Name:
Chemistry 1B
Unit 6:Redox and Equilibrium
So far in Chemistry 1B, we have discussed forward chemical reactions much like the
reaction shown below. In the reaction below, we see that the reactants are combined
to yield the products.
1.) Please label the reactants, products and the yield sign in the reaction below.
N2(g)
+
3H2(g) 
2NH3(g)
We have yet to discuss the idea that reactions can be reversible. A reversible chemical
equation occurs when the reaction can proceed from right to the left side, or from the
left to the right side. A reversible reaction is shown below.
N2(g)
+
3H2(g) 
2NH3(g)
A reversible reaction is often times broken down into two parts; the forward reaction
that proceeds to the right side of the equation, and the reverse reaction that proceeds
to the left side of the equation.
2.) Write out the forward reaction (Left to right) using the reversible reaction above:
3.) Write out the reverse reaction (right to left) using the reversible reaction above:
When a chemical reaction is carried out in a closed container, the reaction eventually
comes to “equilibrium.” Equilibrium occurs when the rate of the forward reaction equals
the rate of the reverse reaction. All reversible reactions at equilibrium have what is
known as an equilibrium constant or a ratio (comparison) of the concentration of the
products and reactants in the chemical equation. This ratio is known as the equilibrium
constant or Keq. Different reactions have different equilibriums. Some may appear to be
completely products; however, all reactions have some reactants present.
It is possible to write an equilibrium constant (Keq) for a reaction. This can be expressed
by concentrations of the products divided by the concentration of the reactants with the
coefficients of each equation acting as exponents. For the reaction:
aA + bB --> cC + dD
the equilibrium expression (Keq) is:
K eq = [C]c[D]d
[A]a [B]b
Where:
K is the equilibrium constant
[A], [B], [C] and [D] are the molar concentrations of A, B, etc.
a, b, c, d are the coefficients of the balanced reaction
4.) Write out the correct equilibrium constant for the following reaction. Use the
example above as a guide. Remember : products/reactants.
N2 (g)
+
3H2(g) 
2NH3(g)
If the reactions equilibrium constant is more than one (+), then the reaction will proceed
in a forward reaction, favoring the products. However, if the equilibrium constant is less
than one (-), then the reaction will proceed in a reverse reaction favoring the reactants.
N2 (g)
+
3H2(g) 
2NH3(g)
5.) If the Keq for the reaction above was equal to 625, would the reaction proceed in the
forward or reverse reaction?
There are three main items that can change or shift the balance of equilibrium in a
reversible chemical equation.
a. Concentration
b. Temperature
c. Pressure
Le Chatelier's principle allows us to predict the effects of changes in temperature,
pressure, and concentration on a system at equilibrium. Le Chatelier’s Principle states
that if stress (change) is applied to the system that is in equilibrium, than the system
itself must change to relieve the stress.
• Changing the concentration- If you lower the concentration or remove a substance,
the system will shift to produce more of that substance. On the other hand, if
you increase the concentration or add a substance, the system will shift to
produce less of that substance.
For example, in the equation: N2 (g) +
3H2(g) 
2NH3(g)
If we decrease the concentration of the H2 by removing some of it, the system will
shift towards the left or towards the reactants (the reverse reaction) to produce
more H2.
6.) What do you think would happen if more we were to increase the concentration of
the H2 by adding more H2 to the reaction?
• Changing the pressure-
When you increase the pressure of the system, the system
will shift so the least number of gas molecules are formed because the more gas
molecules there are, the more collisions there are. These collisions and the
presence of gas molecules are what cause the pressure to increase. Likewise,
when you decrease the pressure, the system will shift so the highest number of
gas molecules is produced.
For example, in the equation: N2 (g)
+
3H2 g) 
2NH3(g)
If the pressure is increased, the system will shift to the right because fewer gas
molecules are produced in the forward reaction than in the reverse reaction.
7.) What would happen if the pressure was decreased? Which way would the system
shift and why?
• Changing temperature- For every reaction which can go forwards and backwards, one
direction is endothermic and the other is exothermic. A reaction is endothermic
if it takes heat from its surroundings. On the other hand, a reaction is exothermic
if it releases heat to the surroundings. If you increase the temperature, then the
endothermic reaction will be favored because that will take in some of the
excess heat. If you decrease the temperature, the exothermic reaction will be
favored because it will produce the heat that was lost.
For example, in the equation: PCl3(g) + Cl2(g) ⇌ PCl5(g) + energy
If the temperature was increased, the system would shift to the left and the
reverse reaction would happen more because that would use some of the extra
energy.
8.) What would happen if the temperature decreased? Which way would the reaction
shift and why?
SummaryFill in the blanks below to complete the summary of Le Chatelier’s principle of
equilibrium shifts.
1.) Le Chatlier’s Principle states that is a
is applied to a reversible
reaction at
than the reaction must shift to relieve the stress.
2.) What is an equilibrium constant?
3.) If the equilibrium constant is
than one, the reaction will occur in the
forward reaction.
4.) If the equilibrium constant is
than one, the reaction will occur in the
reverse reaction.
5.) The three main types of stress applied to a reversible reaction at equilibrium include
,
,
6.) If the concentration of a reactant is decreased, in which way will the equilibrium
shift?
Why?
7.) If the concentration of the reactant is increased, in which way will the equilibrium
shift?
Why?
8.) If the temperature of an endothermic reversible reaction is increased, in which way
will the equilibrium shift?
Why?
9.) If the temperature of an exothermic reversible reaction is increased, in which way
will the equilibrium shift?
Why?
10.) If the pressure is increased in a reversible reaction, how can you predict in which
way the equilibrium will shift?