|Reversibility of Reactions at Equilibrium|
|Once a system has reached equilibrium, any factor which
causes a change in the rate of either the forward or reverse reaction will
disturb or shift the equilibrium. The system will readjust itself
so that a new equilibrium is reached and the rates will again become equal.
|Le Châtelier's Principle|
|When a stress is applied to a system at Equilibrium, the system
readjusts so as to relieve or offset the stress. Stress is any imposed factor
which upsets the balance in rates between the forward and reverse reactions.
Stress factors may be changes in concentrations , total pressure with gases
only, volume changes (which cause the pressure changes), and temperature.
|Stress and Changes in Concentration [ ]|
|Increasing the concentration of the reactants will cause a stress
which is relieved by an increase in the concentration of product, [product].
If the [product] is increased the [reactants] will increase to offset the
|Stress and Changes in total Volume (or related
|eg. N2(g) + 3 H2(g)
<=====> 2 NH3(g) + 92.00 kJ
|A stress imposed by a decrease in volume is actually a stress
caused by the increase in the concentration. The stress is relieved when
the system reduces the number of molecules.
|ie. 4 molecules ------> 2 molecules
|A decrease in total pressure (which is caused by an increase
in volume) will shift the reaction to the reverse. A decrease in to total
volume of a gaseous system (or the accompanying increase in pressure) shifts
an equilibrium in the direction of the fewer molecules as shown by the equation
for the reaction.
|Stress and Changes in Temperature|
|In any reaction a temperature increase favours the reaction
that absorbs heat. i.e. the endothermic reaction.
|eg. N2(g) + 3 H2(g) <=====>
2 NH3(g) + 92.00 kJ
|This reaction is exothermic as written. That means the reverse
reaction or backward reaction is endothermic. Adding heat (demonstrated as
an increase in temperature) will shift the reaction left and more reactants
will be formed as the products get used up. A decease in temperature will
favour the reaction that produces heat. i.e. the exothermic reaction. The
reaction about would switch to the forward direction to produce more heat
|Effect of a Catalyst|
|A catalyst equally favours both the forward and reverse reactions.
Therefore a catalyst does not shift the Equilibrium [ ]'s. It simply
causes the reaction system to reach Equilibrium in a shorter period of time.
For the reaction below:
2 SO2(g) + O2(g)
<=====> 2 SO3(g) + heat
|Indicate the favoured reaction either forward or reverse under
the stress applied below.
a) increase the [SO2]
The reaction will shift to the forward direction and more SO3will be produced.
|b) Partial pressure of SO3 is decreased (Some of
the SO3 is removed fro the system).
The system will shift to the right. As the SO3 is removed its concentration drops. The reactants will react to produce more product.
|c) Decrease in temperature.
The reaction will shift right. As written the reaction is exothermic. As heat is removed the reaction proceeds to produce more heat.
|d) He gas is added at constant volume so that the total pressure
No change takes place. Dalton Law of Partial Pressures comes into play here. Each gas is treated separately from it's neighbours. Because the volume stays constant then the concentrations don't change. Therefore no changes take place.
|e) He gas is added, and the container is allowed to expand so
that the total pressure is kept constant.
To the left or the reactants will be favoured. The volume was allowed to increase to maintain the total pressure. Since the volume increased the concentrations must have dropped for all reactants and products. The reaction will react do produce the maximum number of moles of gas to fill the available space. Hence the reactants are favoured.
|f) A catalyst is added.
No change. The catalyst favours both reactions, forward and reverse equally.
For the reaction below:
heat + 2 H2O(g) <=====> 2 H2(g) + O2(g)
Indicate the direction of the reaction most favoured by each
of the following conditions.