IB Chem Topic 7: Equilibrium

Chemical equilibrium

Forward and reverse reactions occur at the same rate, resulting in no net change in the concentrations of reactants and products.

Characteristics of Equilibrium: Equilibrium is dynamic

No reactions have stopped, just the forward and backward reactions occur at the same rate. Reactants and products are still being produced.

Characteristics of Equilibrium: Equilibrium is achieved in a closed system

No matter can be exchanged with surroundings because reactants and products must be able to react and recombine with eachother

Characteristics of Equilibrium: Concentrations constant

Concentrations of R and P must be constant, but not necessarily equal

Characteristics of Equilibrium: No change in macroscopic properties

Macroscopic properties are easily observable (eg. colour, density) and should remain constant at equilibrium because there is no change in product/reactant concentration

Characteristics of Equilibrium: Can be reached from either direction

Whether the reaction starts on the left or right, or with a mixture of P and R, the same equilibrium will be reached under the same conditions.

Equilibrium position

Proportion of products and reactants at equilibrium; is said to ‘lie to the left’ (reactants) or ‘lie to the right’ (products

Homogeneous equilibrium

Reactants and products are in the same state

Heterogeneous equilibrium

Reactants and products are not all in the same state

Equilibrium constant

Kc, ratio between concentration of products (top) and reactants (bottom) at a specified temperature

Kc expression

Kc= ([C]^c [D]^d…) / ([A]^a [B’]^b)

Kp

The equilibrium constant used for gasses, measured in pressure instead of concentration, derived from the same expression

_____ not included in mass action expression

solids and liquids

A large K value means…

Equilibrium lies to the right in favour of the products therefore reaction has gone almost to completion

K=1 means…

The concentration of reactants and products are equal at equilibrium

A small K value means…

Equilibrium lies to the left in favour of the reactants therefore the reaction has barely happened

The reaction quotient

Qc/Qp, identical to the equilibrium constant expression, but concentrations are not necessarily at equilibrium. Can predict which direction a reaction must go to reach equilibrium.

If Qc>Kc…

There is too much product, the reaction will shift to the left to re-establish equilibrium

If Qc=Kc…

The system is at equilibrium

If Qc<Kc

There is too much reactant, the reaction will shift to the right to re-establish equilibrium

Le Châtelier’s Principle

When a system at equilibrium is subjected to a stress, it will respond in such a way as to minimize the effect of the stress (shift left or right). A new equilibrium will be established with a different composition from earlier equilibrium mixture

Changes in concentration effect on equilibrium

reacts to remove some of the added substance or replenish some of the removed substance.

Changes in volume/pressure effect on equilibrium

System shifts in direction that will balance the number of gas molecules to give original Kc value, ie. increase in pressure will cause equilibrium to shift to side with fewer gas particles (inverse is also true)

Change in temperature

Will shift in direction that disperses some of the heat, Increase in temperature increases the value of Kc for an endothermic reaction and decreases value of Kc for an exothermic reaction

Addition of inert gas effect on equilibrium

no effect, because an inert gas does not change the volume

Addition of a catalyst effect on equilibrium

No effect, lowers the activation energy for forward and backward reaction equally, however it does increase RoR therefore equilibrium is reached faster