2. Equilibrium: “the extent of a reaction”
Recall: for a chemical reaction
actual yield < theoretical yield
Why? Because
• Reactants are not pure
• Not all product is recovered
• Other competing reactions use up
reactant to form alternate products
and some chemical reactions are reversible!
Equilibrium looks at the extent (of completion)
of a reversible chemical reaction.
3. Equilibrium: the extent of a reaction
Equilibrium = no observable changes over time
In chemistry we encounter two types of equilibrium
systems.
Physical/phase equilibrium Chemical equilibrium
H2O (l) H2O (g) N2O4 (g) 2NO2 (g)
4. Equilibrium: the extent of a reaction
Equilibrium = no observable changes over time
In chemistry we encounter two types of equilibrium
systems.
Physical/phase equilibrium
H2O (l) H2O (g)
5. Equilibrium: the extent of a reaction
Equilibrium = no observable changes over time
In chemistry we encounter two types of equilibrium
systems.
Physical/phase equilibrium Chemical equilibrium
H2O (l) H2O (g) N2O4 (g) 2NO2 (g)
6. Equilibrium: the extent of a reaction
A chemical equilibrium is achieved when:
• the rates of the forward and reverse reactions are equal and
• the concentrations of the reactants and products remain constant
for example
• Colorless N2O4 decomposes to brown NO2 at room temperature. The
reaction is reversible.
N2O4(g) 2NO2(g).
• At equilibrium, there is a mixture of N2O4 and NO2.
• The reaction does not stop.
• The color remains constant
7. Key concept 1: at equilibrium, concentrations are constant
8. Constant concentrations imply that the forward
reaction must be proceeding at the same rate as the
reverse reaction.
N2O4(g) 2 NO2(g)
This is the forward reaction
2 NO2(g) N2O4(g).
This is the reverse reaction
The double arrow implies the process is reversible.
N2O4(g) 2NO2(g)
9. Key concept 2: at equilibrium, forward rate = reverse rate
N2O4(g) 2 NO2(g)
2 NO2(g) N2O4(g)
“dynamic” implies that at equilibrium
the reaction continues in both directions
