This document discusses chemical equilibrium, which occurs when the rate of the forward reaction equals the rate of the reverse reaction in a closed system. It defines the equilibrium constant (Kc) and explains that a high Kc value means a high product yield, while a low Kc means a low product yield. It also describes Le Chatelier's Principle, which states that applying stress to a system in equilibrium will cause the equilibrium to shift to reduce the effect of the stress, and discusses how temperature, concentration, and pressure influence the position and value of equilibrium constants.

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8. Chemical equilibrium
Chemistry
Grade 12

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Chemical equilibrium
A reaction is reversible when reactants can react to form products, and products can react
to form the reactants again.
A reaction is in chemical equilibrium when the rate of the forward reaction equals the rate
of the reverse reaction.
In an open system energy and matter can enter and leave the system. In a closed system
energy can enter and leave the system, but matter cannot. In a closed system it is possible
for reactions to be reversible and so to be in chemical equilibrium.
There is a dynamic equilibrium in a reversible reaction when the rate of the forward reaction
equals the rate of the reverse reaction. The amounts of reactants and products remain
constant.

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Equilibrium constant
The equilibrium constant (Kc
) relates to the concentration of the reactants and products at
equilibrium, and can be calculated using the following expression:
Where [C] and [D] are the concentrations of the products and [A] and [B] are the
concentrations of the reactants. a, b, c and d are the stoichiometric co-efficients of the
reactants and products.
We can use a RICE table to help us calculate Kc
for any reaction.
Kc=
[C]c
[D]d
[ A]a
[B]b

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The meaning of Kc
values
A high Kc
value means that the concentration of products at equilibrium is high and the reaction
has a high yield of the products.
A low Kc
value means that the concentration of products at equilibrium is low and the reaction
has a low yield of the products.

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Le Chatelier's Principle
Le Chatelier’s Principle states that if an external stress (change in pressure, temperature or
concentration) is applied to a system in chemical equilibrium, the equilibrium will change in such
a way as to reduce the effect of the stress.
Concentration
If the concentration of one of the reactants is increased or the concentration of one of the
products is decreased the forward reaction is favoured and the equilibrium shifts to the right.
If the concentration of one of the reactants is decreased or the concentration of one of the
products is increased the reverse reaction is favoured and the equilibrium shifts to the left.
Pressure
If the pressure is increased the equilibrium will shift to favour a decrease in pressure.
If the pressure is decreased the equilibrium will shift to favour an increase in pressure.
Temperature
If the temperature is increased the equilibrium will shift to favour the reaction which will reduce
the temperature. The endothermic reaction is favoured.
If the temperature is decreased the equilibrium will shift to favour the reaction which will increase
the temperature. The exothermic reaction is favoured.

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Factors that influence Kc
Although a change in temperature, concentration or total pressure will affect the equilibrium
position, only temperature will affect the equilibrium constant Kc
.
Concentration
Changing the concentration of a reactant or product results in one of the reactions (forward or
reverse) being favoured. This change in reaction rate minimises the effect of the change and
restores the concentration ratio between reactants and products. There will just be more
reactants and products. Kc
will remain the same.
Pressure
Changing the pressure of the system will change the ratio between the reactant and product
concentrations. The equilibrium then shifts to minimise the effect of the change
and restores the ratio between reactant and product concentrations. Kc will remain the same.
Catalyst
Both the forward and reverse reactions rates are increased. Therefore the ratio between
reactant and product concentrations will remain the same. Kc will remain the same.
Temperature
Changing the temperature will favour either the endothermic or exothermic reaction. The ratio
between the concentration of the reactants and products will change. Kc will change.

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Rate-time graphs
A change in temperature or adding/removing a catalyst changes both the rate of the forward
and reverse reactions. Adding a catalyst or removing a catalyst will change both rates equally
while a change in temperature will affect the forward and reverse reactions to different extents.
A change in pressure of the system or concentration of reactants/products will only affect one
rate.

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Concentration-time graphs
A change in concentration of a reactant or product will cause a sharp increase or decrease in the
rate of that compound. The concentrations of the other reactants and products will not change
much.
A change in temperature will affect the forward and reverse rates equally but in opposite
directions.
A change in pressure of the reaction would cause a sharp increase or decrease in all the
reactants and products.
The addition of a catalyst would increase both the forward and reverse reaction rates, meaning
the equilibrium is reached faster.

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