Equilibria happen in phase transitions. For example, if the temperature in a system containing a mixture of ice and water is uniformly 273.15 K, the net amount of ice formed and the melt will be zero. The amount of liquid water will also remain constant, if no vapour escape from the system. In this case, three phases, ice (solid) water (liquid), and vapour (gas) are in equilibrium with one another. Similarly, equilibrium can also be established between the vapour phase and the liquid at a particular temperature. Equilibrium conditions also exist between solid phase and vapour phases. These are phase equilibria.
Chemical reactions may not be as complete as we have assumed in
Stoichiometry
calculations. For example, the following reaction
are far short of completion.
Let us consider only the first reaction in this case. At room temperature, it is impossible to have pure NO2 or N2O4. However, in a sealed tube ( closed system), the ratio
[N2O4] ------- [NO2]2is a constant. This phenomenon is known as chemical equilibrium. Such a law of nature is called the law of mass action or mass action law.
Of course, when conditions, such as pressure and temperature, change, a period of time is required for the system to establish an equilibrium.
Before we introduce the mass action law, it is important for us to identify a system or a closed system in our discussion. The law provides an expression for a constant for all reversible reactions.
For systems that are not at equilibrium yet, the ratio calculated from the mass action law is called a reaction quotient Q. The Q values of a closed system have a tendency to reach a limiting value called equilibrium constant K over time. A system has a tendency to reach an equilibrium state.
When the system under consideration is isolated from its environment
in such a way that there is no energy or mass transferred into
or out of the system, the system is said to be a closed system.
In a closed system, changes continue, but eventually
there is no NET change over time. Such a state is called an
equilibrium state.
For example, a glass containing water is an open system. Evaporation
let water molecules to escape into the air by absorbing energy from the
environment until the glass is empty. When covered and insulated
it is a closed system. Water vapour in the space above water eventually
reaches a equilibrium vapour pressure.
In fact, measuring of temperature itself requires the thermometer
to be at the same state as the system it measures. We read the
temperature of the thermometer when heat transfer between the
thermometer and the system stops (at equilibrium).
Equilibrium states are reached for physical as well as chemical
reactions. Equilibrium is dynamic in the sense that changes continue,
but the net change is zero.
Heat transfer, vapourization, melting, and other phase changes
are physical changes. These changes are reversible and you have
already experienced them.
are reversible chemical reactions.
The mass action law states that if the system is at equilibrium at a
given temperature, then the following ratio is a constant.
The units for K depend upon the units used for concentrations.
If M is used for all concentrations, K has units
Hint The equilibrium constant is
Hint
The expression is
Hint:
Experimentally, it has been shown that
and the equilibrium constant is
= 3.8x10-6 M2
Hint:
Hint:
Answer b Answer yes Answerno Answer 1/5E5 = 2E-6 Answer e Answer b Answer bd
A Closed System for the Equilibrium State
In order to discuss equilibrium, we must define a system, which may be
a cup of water, a balloon, a laboratory, a planet or a universe.
Thus, for discussion purpose, we define an isolated portion of the universe
as a system, and anything outside of the system is called
environment.
Reversible Chemical Reactions
Many chemical reactions are also reversible. For example
N2O4 = 2 NO2
colourless brown
The Law of Mass Action
The law of mass action is universal, applicable under any circumstance.
However, for reactions that are complete, the result may not be very useful.
We introduce the mass action law by using a general chemical reaction
equation in which reactants A and B react to give product C and D.
[C]c [D]d
------------- = Keq
[A]a [B]b
The Reaction Quotients Q and the Equilibrium Constants K
If the system is NOT at equilibrium, the ratio is different from the
equilibrium constant. In such cases, the ratio is called a reaction
quotient which is designated as Q.
[C]c [D]d
------------- = Q
[A]a [B]b
Equilibrium Expressions
The mass action law gives us a general method to write the
expression for the equilibrium constant of any reaction.
At this stage, you should be able to write the equilibrium
expression for any reaction equation. If you are not sure from the
above general theory, here are some examples. It is more important
for you to understand WHY the equilibrium constants are expressed
this way than what is the equilibrium expression.
Examples
The application of the mass action law leads to the method to
write the expression for the equilibrium constant.
The law is given in a general form, and these examples should help
you grasp the method.
[NH4+] [OAc-]
--------------------- = K (unitless constant)
[NH3] [HOAc]
[H+]2 [SO42-]
-------------- = K M2
[H2SO4]
where M = mol/L. Note the unit for K.
[Cu(NH3)62+]
----------------- = K M-6
[Cu2+] [NH3]6
The equilibrium constant depends on the written equation.
[HC2O4-] [H+]
-------------- = K1 = 0.059 M
[H2C2O4]
The second ionization constant is much smaller:
[H+] [C2O42-]
--------------- = K2 = 0.000064 M
[HC2O4-]
The overall ionization can be obtained by adding (1) and (2) to give (3).
[H+]2 [C2O42-]
-------------- = Koverall M2
[H2C2O4]
It is obvious that
[CO] [H2]3
------------------ = 1/4.0 = 0.25
[CH4] [H2O]
[CH4]1/3 [H2O]1/3
------------------------- = 4.01/3 = 1.59
[CO]1/3 [H2]
Confidence Building Questions
Next page:
Special equilibria
a. an open system
b. a closed system
c. not a system
Discussion...
Since it has been in the lab, the temperature of the system is the same as
its environment.
Discussion...
Only a few minute is required for the reaction to reach an
equilibrium state. NO2 is a brown gas
Discussion...
Heat will be extracted from the container, and that cause the equilibrium to shift.
At 0 deg C, the equilibrium is shifted to have lots of N2O4.
Discussion...
Since the reaction equation is reversed, use the relationship
(a) C = RT/(PV)
(b) C = RT/P
(c) C = RT/V
(d) C = RT/(PM)
(e) C = P/(RT)
(f) C = PV/(RT).
Discussion...
By definition, C = n / V. Derive it from n R T = P V.
(a) Kc = Kp
(b) Kc is proportional to Kp
(c) Kc = 1/Kp
(d) Kc is inversely proportional to Kp
(e) Kc * Kp = RT
Discussion...
Statements (c), (d), and (e) may be true for special cases, but they are not
generally true. In this list, Kc is proportional to Kp is true,
but a more quantative relationship will be derived.
(a) 2 H2 + O2 = 2 H2O
(b) 2 NO = N2 + O2
(c) COCl2 = CO + Cl2
(d) CO + H2O = CO2 + H2.
(Give the letter(s) of your choice(s), e.g., ad).
Discussion ...
Both reaction equations in (b) and (d) have equal numbers of reactants and
products.