1. Briony Brundidge AP Chemistry
Chemical Equilibrium
Briony Brundidge
AP Chemistry

2. Key Terms Chemical equilibrium Reversible reaction Law of mass action
Equilibrium constant
Equilibrium position
Equilibrium expression
Homogeneous equilibria
Heterogeneous equilibria
Reaction quotient
Le Chatelier’s principle

3. What is it?
The state where the concentrations of all reactants and products remain constant with time
chemical reaction and its reverse proceed at equal rates

4. Types of Equilibrium Homogeneous: one phase
Normally gas
Heterogeneous: at least 2 phases
solid-liquid, solid-gas, liquid-gas
Gaseous:
involving molar concentrations or partial pressures
Aqueous:
Acid/base
Precipitate
Buffers

5. Chemical Equilibrium
Chemical: bonds breaking and bonds forming (change in chemical composition)
Ex: dissociation of acetic acid
HC2H3O2+ + H2O  H3O+ + C2H3O2-

6. Physical Equilibrium
Physical: change in state (not involving change to chemical properties)
Ex: sealed bottle of water- partly filled
Rate of water evaporating= rate of water condensing

7. Dynamic Equilibrium Rate of forward rxn= rate of reverse rxn
No net change in concentration of products or reactants
Same total number of particles
Both some of products and reactants left over after reaction occurs

9. Law of Mass Action *General description of the equilibrium condition
Law of Mass Action: states that the molecularity of a balanced chemical equation can be used to find a constant
Principle that the rate of a chemical reaction is proportional to the masses of the reacting substances
*General description of the equilibrium condition

10. Equilibrium Constants
aA + bB  cC + dD
Concentrations can be found in Appendix
Kc = [C]c[D]d
[A]a[B]b
Kp = (PC)c (PD)d
(PA)a (PB)b
 Represents coefficient in chemical equation
Concentration of products
Concentration reactants
Partial pressure of products
Partial pressure of reactants

11. Practice
Write the equilibrium constant expression for the following reaction: 4NH3 (g) + 7O2 (g)  4NO2 (g) + 6H2O (g)

12. Calculate the value of the equilibrium constant:
Practice
Calculate the value of the equilibrium constant:
2NH3 (g)  N2 (g) + 3H2 (g)
[NH3]= 3.1 X 10-2 , [N2]= 8.5 X 10-1, [H2]= 3.1 X 10-3

13. More on Keq…
NO Units!!!
The K value of the reverse reaction is the reciprocal of the K value of the original reaction
If rxn is multiplied x2, Keq is squared, etc.
Dependent on temperature
Defined for only 1 temperature

14. Gaseous Equilibrium So… Kp = Kc (RT)Δn(gas)
Molarity is directly related to the pressure for gaseous equilibrium
PV = nRT
P = (n/V)RT
So…
Kp = Kc (RT)Δn(gas)
Δng = change in moles of gas
Products - Reactants

15. Kp = 4.34 x 10-3 calculate Kc at 300°C
Practice
N2 (g) + 3H2 (g)  2NH3 (g)
Kp = 4.34 x 10-3 calculate Kc at 300°C

16. Determining Concentrations at Equilibrium…
Method: use ICE table
I initial
C change
E equilibrium
Additional Method: Neglect x
When k is very small, less than 5%

17. Practice H2(g) + I2(g)  2HI(g)
At a particular temperature, K = 1.00 x 102 for the reaction:
H2(g) + I2(g)  2HI(g)
In an experiment, 1.00mol H2, 1.00mol I2, and 1.00mol HI are introduced into a 1.00-L container. Calculate the concentrations of all species when equilibrium is reached.

18. Practice
At 25°C, Kc, for the following equilibrium is 1.00 x if the initial concentrations of IO3- and H+ are, M and 0.065M respectively what is the equilibrium concentration of IO3-? 7IO3(aq)- + 9H2O(l) + 7H-(aq)  I2(g) + 5H5IO6(aq)

19. Le Chatelier‛s Principle
Equilibrium shifts in the opposite direction of the substance whose concentration is increased and towards the substance of decreasing concentration.
Adding a reactant or product
Increase reactant [ ] = equil shifts right
Increase product [ ] = equil shifts left
Removing a reactant or product
Decrease reactant [ ]= equil shift left
Decrease product [ ] = equil shifts right

20. Common Ion Effect
States that if the concentration of any one of the ions in solution is increased, then according to Le Chat’s principle, the excess number of ions should combine with those of opposite charges

21. Le Chatelier’s Principle
Changing pressure
Adding an inert gas (that isn't involved in the reaction)
No effect on equilibrium position, total pressure, or partial pressures of reactants and products because inert gas already does not participate in reaction
Changing the volume of the container
Decrease in volume = equil shifts right because System decreases its own volume as a result and therefore its total number of gaseous molecules
Can be further proved by ideal gas law
V = (RT/P)n

22. Le Chatelier’s Principle
Adding a catalyst
Decreases energy needed for reaction to occur (activation energy) but does not affect equilibrium
Only speeds up the rate for which a reaction reaches dynamic equilibrium

23. Le Chat’s Principle Changing temperature
Only stress that changes equilibrium constant, K
Equilibrium shifts towards direction that consumes energy
Ex: N2 (g) + 3H2 (g)  2NH3 (g) + 92kj
Exothermic so equilibrium shifts left
Value of K decreases as result of increase in reactant [ ]’s

24. NH4NO2 (s) + O2 (g)  2NO2 (g) + 2H2O(g)
Practice
NH4NO2 (s) + O2 (g)  2NO2 (g) + 2H2O(g)
What is the effect of each of the following on the equilibrium position?
Increase Volume
Decrease [O2]
Increase [NO]
Decrease Temperature
Add inert gas
Increase NH4NO2

25. Reaction Quotient
When the concentration of one of the substances in a reaction is zero, equilibrium shifts towards the direction of the missing component
Hard to determine shift when all of the initial concentrations are zero, so the reaction quotient is used to determine the shift

26. Reactant Quotient If K = Q, then no shift in equilibrium
Ex: N2 (g) + 3H2 (g)  2NH3 (g)
Q = [NH3]02
[N2]0[H2]0
If K = Q, then no shift in equilibrium
If K > Q, then equil shifts right because ratio of [ ] of products to reactants is too large, products favored
If K < Q, then equil shifts left because ratio of [ ] of products to reactants is too small, reactants favored

27. Labs Determination of Equilibrium Constant, K Lab Fun with Cobalt Lab
Finding value of Keq for the reaction after determining the molar [ ]’s of ions present and testing solutions of unknown molar [ ]’s
Fun with Cobalt Lab
Determining effects on the reactants and products, observe shifts in equilibrium

28. Works Cited…