A + B ↔ C + D
Reactants: A and B
Products: C and D
Rate: How fast the reaction reaches equilibrium.
Forward reaction: A + B → C + D
Backward reaction: A + B ← C + D
A + B ↔ C + D
The equilibrium position is to the left: The mixture at equilibrium contains mostly reactants (A and B)
The equilibrium position is to the right: The mixture at equilibrium contains mostly products (C and D)
Yield: How much is produced in the reaction.
In all reactions, there are in fact two reactions occurring, one where the reactants produce the products,
and the other where the products react to form the reactants. In some reactions, this second reaction is insignificant,
but in others there comes a point where the two reactions exactly cancel each other out. Thus the reactants and
products remain in constant proportions, though both are continually being used up and produced at the same time.
The equilibrium constant Kc is a constant which represents how far the reaction will proceed at a given temperature.
Kc > 1 : There is more products than reactants (at equilibrium)
Kc >> 1 : The reaction goes almost to completion (the reactants almost disappear)
Kc < 1 : There is more reactants than products (at equilibrium)
Kc << 1 : The reaction hardly occurs at all
The only thing which can change the value of Kc for a given reaction is a change in temperature.
NOTE: A CHANGE OF CONCENTRATION OR PRESSURE WILL NOT CHANGE Kc, ONLY THE TEMPERATURE WILL.
The position of equilibrium, however, can change without a change in the value of Kc.
Effect of Temperature : The effect of a change of temperature on a reaction will depend on
whether the reaction is exothermic or endothermic. When the temperature increases, Le Chatelier’s
principle says the reaction will proceed in such a way as to counteract this change, i.e. lower the temperature.
Exothermic reactions:ΔH = negative → the forward reaction will produce energy → which
will increase temperature → an increase of temperature from the outside will make the reaction go backwards →
the yield goes down → lower Kc (Remember: Higher temperature + Exo → ΔH = - → backwards → lower yield)
Endothermic reactions:ΔH = positive → the forward reaction will absorb energy →
which will decrease temperature → an increase of temperature from the outside will make the reaction go forwards →
the yield goes up → higher Kc (Remember: Higher temperature + Endo → ΔH = + → forwards → higher yield.)
Effect of Concentration : When the concentration is increased on one side of the reaction it will go
more to the other side in order to compensate and lower the concentration of the chemical that was increased. When
for example the concentration of a reactant is increased, the reaction proceeds forward to decrease
the concentration of reactants.
Kc never change when the concentrations are changed. The equilibrium position move to the right.
Effect of Pressure : Look how many moles of gas you have on both sides of the reaction. An increase
of the pressure will make the reaction goes away from the side with most moles towards the side with fewer moles.
2A(g) + 8B(aq) ↔ C(g) + 2D(g)
There is 2 moles of gas on the left side and 3 moles of gas on the right side. An increase of pressure will
make the reaction go to the left so the equilibrium position will move to the left. Kc do not change when the pressure
is changed.