1. Equilibrium
Chemistry 30

2. Equilibrium in Chemical Systems
Reactions do not always go forward to completion (reactants do not all react)
Instead reach equilibrium: concentration of reactants and products becomes constant (but not equal)
Use an arrow in the equation to show equilibrium

3. Ex)
Colorless Violet Colorless

4. Dynamic equilibrium – a balance between the forward and reverse reactions occurring at the same rate with constant macroscopic properties (temp, ph, color etc).

5. Although rates are the same, concentrations of reactants and products are different at equilibrium
equilibrium is reached when concentrations stop changing

6. Demonstration of simulated chemical equilibrium

7. Two Types of Equilibrium
Phase change equilibrium
Solid in a saturated solution ex) sugar and sugar water
Gas above a liquid ex) pop
Vapour above a solid ex) mothballs in a dresser

8. Chemical equilibrium Ex) CaCO(s) CaO(s) + CO2(g)
Homogeneous: reactants and products are in the same phase or state
Ex) C2H5OH(l) + CH3COOH(l)   CH3COOC2H5(l) + H2O(l)
Heterogeneous: reactants and products are in different phases or states
Ex) CaCO(s)   CaO(s) + CO2(g)

9. 4 Conditions for Equilibrium
Forward rate and reverse rate of reaction are the same
Macroscopic properties are constant
Closed system
Equilibrium can be approached from either direction

10. Classes of Chemical Reaction Criteria
Percent Reaction
Description at Equilibrium
Position at equilibrium
<50%
Reactants favored
>50%
Products favored
>99%
No eq’b

11. Equilibrium Constant
Ratio of the product and reactant concentrations is a constant value.
This value is called the equilibrium constant Kc

12. Finding Kc For the reaction: aA + bB cC + dD Kc= [C]c[D]d = products
[A]a[B]b reactants
*Only substances whose concentrations or pressures change can be included in the equilibrium expression. Liquids and solids are not included.

13. If K>1 the reaction favors products
If K<1 the reaction favors reactants
If K=1 there are equal concentrations of reactants and products

14. Calculations with Equilibrium Constants
Case 1 – Given all the amounts of species at equilibrium, calculate the equilibrium constant.
Case 2 – Given the initial and final conditions, calculate the equilibrium constant.

15. In each case, you will need:
The balanced chemical equation for the system at equilibrium
The equilibrium constant expression (remember to include only those that are solutions or gases, not liquids or solids)

16. Case 1 Examples
Calculate the Kc for the following system at a constant temperature:
2NO(g) + O2(g) NO2(g)
[NO]: 1.0 mol/L [O2]: 6.0 mol/L [NO2]: 2.0 mol/L

17. For the reaction CO(g) + Cl2(g) COCl2(g) at
a particular temperature, Kc = 5.1 x 105. At
equilibrium, there are 0.30 mol of Cl2(g) and 0.072
mol of COCl2(g) in a 2.0L container. What is the equilibrium
concentration of CO(g)?

18. Case 2 Examples Use the ICE tables I= initial C=change E= equilibrium
Need to use molar ratio as well

19. For the reaction given below, 3. 00 mol of A and 4
For the reaction given below, 3.00 mol of A and mol of B are placed in a 5.00 L container.
2A(g) + B(g) C(g)
At equilibrium, the concentration of A is 0.40 mol/L. Determine the value of Kc.

20. Initially 2. 0 mol of N2 and 4. 0 mol of H2 were added to a 1
Initially 2.0 mol of N2 and 4.0 mol of H2 were added to a 1.0 L container and the following reaction occurred.
3H2(g) + N2(g) NH3(g)
The equilibrium concentration of NH3 is 0.68 mol/L. What is the Kc?

21. Le Chatelier’s Principle
when a system at equilibrium is disturbed by a change in a property, the system adjusts in a way that opposes this change.
Change can occur to:
Concentration
Pressure or volume
Temperature (this will also change Kc)
Add a Catalyst

22. The application of Le Chatelier’s Principle involves three stages:
The initial state of equilibrium in the system
Stress exerted on the system by some change in property (temp, conc, pressure) that creates dis- equilibrium
Shift towards reactants or products, to re-establish a new state of equilibrium
Le Chatelier’s Principle can provide a method of predicting the reaction’s response to the change.
y/flash/lechv17.swf

23. Concentration Changes
The addition of more reactant or the removal of a product will increase the yield of the product, shifting the equilibrium towards the right (products)
A + B C
If the opposite occurs, the shift will be towards the left (reactants).

24. Concentration change can occur by adding something that makes a precipitate.
Ex) AgNO3(s) Ag+(aq) + NO3-(aq)
If you add NaCl, the Cl-(aq) reacts with Ag+ (aq) to form AgNO3(s)
Response is to shift to the right
AgNO3(s) Ag+(aq) + NO3-(aq)

25. 2. Pressure / Volume Change
Gases only
If pressure decreases, response is to increase pressure by shifting to the side with more moles of gas
Ex) Decrease pressure and :
A(g) + 2B(g) C (g)

26. In a gaseous system, decreasing the volume of the container increases the pressure.
Adding more gas increases the pressure
Pressure change has no effect on equilibrium if moles are the same on both side.

27. 3. Temperature Change
The energy in a chemical reaction is treated as if it were a reactant or product.
Endothermic: reactants + energy products
Exothermic: reactants products + energy
Energy can be added or removed by heating or cooling the system. Equilibrium shifts to minimize the change.
Will change Kc

28. Ex) 2 SO3(g) + 97 kJ 2 SO2(g) + O2(g)
If you heat it up (the stress), response is to cool down and equilibrium shifts right. Kc increases
If you cool it down (the stress), response is to heat up and equilibrium shifts left. Kc decreases.

29. 4. Catalyst Decreases time to reach equilibrium
Lowers activation energy by the same amount in both the forward and reverse directions
Does not effect final equilibrium position

30. Summary Variables Change Response of the system Concentration Increase
Shift to consume added reactant or product
Decrease
Shift to replenish some of the removed reactant or product
Temperature
Shifts to consume some of the added heat
Shifts to replenish some of the removed heat
Volume (overall pressure)
Increase volume / decrease pressure
Shifts toward the side with the larger number of gaseous molecules
Decrease volume / increase pressure
Shifts toward the side with the smaller number of gaseous molecules