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Chapter 15
Chemical Equilibrium
15.1 The Concept of Equilibrium
Consider the following chemical equilibrium A
B
If the process A
B is an elementary process then , the rate is
expressed as rate = kf [A]. Assuming that the reverse process B A is also
an elementary process, its rate is expressed as kr[B].
If the reverse process rate is equal the forward process rate, a dynamic
chemical equilibrium would be established. At the equilibrium time the
concentrations of A and B do not change
Example:
N2O4 (g)
NO2 (g).
As N2O4 is warmed above its boiling point, it starts to dissociate into brown
NO2 gas. (c) Eventually the color stops changing as N2O4(g) and NO2(g)
reach concentrations at which they are interconverting at the same rate. The
two gases are in equilibrium.
15.2 The Equilibrium Constant
The production of ammonia from nitrogen and hydrogen gases is an
important industrial process called the Haber Process. This process is used
to manufacture ammonia to be applied directly to soil as a fertilizer.
N2 (g) + H2 (g)
NH3 (g)
The reaction is carried out under conditions of high pressure and high
temperature. Equilibrium can be established either by starting with N2 and
H2 or by starting only with NH3.
For the general reaction aA + bB
cC + dD, the concentrations of
reactants and products at equilibrium are related by equilibrium constant:
where Kc is the equilibrium constant . The equilibrium expression is
essentially products over reactants, each raised to its coefficient in the
balanced equation.
The equilibrium expression for the Haber process is
The equilibrium expression of a chemical reaction depends only upon the
stoichiometry of the equation, and not on the mechanism by which the
reaction takes place.
The value of the equilibrium constant depends only on the particular
reaction and on temperature. It does not depend on starting concentrations of
reactants or products.
Example:
N2O4 (g)
NO2 (g).
.
the value of Kc is the same, within experimental error, regardless of the
starting concentrations of N2O4 and NO2.
When all of the species in the equilibrium expression are gases, it is possible
to write the expression in terms of partial pressures, rather than molar
concentrations. For the Haber process the equilibrium expression in terms of
gas pressures is:
Both Kc and Kp expressions can be written for equilibria involving only
gases. .
We can convert from pressure to molarity using the ideal-gas equation
If we replace each of the partial pressures in the Kp expression for the Haber
process with MRT we get
This expression reduces to a form that relates the Kp to the Kc of the Haber
process.
In general, Kc and Kp are related by the equation:
where n is the change in the number of moles of gas in the balance equation
The numerical value of the equilibrium constant tells us something about the
nature of the reaction. When K (Kc or Kp) is a very large number, we say that
the equilibrium lies to the right. That means that products predominate in
the equilibrium mixture. When K is a very small number, we say that the
equilibrium lies to the left and reactants predominate in the equilibrium
mixture.
Remember that equilibrium can be achieved whether we start with only
reactants or with only products. It is therefore possible to write the reaction
in either direction.
15.3 Heterogeneous Equilibria
Equilibria that involve more than one phase are called Heterogeneous
equilibria.
When writing the equilibrium expression for a heterogeneous equilibrium,
we do not include the concentrations of pure liquids or solids . Regardless
of how much liquid or is present, its concentration in terms of moles per
liter remains constant.
SnO2 (s) + 2CO (g)
Zn (s) + Cu2+(aq)
2CO2(g) + Sn (g)
Kc=? or Kp =?
Cu(s) + Zn2+(aq) Kc=?
15.4 Calculating Equilibrium Constants
0.100 mol of N2O4 gas is placed in a 2.00-L vessel and allowed to
equilibrate at 110°C. At equilibrium the concentration of NO2 is 0.072 M.
Calculate the value of Kc for the reaction at this temperature.
Chatelier Principle:
If a system at equilibrium is disturbed by a change of temperature , pressure,
or the concentration of one of the components, the system will shifts its
equilibrium position so as to counteract the effect of the disturbance.
a. Change in Reactant or Product Concentration:
If a system at dynamic equilibrium is disturbed by adding or removing a
reactant or product then the system will be shifted to a new equilibrium
position.
Example: N2 (g) + H2 (g)
NH3 (g)
Adding H2 will cause the system to shift as to reduce the concentration of H2
to its original value. This will caus the system to produce more of NH3
b. Effect of volume and pressure
If a system at equilibrium is disturbed by decreasing the volume (increasing
the total pressure), the system respond by decreasing the pressure. A system
can reduce its pressure by reducing its total number of gas molecules. Thus
at constant temperature, reducing the volume will cause the system to shift
in the direction that reduce the number of moles of gases.
c. Effect of temperature:
Reactants + Heat
products (endo)
Reactants
heat + products (exo)