Grade 11 Chemistry

1. Chemical Equilibrium

2. Reversible Reactions
• So far, we’ve treated chemical reactions as a sure
thing, progressing from beginning to end.
However, it’s not always that simple…
• If conditions are correct, a reaction can be
made to go in reverse. In theory, this is true for
all reactions. Thus, all reactions are reversible.
Often, reactions proceed in one direction, with a
small amount of the reverse reaction occurring at
the same time. Reversible reactions can be
written as follows:
• A double sided arrow indicates that the reaction
goes in both directions

3. How do reactions reach Equilibrium?
• Equilibrium is reached when
the amount of reactants
going forward equals the
amount of product going
backward, in other words,
when there is no net change
in the concentration of
reactants and products.
• Note that this does not
mean that the reactions
stop, only that the forward
rate balances the reverse
rate.
Rateforward reaction = Ratereverse reaction

4. The Law of Equilibrium
• When a chemical reaction is at equilibrium,
there is no NET change in the concentration of
all components in the reaction. This can be
expressed mathematically. Consider the
following reaction:
• (The lower case letters are the coefficients
from the balanced equations)

5. Equilibrium Expressions
• This allows us to look at a reaction at equilibrium as an
expression of a constant (the equilibrium constant, keq).
• keq =  products 
 reactants
• The brackets denote concentrations in Mol/L
• Or keq =  C c  D d
 A a  B b
We only included gases in the equilibrium expression
because the concentrations of solids and liquids are
constant at a constant temperature

6. Write an Equilibrium Expression
• Homogeneous: All
participants in the
reaction are in the same
state:
• Heterogeneous: Some
participants are in
different states.

7. Calculating keq
What is the value for the equilibrium constant (keq) if the equilibrium
concentrations are:
 CH4  = 0.354 mol/L
 CO = 0.05 mol/L
C2H4O = 0.05 mol/L
(.354) (.05) = .354
.05

8. Keq and what it means
• Note that large values
for keq indicate that the
product is favored in
this reaction
(Numerator)
• If the value of keq < 1,
then the reactants are
favored (the reaction
goes toward reactants).

9. Practice
• Calculate keq for the following reaction and
determine if the products or reactants are
favored and why.
• [NO] = .89 M
• [Br2] = .45 M
• [NOBr] = .56 M
• (.56)2 = .88
(.89)2 (.45)
Since keq is less than 1, the reactants are favored
in this reaction (slightly)