Equilibrium and Le Chatlier

What is equilibrium?

Reactions that occur in both directions (reversible reactions)

At equilibrium, do the forward and reverse reactions occur at the same rate?

Yes

At equilibrium, do the forward and reverse reactions produce equal amounts?

No, some amounts of reactants and products must be present, but generally not equal amounts of each.

What is Le Chatlier’s principle?

When a system at equilibrium is subjected to a stress, the equilibrium will shift in a direction that tends to reduce that stress

Describe the changes in concentration when something is added or removed at equilibrium.

• When something is added, shift away from that thing

• When something is removed, shift towards that thing

• Only amounts of (aq) and (g) count; (s) and (l) have NO EFFECT

• Adding a substance not in the reaction will have NO EFFECT

Describe the effect of a change in volume or pressure when a reaction is at equilibrium.

• If volume is reduced (or pressure is increased), shift to side with fewer gas particles

• If volume is increased (or pressure is decreased), shift to side with more gas particles

Describe the effect of a change in temperature when a reaction is at equilibrium.

• If temperature is increased, shift to side without heat

• If temperature is decreased, shift to side with heat

Describe the effect of other changes (not temperature, pressure, or volume) when a reaction is at equilibrium.

Other changes that may happen to the reaction at equilibrium, will NOT AFFECT it (for example, catalysts)

What is the equilibrium constant?

K (or Keq)

What does it mean if Keq is less than one?

The reactants are favored over the products

What does it mean if Keq is greater than one?

The products are favored over the reactants

What does it mean if Keq is equal to one?

Reactants and products are both present in significant concentrations at equilibrium

What are the types of stress a reaction can experience?

• Changes in concentration

• Catalyst

• Particle Size

• Changes in pressure

• Changes in temperature

What does the graph of a reaction at equilibrium look like?

(could also be shown as two separate lines, one being the forward reaction and the other being the reverse reaction, converging into one at the equilibrium state)

What does K measure?

The ratio of product concentrations in molarity (M), or pressures (in atm) to reactant concentrations or pressures

What is the formula for equilibrium expression in moles or liters?

In a reaction where aA+bB—>cC+dD

Keq= [C]^c[D]^d/[A]^a[B]^b

What is the formula for equilibrium expression in pressure?

In a reaction where aA+bB—>cC+dD

Kp=(P_D)^d(P_C)^c/(P_A)^a(P_B)^b