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.
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.
COMPLETION
Reaction goes to completion• When a reaction results in almost complete
conversion of reactants to products
Rarely happens
REVERSIBLE REACTION
Most reactions• 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
REVERSIBLE REACTIONS
R AT E O F R E A C T I O NDepends on concentration of the reactants
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.
N2 (g) + 3 H2 (g) < -- > 2NH3 (g)
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!
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
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
HOMOGENEOUS EQUILIBRUM
All the products and reactants are in the same
physical state
Must use ALL CONCENTRATIONS for Keq
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
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.
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
Changes equilibrium
POSITION• Shifts left or right
Does NOT change
equilibrium constant (Keq)
CHANGE IN CONCENTRATION
Add reactant = shift
right
Remove reactant =
shift left
Add product = shift
left
Remove product =
shift right
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
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
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
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
CHANGE IN TEMPERATURE
Exothermic- releases heat
Reactants <--> Products + heat
Endothermic- absorbs heat
Reactants + heat <--> Products
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
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.
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)
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
CALCULATING EQUILIBRIUM CONCENTRATIONS
Some dissolve readily in
water
Ex NaCl(s)• High solubility
Some barely dissolve at
all
Ex BaSO4(s)• low solubility
SOLUBILITY EQUILIBRIA
Ionic Compounds
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
SOLUBILITY PRODUCT CONSTANT
Example: (Remember it depends ONLY on
[ IONs])
REVIEW: SOLUBILITY
Solubility in water• The amount of the substance (moles) that will
dissolve in a given volume of water (Liter)
•
MOLAR SOLUBILITY
To determine solubility of a sparingly soluble
compound X X 2X
S = = (x)(2x)2
MOLAR SOLUBILITY
CALCULATING ION CONCENTRATION FROM K S P
Write the Ksp expression in terms of X
Solve for X
Once you know what X is you can find the
concentration of ions
CALCULATING ION
CONCENTRATION FROM KSP
PREDICTING PRECIPITATES
