Equilibrium

Equilibrium
Equilibrium exists when the rate of the forward
reaction is equal to rate of the reverse reaction.

When the sample is at equilibrium you will not
see any visible change or movement because for each
A that becomes B there is a B that becomes A.

A. Mass action expression:

If my original equation looks like this:
mA + nB

pC + qD
where A, B, C & D are gases or aqueous solutions
and m, n, p & q are their coefficients. If A, B, C or D
are solids or liquids we use a 1 in the mass action expression.

Then the mass action expression would look like this:

= K_eq (the equilibrium constant)

We divide the concentration of the products by the concentration
of the reactants and the coefficients become the superscripts.

If your K_eq value is > 1 the reaction will favor pruduction of the products
because the products are on the top (numerator) of the fraction.
A large numerator means the fraction has a value greater than one.

If your K_eq value is < 1 the reaction will favor pruduction of the reactants
because the reactants are on the bottom (denominator) of the fraction.
A large denominator means the fraction has a value less than one.

B. LeChatelier’s Principle:

“When a system at equilibrium is subjected to a stress the
system shifts in order to relieve the effects of that the stress
and restore equilibrium conditions as closely as possible.”

If we increase the concentration of any one reactant or of any one product...

the reaction shifts away from side to which you add. If I add A it will shift to the right. If I add A₂B it will shift to the left.

There is no change in the K_eq value for this reaction.

If we increase the reaction temperature we will cause the endothermic reaction to take place...

the reaction shifts away from side with the heat term. The heat term takes the form of kilojoules, calories or the words heat or energy.

This is the only change that causes a change in the K_eq

If we decrease the reaction temperature we will cause the exothermic reaction to take place...

the reaction shifts toward side with the heat term. The heat term takes the form of kilojoules, calories or the words heat or energy.

This is the only change that causes a change in the K_eq

If we decrease the pressure on a reaction, we decrease pressure on both sides of the reaction...

the reaction will shift to the side with the most moles. In this reaction we will shift to the left because there are 6 moles of reactant.

There is no change in the K_eq value for this reaction.

If we add a catalyst to the reaction it will speed up both the forward and the reverse reactions equally...

therefore we will not see any change in our reaction or our equilibrium.

There is no change in the K_eq value for this reaction.

Effect of Shift

If you know the direction the reaction will shift
you also now what happens to the consentration
of both the reactants and the products.

	If you shift to the right the reactants will decrease and the products will increase in concentration.

	If you shift to the left the reactants will increase and the products will decrease in concentration.