 2 of the three factors that affect Equilibrium
Namely using le Chatlier’s principle:
1)Changing temperature
2)Changing concentration
The effect of the last factor, PRESSURE, we’ll leave
For discussion in the next lesson.