SCH 4U1
UNIT 4
EQUILIBRIUM
Prior Knowledge Q’s:

Negative enthalpy change indicates an
exothermic reaction.

Positive enthalpy change indicates an
endothermic reaction.
Prior Knowledge Q’s:
Which line represents an endothermic
reaction (blue or green)?
 Blue=exothermic
 Green= endothermic

Prior Knowledge Q’s:

In words, describe what a rate of a
reaction is
rate of reaction: the change in the
amount of reactants or products over time
Prior Knowledge Q’s:

Using the previous definition, write an
equation for the rate of reaction:
rate of reaction = Δ [A]
Δt
Note: [A] is the concentration of either the reactants or the
products
LESSON 1:
INTRODUCTION TO EQUILIBRIUM
A+B → AB + energy
This symbol means that >99% of the product
AB is formed. (Spontaneous rxn)
AB + energy → no reaction
Therefore, in the opposite direction <1% of
AB reacts. (Non-spontaneous)
What is EQUILIBRIUM?

At equilibrium, reactions actually take place in
both the forward and reverse directions at the
same time. These reactions can be shown
together using a double arrow,
A + B ↔ AB
Note: Equilibrium is not equalibrium. The amount
of products and reactants is not necessarily
equal.
Equilibrium Mini Lab
-
Transfer of water using straws between
two graduated cylinders:
cylinder A starting volume = 10mL
cylinder B starting volume = 0mL
Mini Lab Results
- Equilibrium does not
mean equal volume
(equal concentration).
When the experimental
system is at equilibrium,
the volumes are
different.
- Chemical equilibrium
is reached when the
proportion of reactants
and products stays
constant.
An Example of Equilibrium

A sealed flask containing water
will contain a mixture of liquid
and vapour water that will
eventually establish a stable or
balanced condition known as
equilibrium.
H2O (l) ↔ H2O (g)
Characteristics of a System at Equilibrium
1.
The system is closed.
2.
The rate of the forward reaction equals the rate of the
reverse reaction.
3.
The concentrations of the reactants and products are
constant.
4.
The temperature and pressure remains constant.
5.
The same equilibrium state can be reached by starting
with reactants or products.
STEADY STATE SYSTEMS
-
-
Steady states should not be confused with
equilibrium states
These states appear to be in equilibrium but in
reality they are not
A steady state is an irreversible process
occurring in an open system in which input of
energy and materials equals the output of
energy and materials.
bathtub example
Which of the following systems constitute steadystate situations, and which are at equilibrium? For
each, a constant property is indicated.
a)
An open pan of water is boiling on a
stove. The temperature of water is
constant.
Steady State
- not an equilibrium; in order for
equilibrium to exist, the system must be
closed.
b) A balloon contains air and a few drops of
water. The pressure in the balloon is
constant.
equilibrium
c) A Bunsen burner burns in a laboratory to
give a well-defined flame. Supplies of gas
and air are constant.
steady state
d) The liquid mercury in a thermometer and
the mercury vapour above it in a closed
thermometer at constant temperature.
equilibrium
e) A plant manufacturing CaO from CaO3 by
the reaction CaO3  CaO + CO2. The plant
produces 200 tonnes of CaO each day.
steady state
Check your understanding
When a chemical system is in equilibrium, a number of items are
true:
1. The rates of the forward and reverse reactions are_________.
2. The concentrations of all the substances involved are
_________.
3. This happens because the products and reactants are being
formed __________.
4. There will be both ____________present but the concentrations
of them will not necessarily be equal.