Chemical equilibrium

Definition of: closed system

One in which mass is conserved inside the system but energy can enter or leave the system freely

Definition of: open system

One in which both energy and matter can be exchanged between the system and its surroundings

Definition of: reversible reaction

One in which products can be converted back into reactants

Definition of: dynamic chemical equilibrium

A reversible reaction in which the forward and reverse reactions take place simultaneously and at the same rate, hence the concentrations of reactants and products are constant

Definition of: equilibrium constant (Kc)

An expression of the concentration of the products over the concentration of the reactants for a reaction that is in equilibrium

Definition of: Le Chatelier's principle

When an external stress (change in pressure, temperature or concentration) is applied to a system in dynamic chemical equilibrium, the equilibrium point will change in such a way as to counteract the stress

Definition of: yield

A measure of the extent of a reaction, generally measured by comparing the amount of product formed against the amount of product that is possible

How to define Kc (Generally)

How does dynamic equilibrium look on a graph in terms of rate?

How does dynamic equilibrium look on a graph in terms of concentration / amount of mol?

FACTORS AFFECTING THE EQUILIBRIUM POSITION

1. Concentration (Reactants or products)
2. Temperature
3. Pressure
4. The common ion effect

How does the following affect the equilibrium position: Concentration (Reactants or products)

The reaction that favours counteracting the change will be favoured. Take note: [] of a solid or liquid is given by its density, meaning that if a solid or liquid is added there is no change in the equilibrium position.

How does the following affect the equilibrium position: Temperature

An increase in temperature always favours the endothermic reaction. A decrease in temperature always favours the exothermic reaction.

How does the following affect the equilibrium position: Pressure

An increase in pressure favours the reaction that proceeds towards the least number of moles of gas. A decrease in pressure favours the reaction towards the most number of moles of gas.

How does the following affect the equilibrium position: A catalyst

Does not affect the equilibrium position. If a catalyst is added to a system that is not yet in equilibrium, the equilibrium would be reached sooner with a catalyst, compared to a system without a catalyst.

How does the following affect the equilibrium position: The common ion effect

If a compound is added to a system in an aqueous solution in equilibrium and that compound contains an ion that is already present in the system, then the reaction that uses up that ion will be favoured.

What are the 3 examinable industrial processes which are affected by equillibrium?

THE HABER PROCESS (preparing ammonia)

THE OSTWALD PROCESS (preparing nitric acid)

THE CONTACT PROCESS (preparing sulfuric acid)

THE HABER PROCESS

THE OSTWALD PROCESS

THE CONTACT PROCESS

What factors can affect the K_c constant value

What do the different K_c values indicate?

What format should be used to calculate questions about changes in mol amounts in equilibrium questions.

What is K_sp?

What is K_a?

What is K_b?

What is K_w?

How to answer questions with Le Châtelier's principle

1. S — Stress: State exactly what changed.
   "The [pressure/temperature/concentration of X] is [increased/decreased]."

2. R — Response: Apply Le Châtelier's principle — state that the system will shift to counteract that stress.
   "By Le Châtelier's principle, the equilibrium will shift to oppose this change, i.e. to [decrease/increase] the [pressure/temperature/concentration]."

3. F — Favoured reaction: State which direction (forward or reverse) is favoured as a result.
   "This favours the [forward/reverse] reaction, i.e. the reaction that [produces more moles of gas / absorbs heat / consumes X], because..."

4. E — Effect: State the actual outcome — on concentrations/moles of reactants and products, and (if asked) on Kc. MENTION IF IT SHIFTS TO THE LEFT OR TO THE RIGHT