Chemical Equilibrium :)

Define reversible reactions

Reversible reactions are reactions that proceed in both forward and backward reactions

Define dynamic equilibrium

When a reversible reaction in a closed system reaches dynamic chemical equilibrium, reaction continues to occur and rates of the forward and backward reaction are equal, hence there is no net change in the concentrations of reactants and products

Define equilibrium constant Kc

Kc is the ratio of the product of the concentrations of products raised to the appropriate power to the product of the concentrations of reactants raised to the appropriate powers at equilibrium at a given temperature

What does the Kc expression not include?

1. Solids

2. In an aqueous system where water is the solvent, Kc does not include the concentration of water (Water is included when there is no aq)

What is the significance of Kc?

Kc measures the extent to which a reversible reaction takes place. It is independent of the initial composition of the reaction mixture

What happens when Kc>1?

When Kc»»»1, reaction proceeds from left to right to a greater extent and formation of products is favoured. The reaction is “effectively complete”

What happens when Kc<1?

When Kc«««1, the reaction proceeds from right to left to a greater extent and formation of reactants is favoured. The reaction is “negligible”

Define Le Chaterlier’s Principle

When a system at dynamic equilibrium is subjected to a change which disturbs the equilibrium, the system will react in a way so as to counter-act the effect of the change to re-establish equilibrium

What happens to the equilibrium position and equilibrium composition when more reactant 1 is added?

By Le Chatelier’s Principle, equilibrium position shifts right to decrease the concentration of reactant 1. The new equilibrium mixture will contain more products, more reactant 1 and less reactant 2

What happens to the equilibrium position and equilibrium composition when concentration of reactant 1 decreases?

By Le Chatelier’s Principle, equilibrium position shifts left to increase the concentration of reactant 1. The new equilibrium mixture will contain more reactant 2, less products and less reactant 1

Is Kc affected by changes in concentration?

No

Considering a reversible reaction from colourless N2O4(g) to form brown 2NO2(g), what happens to equilibrium position and equilibrium composition when pressure increases?

By Le Chatelier’s Principle, equilibrium position shifts left to decrease the total number of moles of gas, hence decreasing pressure. The new equilibrium mixture will contain more reactant N2O4 and less products NO2. Therefore, the brown colour lightens

Considering a reversible reaction from colourless N2O4(g) to form brown 2NO2(g), what happens to equilibrium position and equilibrium composition when pressure decreases?

By Le Chatelier’s Principle, equilibrium position shifts right to increase the total number of moles of gas, hence increasing pressure. The new equilibrium mixture will contain more product NO2 and less reactant N2O4. Therefore, brown colour deepens

Consider a reversible reaction between N2(g) + O2(g) to form 2NO(g), what happens to equilibrium position and equilibrium composition when pressure increases?

Since total number of moles of gaseous reactant is equal to the total number of ones of gaseous products, changes in pressure would not affect the equilibrium position and hence composition of the equilibrium mixture

Considering a reversible reaction is endothermic in the forward reaction, what happens to equilibrium position and equilibrium composition when surrounding temperature increases?

By Le Chatelier’s Principle, equilibrium position shifts right to towards the endothermic reaction to absorb excess heat. The new equilibrium mixture will contain more products and less reactants

Considering a reversible reaction is exothermic in the backward reaction, what happens to equilibrium position and temperature when surrounding temperature decreases?

By Le Chatelier’s Principle, equilibrium position shifts left towards the exothermic reaction to release heat. The new equilibrium mixture will contain more reactants and less products

Is Kc temperature dependent?

Yes

What happens to equilibrium position and equilibrium composition when a catalyst is added?

Catalyst increases the rate of both forward and backward reactions to the same extent hence equilibrium position remains unchanged and equilibrium composition is the same. Catalyst increase the rate of reaction so that dynamic equilibrium is established more quickly

What is the chemical equation describing the haber process?

N2(g) + 3H2(g)→ 2NH3(g) (enthalpy change= -92 kJmol^-1)

What are the 4 operating conditions for the haber process?

1. High pressure of 250 atm

2. Moderate temperature of 450 degrees celcius

3. Iron catalyst

4. Molar ratio of N2: H2 is 1:3

With reference to the haber process, why is pressure at 250 atm?

By Le Chatelier’s Principle, high pressure leads to equilibrium position shifts right to decrease the total number of moles of gas to decrease pressure. The new equilibrium mixture will contain more product NH3 and less reactants N2 and H2. However, too high a pressure would be too expensive to maintain as thicker pipes would need to be built to withstand the higher pressure. Therefore, a pressure of 250 atm is used

With reference to the haber process, why is a moderate temperature of 450 degree celcius used?

By Le Chatelier’s Principle, low temperature results in equilibrium position shifts right towards the exothermic reaction to release heat. The new equilibrium mixture will contain more product NH3 and less reactants N2 and H2. However, too low a temperature would decrease the rate of reaction, making it not feasible industrially. Therefore, moderate temperature of 450 degrees celcius is used

With reference to the haber process, why is an iron catalyst required?

Iron catalyst increases reaction rate so that dynamic equilibrium can be established more quickly but does not affect the yield of NH2

What does changes in gibbs free energy measure?

Changes in gibbs free energy is a measure of the spontaneity of a reaction

What does a negative change in gibbs free energy mean?

The reaction is spontaneous in the forward reaction

What does a positive change in gibbs free energy mean?

The reaction is spontaneous in the backward reaction

What does change in gibbs free energy=0 mean?

The reaction is at equilibrium

Compare reaction quotient, Q to equilibrium constant, Kc

Both the reaction quotient, Q and equilibrium constant, Kc have the same expression. However, the reaction quotient represents the quantity of reactant and products at any composition while the equilibrium constant represents the quantity of reactants and products at equilibrium at a given temperature