1. Chemical Equilibrium Reversible Reactions
Mr. Shields Regents Chemistry U13 L01

2. Reversible Reactions What do we mean when we say something is
And what does it mean in terms of chemical reactions?
For example if the following chemical reaction
Occurred what would be the reverse reaction?
2H2 + O2  2 H2O + Energy (exothermic rxn)
Energy + 2H2O  2H2 + O2 (endothermic rxn)
Heat would be released in the 1st rxn and
Elec. could provide the energy to drive the 2nd

3. Many reactions in chemistry can proceed in either
Direction depending upon the conditions applied.
These reversible rxns are designated with either a
double split arrow
Or double tipped arrow 
The former is more common but we’ll use also use
From time to time the 2nd Format ( )
So our previous example would be written …
2H2 + O2  2 H2O + Energy

4. Equilibrium Now let’s discuss what we mean when we use the
Word “Equilibrium”.
For example, what does it mean when you say …
“I got dizzy all of a sudden and lost my equilibrium”?
You might re-explain it as follows:
I was balanced on my feet and thus standing straight.
Then I got dizzy which caused my center of balance to
Shift driving me in a direction away from “upright”

5. At the point in time when we had not yet lost our
Balance we stood straight. Forces worked to
Overcome gravity so we did not fall one way or
another .
As soon as these forces became unbalanced our
Body began moving in a particular direction.
Equilibrium only occurs when all
opposing forces are…
Precisely balanced.

6. Solution Equilibrium Recall our earlier discussions of solution ...
What is the definition of a saturated sol’n ?
“a saturated sol’n is one in which the
solvent can not dissolve any more solute”
If we add more solute to the solution
It should fall to the bottom and just lay there. Right?
Wrong! The process is not static!

7. So what is exactly happening?
Excess solute goes into sol’n
At a rate equal to solute
Coming out of sol’n
Solute (s)  Solute (aq) = rate1
Solute (aq)  Solute (s) = rate2
When rate1 = rate2 the system
is said to be in Equilibrium
Notice that even though solute is going into sol’n it is also
Coming out of sol’n at an equal rate. So…
The overall concentration of solute in sol’n does not change.

8. Equilibrium in the gas phase
Now Consider the following equilibrium reaction:
N2O4 (g) + heat  2NO2 (g) (at a given T)
At first the conc. Of
Gas is all N2O4 (a)
As temp inc. the conc.
Of NO2 begins to inc.
& N2O4 dec. (b)
With time an new
equilibrium is estab-
lished between NO2
N2O4 (c) T1 T2

9. The reaction continues until there is a new equilibrium
Established between the concentration of N2O4
and NO2 at the new temperature
NOTE THAT THIS DOES NOT IMPLY THEIR CONC.
NEEDS TO BE EQUAL
% Conc.
N2O4 NO2
100 0
85 15
75 25
71 29
68 32
66 34
All N2O4 at T1
2/3 N2O4 at equilibrium
At equilibrium the
Concentration of
Both reactant and
Product remains
constant

10. At the same time the concentrations of reactants
And products are reaching equilibrium the Rate of
Change of reactant and product concentration
is approaching one another
Rate 1: N2O4  2NO2
Rate 2: 2 NO2  N2O4
Rate 1 = Rate 2
At equilibrium the
Rate of the forward
Reaction equals the
Rate of the reverse
reaction
The rates are
Balanced!
Note: the rate of change of rxn 1 and rxn 2
Do not have to be the same at any time
EXCEPT when they are at equilibrium