Chemical equilibrium
- Reaction must be reversible
[acid + metal] and [carbonate + acid] are not reversible reactions
- Reaction must take place in a closed system
E.g., With a lid over a beaker
Rate of forward reaction = rate of reversible reaction
Equilibrium constant (Kc)
The equilibrium constant Kc indicates the relationship between the product and
reactant concentrations at equilibrium and is a constant at a specific temperature.
The Kc value indicates to what extent reactants have changed into products by the
time equilibrium is reached.
This is of particular importance in the evaluation of the efficiency of industrial
processes:
- A high Kc value (Kc > 1) indicates that a lot of product has been formed and there
is a higher concentration of products than reactants (equilibrium lies to the right)
- A low Kc value (Kc <1) indicates that a higher concentration of reactants than
products are present (the equilibrium lies to the left)
Large value Kc: equilibrium point shifts right
Small value Kc: equilibrium point shifts left
Kc = 1 indicates equal concentrations of reactants and products
It is a number without units
[ product ]
Kc =
[ reactants ]
Example: N2 (g) + 3H2 (g) ⇌ 2NH3
2
[ NH 3 ] Coefficient becomes exponent
Kc =
[ H2]¿ ¿
Kc is temperature dependent
Therefore, Kc only changes when temperature changes
The Kc value is only calculated using concentrations of the gasses and the dissolved
substances in liquids in the equation, and not those of solids and pure liquids.
i.e., ignore solids and liquids when working out Kc
CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Kc = [CO2]
, All concentrations must be equilibrium values!
Do ICE table to figure out equilibrium values
I stands for initial concentration. This row contains the initial concentrations of products and
reactants.
C stands for the change in concentration. This is the concentration change required for the
reaction to reach equilibrium. It is the difference between the equilibrium and initial rows.
The concentrations in this row are, unlike the other rows, expressed with either an
appropriate positive (+) or negative (-) sign and a variable; this is because this row
represents an increase or decrease (or no change) in concentration.
E is for the concentration when the reaction is at equilibrium. This is the summary of the
initial and change rows. Once this row is completed, its contents can be plugged into the
equilibrium constant equation to solve for Kc
Draw up a table and use the mole ration of the reaction equation to get the number
of moles and the concentration of all the substances at equilibrium then calculate
the Kc value
Worked example: 2X(g)⇌3Y(g)+4Z(g)
Follow the steps below:
A sample consisting of 0.5 mol of x is placed into a system with a volume of
0.75cm3.
This implies that there are no initial amounts of Y and Z. For the I row of the Y and Z
columns, 0 mol will be entered.
At equilibrium, the amount of sample x is known to be 0.35 mol.
For the equilibrium row of X, 0.35 mol will be entered.
X Y Z
Mole ratio 2 3 4
Initial (mol) 0.5 (given) 0 0
Change (mol) ? ? ?
Equilibrium (mol) 0.35 (given) ? ?
[ ] (mol.dm-3) ? ? ?
- Reaction must be reversible
[acid + metal] and [carbonate + acid] are not reversible reactions
- Reaction must take place in a closed system
E.g., With a lid over a beaker
Rate of forward reaction = rate of reversible reaction
Equilibrium constant (Kc)
The equilibrium constant Kc indicates the relationship between the product and
reactant concentrations at equilibrium and is a constant at a specific temperature.
The Kc value indicates to what extent reactants have changed into products by the
time equilibrium is reached.
This is of particular importance in the evaluation of the efficiency of industrial
processes:
- A high Kc value (Kc > 1) indicates that a lot of product has been formed and there
is a higher concentration of products than reactants (equilibrium lies to the right)
- A low Kc value (Kc <1) indicates that a higher concentration of reactants than
products are present (the equilibrium lies to the left)
Large value Kc: equilibrium point shifts right
Small value Kc: equilibrium point shifts left
Kc = 1 indicates equal concentrations of reactants and products
It is a number without units
[ product ]
Kc =
[ reactants ]
Example: N2 (g) + 3H2 (g) ⇌ 2NH3
2
[ NH 3 ] Coefficient becomes exponent
Kc =
[ H2]¿ ¿
Kc is temperature dependent
Therefore, Kc only changes when temperature changes
The Kc value is only calculated using concentrations of the gasses and the dissolved
substances in liquids in the equation, and not those of solids and pure liquids.
i.e., ignore solids and liquids when working out Kc
CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Kc = [CO2]
, All concentrations must be equilibrium values!
Do ICE table to figure out equilibrium values
I stands for initial concentration. This row contains the initial concentrations of products and
reactants.
C stands for the change in concentration. This is the concentration change required for the
reaction to reach equilibrium. It is the difference between the equilibrium and initial rows.
The concentrations in this row are, unlike the other rows, expressed with either an
appropriate positive (+) or negative (-) sign and a variable; this is because this row
represents an increase or decrease (or no change) in concentration.
E is for the concentration when the reaction is at equilibrium. This is the summary of the
initial and change rows. Once this row is completed, its contents can be plugged into the
equilibrium constant equation to solve for Kc
Draw up a table and use the mole ration of the reaction equation to get the number
of moles and the concentration of all the substances at equilibrium then calculate
the Kc value
Worked example: 2X(g)⇌3Y(g)+4Z(g)
Follow the steps below:
A sample consisting of 0.5 mol of x is placed into a system with a volume of
0.75cm3.
This implies that there are no initial amounts of Y and Z. For the I row of the Y and Z
columns, 0 mol will be entered.
At equilibrium, the amount of sample x is known to be 0.35 mol.
For the equilibrium row of X, 0.35 mol will be entered.
X Y Z
Mole ratio 2 3 4
Initial (mol) 0.5 (given) 0 0
Change (mol) ? ? ?
Equilibrium (mol) 0.35 (given) ? ?
[ ] (mol.dm-3) ? ? ?
