Equilibria Notes

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  • Created on: 25-04-14 00:06
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Equilibrium ­ Answers
1. In a dynamic equilibrium, the rates of the forward and reverse reactions are the same. Therefore, there is no
further change in the concentrations of the reactants and products.
2. The law of mass action states that when reactions reach equilibrium, the equilibrium concentrations of the
products multiplied together and divided by the equilibrium concentrations of the reactants also multiplied
together, with the concentration of each substance raised to the power appropriate to the reaction
stoichiometry, are a constant at a given temperature.
I.e. for a given equation: xA + yB nC + mD , Kc = [A]x[B]y
The right hand side of this expression is called the `reaction quotient' and given the symbol Q.
3. Kc is the equilibrium constant (a fixed value at a particular temperature). It is measured in terms of concentrations,
i.e. mol dm-3.
4. The value of the equilibrium constant depends on Stotal:
Stotal = RlnK, where R is the gas constant which equals 8.31 J K-1 mol-1.
The value of Stotal depends on the entropy change of the system and the entropy change of the surroundings:
Stotal = Ssystem + Ssurroundings
Ssurroundings is calculated by ­H/T
Thus Stotal and hence K depend on the nature of the reaction, which determines H and Ssystem as well as the
temperature at equilibrium. The value of Kc does not depend on the pressure or the presence of a catalyst.
5. The equilibrium constant is a thermodynamic quantity, the value of which depends on the total entropy change of
the system:
Q = K: The system is in equilibrium and there will be no further change in concentration of the reactants
and products
Q > K: The system is not in equilibrium and will react to reduce the value of the numerator. Thus products
will be converted into reactants (the position of equilibrium will shift to the left.)
Q < K: The system is not in equilibrium and will react to increase the value of the numerator. Thus,
reactants will be converted into products (the position of equilibrium will shift to the right).
6. When water is a reactant but not eh solvent, the term [H2O] must always appears in the expression for the
equilibrium constant. Additionally when water is in the gaseous state, [H2O] must appear in the equilibrium
constant expressions. However, when water is the solvent, even if it is also a reactant or product, [H2O] does not
appear in the expression for the equilibrium constant. This is because its concentration remains constant and its
value is incorporated into Kc.
7. The calculation requires the use of a table:
Write the chemical equation
Construct a suitable table and write in the following:
The initial amounts (in moles) of the reactants and of the products
The amounts by which the reactants and the products change in reaching equilibrium ­ use the
stoichiometry of the equation
The amount, in mole, of each substance at equilibrium
The equilibrium concentration in mol dm-3 ­ divide the equilibrium number of moles by the total volume
Below the table, write the expression for the equilibrium constant
Substitute the equilibrium concentrations into the expression and calculate its value. At the same time,
determine the units of Kc and include them in your answer.
8. If you have two immiscible liquids like hexane and water, and shake them up in a separating funnel, they
obviously form two layers. The hexane is less dense than water, and so forms the top layer. If you were to add
iodine to the mixture, an iodine equilibrium is set up between the two layers as it is soluble in both:
[I (hexane)]
I 2(aq)I 2(hexane) which gives the Kc expression: K c = 2 [I2(aq)]
This particular equilibrium constant is called a partition coefficient. It can be used to determine the value of the
equilibrium constant of a reaction.
9. In heterogeneous equilibria, different phases are present. The concentration of a pure liquid or a pure solid is
itself a constant. Therefore, we ignore pure solids and pure liquids for heterogenous equilibrium in a Kc/ Kp

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E.g. Mg (s) + Cu (aq) Mg2+(s) + Cu(s) giving the Kc expression: K c = [
Mg ][Cu]
[Mg][Cu2+], which cancels to give:
K c = [Mg ]
10. The partial pressure of a gas A in a mixture of gases is the pressure that the gas A would exert if it were alone in
the container at the particular time. The sum of the partial pressure of the gases in a mixture equals the total
pressure.…read more


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