Page 243 - Theory and Problems of BEGINNING CHEMISTRY

232 RATES AND EQUILIBRIUM [CHAP. 16

16.3. CHEMICAL EQUILIBRIUM
Many chemical reactions convert practically all the reactant(s) (at least the limiting quantity) to products
under a given set of conditions. These reactions are said to go to completion. In other reactions, as the products are
formed, they in turn react to form the original reactants again. This situation—two opposing reactions occurring
at the same time—leads to formation of some products but the reactants are not completely converted to products.
A state in which two exactly opposite reactions are occurring at the same rate is called chemical equilibrium. (In
fact, all chemical reactions are equilibrium reactions, at least theoretically.) For example, nitrogen and hydrogen
◦
gases react with each other at 500 C and high pressure to form ammonia; under the same conditions, ammonia
decomposes to produce hydrogen and nitrogen:
3H 2 + N 2 −→ 2NH 3
2NH 3 −→ 3H 2 + N 2
To save effort, we often write these two exactly opposite equations as one, with double arrows:
−→ −→
3H 2 + N 2 ←− 2NH 3 or 2 NH 3 ←− 3H 2 + N 2
We call the reagents on the right of the chemical equation as it is written the products and those on the left the
reactants, despite the fact that we can write the equation with either set of reagents on either side.
With the reaction just above, if you start with a mixture of nitrogen and hydrogen and allow it to come to
500 C at 200-atm pressure, some nitrogen and hydrogen combine to form ammonia. If you heat ammonia to
◦
500 C at 200-atm pressure, some of it decomposes to nitrogen and hydrogen. Both reactions can occur in the
◦
same vessel at the same time.
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What happens when we first place hydrogen and nitrogen in a container at 500 C and allow them to react?
At first, there is no ammonia present, and so the only reaction that occurs is the combination of the two elements.
As time passes, there is less and less nitrogen and hydrogen, and the combination reaction therefore slows down
(factor 4 or 5, Sec. 16.2). Meanwhile, the concentration of ammonia is building up, and the decomposition rate of
the ammonia therefore increases. There comes a time when both the combination reaction and the decomposition
reaction occur at the same rate. When that happens, the concentration of ammonia will not change any more.
The reaction apparently stops. However, in reality both the combination reaction and the decomposition reaction
continue to occur with no net change taking place. A state of equilibrium has been achieved.

Le Chˆatelier’s Principle
If we change the conditions on a system at equilibrium such as the N 2 ,H 2 ,NH 3 system at equilibrium,
for example by changing the temperature, we can get some further net reaction. Soon, however, the system will
achieve a new equilibrium at the new set of conditions.
Le Chˆatelier’s principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift
in a tendency to reduce that stress. A stress is something done to the system (not by the equilibrium reaction).
The stresses that we consider are change of concentration(s), change of temperature, change of pressure, and
addition of a catalyst. Let us consider the effect on a typical equilibrium by each of these stresses.

The Effect of Concentration
An increase in concentration of one of the reactants or products of the equilibrium will cause the equilibrium
to shift to try to reduce that concentration increase.

EXAMPLE 16.2. How will addition of hydrogen gas affect the following equilibrium system?
−→
3H 2 + N 2 ←− 2NH 3
Ans. Addition of hydrogen will at first increase the concentration of hydrogen. The equilibrium will therefore shift to
reduce some of that increased concentration; it will shift right. That is, some of the added hydrogen will react with
some of the nitrogen originally present to produce more ammonia. Note especially that the hydrogen concentration
will be above the original hydrogen concentration but below the concentration it would have if no shift had taken
place.