1. Chemical equilibrium Chapter 18
I will discover that many reactions and processes reach a state of chemical equilibrium.
I will use Le Chatelier’s Principle to explain how various factors affect chemical equilibrium.
I will calculate equilibrium concentrations of reactants and products using the equilibrium constant expression.
I will determine the solubilities of sparingly soluble ionic compounds.

2. Equilibrium: A state of dynamic balance
I will recognize the characteristics of chemical equilibrium.
I will write equilibrium expressions for systems that are at equilibrium.
I will calculate equilibrium constants from concentration data.

3. completion Reaction goes to completion Rarely happens
When a reaction results in almost complete conversion of reactants to products
Rarely happens

4. Reversible reaction Most reactions Reversible reaction
Do NOT go to completion
Appear to stop
Are reversible
Reversible reaction
One that can occur in both the forward and the reverse directions
Denoted with a double arrow to show that both reactions occur
Forward reaction = reactants on left
Reverse reaction = reactants on right

5. Reversible Reactions

6. Rate of reaction Depends on concentration of the reactants
N2 (g) + 3 H2 (g) < -- > 2NH3 (g)
The concentrations of reactants decrease at first.
The concentrations of the product increases at first.
Then, before all the reactants are used up, all concentrations become constant.

7. Chemical equilibrium
A state in which the forward and reverse reactions balance each other because they take place at equal rates
Rate forward reaction = Rate reverse reaction
Concentrations of reactants and products are constant
HOWEVER!
The amounts or concentrations of reactants and products
Are NOT usually equal
MAY even differ by a factor of a million or more!

8. Equilibrium expressions and constants
Majority of reactions reach equilibrium with varying amounts of reactants unconsumed
NOT all our predicted moles of product gets produced
Law of Chemical Equilibrium
At a given temperature, a chemical system may reach a state in which a particular ratio of a reactant and product concentrations has a constant value
aA + bB <--> cC + dD

9. Equilibrium Constant
The numerical value of the ratio of product concentrations to reactant concentrations
Constant only at a SPECIFIC TEMPERATURE
Products on top, reactants on bottom
Keq > 1: MORE products than reactants at equilibrium
Keq < 1: LESS products than reactants at equilibrium
Keq = [C]c[D]d
[A]a[B]b

10. HOMOgeneous equilibrum
All the products and reactants are in the same physical state
Must use ALL CONCENTRATIONS for Keq

11. Heterogeneous Equilibrium
Reactants and products of a reaction are present in more than one physical state
Do NOT count concentrations of solids or liquids when calculating Keq
Can be OMITTED from the Keq expressions
If a solid or liquid state of a substance is present in addition to the gas state….LABEL the gas concentration in your expression to distinguish between the two

12. Factors affecting chemical equilibrium
I will describe how various factors affect chemical equilibrium.
I will explain how Le Chatelier’s Principle applies to equilibrium systems.

13. Le Chatelier’s Principle
Apply stress to a system at equilibrium
System will shift in the direction that relieves the stress
Stress
Any kind of change in a system at equilibrium that UPSETS equilibrium
Types:
Change in concentration
Change in volume (pressure)
Change in temperature

14. Change in Concentration
Changes equilibrium POSITION
Shifts left or right
Does NOT change equilibrium constant (Keq)
Add reactant = shift right
Remove reactant = shift left
Add product = shift left
Remove product = shift right

15. Change in Volume (Pressure)
Changes equilibrium POSITION
Shifts left or right
ONLY if # moles of gaseous reactants is DIFFERENT than # moles gaseous products
Does NOT change equilibrium constant (Keq)
Volume Pressure

16. Change in Volume (Pressure)
Decrease volume (increase pressure)
Situation 1: more moles gas reactants & less moles gas products
Shift right
Situation 2: moles gas reactants = moles gas products
NO shift
Situation 3: less moles gas reactants & more moles gas products
Shift left
4 moles gas reactants > 2 moles gas products
2 moles gas reactants = 2 moles gas products
2 moles gas reactants < 3 moles gas products

17. Change in Temperature Changes equilibrium POSITION
Shifts left or right
CHANGES equilibrium constant (Keq)
Large Keq = more product in equilibrium mixture
Small Keq = less product in equilibrium mixture

18. Change in Temperature Exothermic- releases heat
Reactants <--> Products + heat
Endothermic- absorbs heat
Reactants + heat <--> Products
Hot
-∆H° (lose heat)
Forward reaction = exo, backward = endo
↑temp = shift left, ↓Keq
↓temp = shift right, ↑Keq
Cold
+∆H° (gain heat)
Forward reaction = endo, backward = exo
↑temp = shift right, ↓Keq
↓temp = shift left, ↑Keq
ADDING HEAT to an exothermic reaction causes STRESS
REMOVING HEAT to an endothermic reaction causes STRESS

20. Catalysts Speeds up a reaction
Speeds it up EQUALLY in BOTH directions (Right & Left)
Helps a reaction reach equilibrium quickly
But NO CHANGE in the AMOUNT of PRODUCT formed

21. Using equilibrium constants
I will determine equilibrium concentrations of reactants and products.
I will calculate the solubility of a compound from its solubility product constant.
I will explain the common ion effect.

22. Using Equilibrium constants
Review:
Large Keq = Products favored
Small Keq = Reactants favored
Knowing the size of the Keq helps a chemist
Decide whether a reaction is practical for making a particular product
Calculate the equilibrium concentration of ANY substance involved in the reaction
(if the concentrations of all other reactants/products are known)

23. Calculating Equilibrium Concentrations
Write the equilibrium constant (Keq) expression
Solve the equation for the unknown (using algebra skills)
Substitute in all known concentrations and the Keq value
Use calculator to find unknown concentration
Chemists would then use this concentration to determine if enough of their desired unknown could be produced in the reaction

24. Calculating Equilibrium Concentrations

25. Solubility Equilibria
Ionic Compounds
Some dissolve readily in water
Ex NaCl(s)
High solubility
Some barely dissolve at all
Ex BaSO4(s)
low solubility

26. Solubility Product Constant
An equilibrium constant for the dissolving of a sparingly soluble ionic compound in water
Ksp = the product of the concentrations of the ions each raised to the power equal to the coefficient of the ion in the chemical equation
Small Ksp = Products NOT favored at equilibrium

27. Solubility Product Constant
Example: (Remember it depends ONLY on [ IONs])

28. Review: Solubility 𝑆= 𝑀𝑜𝑙𝑒𝑠 𝐿 Solubility in water
The amount of the substance (moles) that will dissolve in a given volume of water (Liter)
𝑆= 𝑀𝑜𝑙𝑒𝑠 𝐿

29. Molar Solubility
To determine solubility of a sparingly soluble compound
X X X
S = = (x)(2x)2

30. Molar Solubility

31. Calculating ion concentration from Ksp
Write the Ksp expression in terms of X
Solve for X
Once you know what X is you can find the concentration of ions

32. Calculating ion concentration from Ksp

33. Predicting Precipitates