Module 3
Lesson 12/13 Reversible
Reactions & Dynamic Equilibria
Objectives
Must
State that some chemical reactions are
reversible
Should
Describe how reversible reactions reach a state
of equilibrium
Could
Explain how at dynamical equilibrium the rate of
the forward reaction is equal to the rate of the
Reversible or not reversible
Until now, we were careful to say that most
chemical reactions were not reversible –
They could not go back to the reactants
once the products are formed.
Example
In the case of the vast majority of chemical
reactions this is true, the reaction of
methane and oxygen for example:
It is almost impossible to return the carbon
dioxide and water to the original methane
and oxygen.
Reversible
• Some chemical reactions, however, will go
backwards and forwards depending on the
conditions.
• CoCl2·6H2O(s)  CoCl2(s) + 6H2O(l)
pink
blue
How do we write them down?
• This is the symbol for used for reversible
reactions.
CoCl2·6H2O(s)
CoCl2(s) + 6H2O(l)
What is equilibrium?
• Reversible reactions reach a balance point,
where the amount of reactants and the
amount of products formed remains
constant.
General equation
• This is the equation for a general
reaction which has reached dynamic
equilibrium:
• How did it reach this balanced state
• At the beginning of the reaction, the
concentrations of A and B were at their
maximum.
A
+
B  C + D
As A and B react, their concentrations fall. That
means that they are less likely to collide and
react, and so
.
• In the beginning, there isn't any C and D, so
there can't be any reaction between them.
• As time goes on, though, their concentrations in
the mixture increase and they are more likely to
collide and react.
• Eventually, the rates of the two reactions will
become equal.
• A and B will be converting into C and D at
exactly the same rate as C and D convert back
into A and B again
• At this point there won't be any further change in
the amounts of A, B, C and D in the mixture.
• We have reached a position of dynamic
equilibrium.
Dynamic Equilibrium.
• In
the forward and
backwards reactions continue at equal rates
so the concentrations of reactants and
products do not change.
• On a molecular scale there is
.
• On the macroscopic scale
. The system needs to be closed
– isolated from the outside world.
Other examples
2NO2  N2O4
Briggs Rauscher – an awesome set of 10
competing chemical reactions in equilibrium
Plenary - Examination questions
• As a warm up we will use some GCSE level
answers.
Mark scheme
Summary
State that some chemical reactions are
reversible
Describe how reversible reactions reach a state
of equilibrium
Explain this using dynamic equilibrium model.