Name_____________________________Period_________Chem. Hons.
Chemical Equilibrium
Notes
Chem. Hons.
Reversible Reactions: A reaction in which products can re-react to form reactants.
Ex: A+ B > C+D
In this reaction, A+B can react to from C+D (forward reaction) and C+D can also react to
form A+B (reverse reaction)
Chemical Equilibrium: In a reversible reaction, chemical equilibrium occurs when
rate of forward reaction becomes equal to rate to reverse reaction, as a result of
which the concentrations of the reactants and the products remain constant. But it
does not mean that the reactants stop reacting. What actually happens is that at
equilibrium the amount of reactants converted into products is equal to the amount of
products that convert back to the reactants. Therefore, the chemical equilibrium is
dynamic.
Ex. Lets say a chemical reaction goes like this,
Forward reaction: A-- B
Reverse reaction: B--A
One of the following graphs shows the change in concentration of the reactants with time
and other shows the rate of reaction changing with time. Can you tell which one is
which?
[A0]
Equilibrium
Achieved
[A]
[B]
Equilibrium
Achieved
[A]
[B]
[B0]
Time--
Time--
The Equilibrium Constant
According to law of mass action, which expresses the relationship between the
concentration of the reactants and the products at equilibrium in any reaction, for the
general reaction,
aA + bB
p P + q Q , where A,B,P,Q are the chemical species involved and
a,b,p,q are their coefficients in the balanced chemical equation, the equilibrium condition
can be expressed by the equation,
Kc = [P]p [Q]q
The square brackets specify the molar concentrations o
----------- ,
reactants and products.
[A]a[B]b
This relationship is called equilibrium expression .
The constant Kc is called as equilibrium constant. The value of equilibrium constant at
any given temperature is constant. It does not depend on the initial concentrations of
reactants and products. It also does not matter if other substances are present, as long as
they do not react with reactants or products.
Example: For the following reaction, write the equilibrium expression and equilibrium
constant.
N2 (g) + 3H2(g)---2NH3(g)
Kc=
Do problems #6 and #7 from the section review on page 559.
#6.
#7.
Expressing Equilibrium Constant in Terms of Pressure Kp
When the reactants and products are gases, we can formulate the equilibrium expression
in terms of partial pressure instead of molar concentrations. For the general reaction
written above, we can denote the equilibrium expression as,
Kp= (Pp)p (Pq)q
----------- , where PA is the partial pressure of A and so on.
(PA)a (Pb)b
The numerical values of Kp and Kc will generally be different.
Magnitude of Equilibrium Constants
Equilibrium constant can be large or small.
K>>1, equilibrium lies to the right; products favored
K<<1, equilibrium lies to the left; reactants favored
Heterogeneous Equilibrium
When substances at equilibrium are in different phases, then the equilibrium is called as
heterogeneous equilibrium.
Example: CaCO3(s)--- CaO(s) +CO2(g)
This system involves a gas in equilibrium with two solids.
If a pure solid or a pure liquid is involved in a heterogeneous equilibrium, its
concentration is not included in the equilibrium expression. This is because the
effective concentration of a pure solid or liquid is a constant. So, the equilibrium
expression for the above reaction would be,
Kc= [CaO] [CO2]
--------------[CaCO3]
Since CaO and CaCO3 are solids, their concentration will remain unchanged, so the
equilibrium expression will look like this,
Kc=[CO2]
Questions on heterogeneous equilibria
Write the equilibrium expressions for the following reactions:
a. CO2(g) + H2(g)
CO(g) + H2O(l)
b. SnO2(s) + 2 CO(g)
Sn(s) + 2 CO2(g)
Homework: Do problems 24,25,26 and 27 on page 587.
Some more practice problems
1. A mixture of 5.000 X 10-3mol of H2 and 1.000 X 10-2 mol of I2 is placed in a 5.000 L
container at 4480C and allowed to come to equilibrium. Analysis of the equilibrium
mixture shows that the concentration of HI is 1.87 X10-3M. Calculate Kc at 4480C for the
reaction H2(g) + I2(g)-- 2HI(g)
Ans. 51
2. For the Haber process, N2(g) + 3H2(g)
2NH3(g), Kp= 1.45 X10-5 at 5000C. In an
equilibrium mixture of the three gases at 5000C the partial pressure of hydrogen gas is
0.928 atm and that of nitrogen gas is 0.432 atm. What is the partial pressure of ammonia
in this equilibrium mixture?
Ans. 2.24 X10-3 atm
3. A 1.000 L flask is filled with 1.000 mol of H2 and 2.000 mol of I2at 4480C. The value
of equilibrium constant Kc, for the reaction H2(g) + I2(g)-- 2HI(g) at 4480 C is 50.5.
What are the concentrations of hydrogen, Iodine and hydrogen iodide in the flask at
equilibrium?