Topic 7: Chemistry, Equilibrium

Define a Reversible Reaction

a reaction that can go either direction and can come to a state of equilibrium

Define Dynamic Equilibrium

concentrations of all reactants and products remain constant and the rate of the forward reaction is equal to the rate of the reverse reaction

Define a Closed System

no exchange of matter with the surroundings and equilibrium can only be reached in a closed system

Define Physical Equilibrium

equilibrium involving a physical process such as melting/freezing/evaporating/condensing

How do you notate the Equilibrium Constant?

Kc

What does the equilibrium constant tell us?

provides information about how far a reaction proceeds at a particular temperature

Where do the products go when forming a Kc expression?

numerator

Where do the reactants go when forming a Kc expression?

denominator

What number do you raise your concentrations to in a Kc expression?

co-efficients in reaction

How do you distinguish between the two Kc values obtained from the same reaction, one being the forward reaction value and one being the reverse reaction value?

Kc = forward Kc’ = reverse

A high Kc value =

equilibrium position lies to the right as you have mostly products in reaction mixture

A low Kc value =

equilibrium position lies to left as you have mostly reactants in reaction mixture

Greater reactants, equilibrium lies to

left

Greater products, equilibrium lies to

right

Kc>> 1 means the reaction…

proceeds almost totally towards the products

Kc<

hardly proceeds at all towards products

State Le Chatelier’s Principle

if a system at equilibrium is subjected to some change the position of equilibrium will shift in order to minimise the effect of the change

What happens to the equilibrium position when you increase pressure?

shifts to side with fewer moles

What happens to equilibrium position when you decrease pressure?

shifts to side with more moles

Does changing pressure effect Kc value?

no

What happens to equilibrium position when you increase temperature?

shifts to endothermic side (decrease temp)

What happens to equilibrium position when you decrease temperature?

shifts to exothermic side (increase temp)

What happens to Kc value when temperature increases in an endothermic reaction?

Kc increases

What happens to Kc value when temperature increases in an exothermic reaction?

Kc decreases

What happens to Kc value when temperature decreases in an endothermic reaction?

Kc decreases

What happens to Kc value when temperature decreases in an exothermic reaction?

Kc increases

What happens to equilibrium position when the concentration of one species in a mixture is increased?

shifts to side when chemical has not been added so it is used up

Does increasing the concentration of a species in a reaction affect the Kc value?

no

How does the introduction of a catalyst affect equilibrium position?

it doesn’t however the rate of forward and reverse reaction increase equally, has no effect on Kc

How do you notate the Reaction Quotient?

Q

What does the reaction quotient tell us?

How far a reaction has proceeded at any given time before, after or at equilibrium

How does Kc differ to Q?

Q does not use equilibrium concentrations

What does it mean if Q>Kc? Where does the equilibrium position shift?

reaction mixture contains more products, therefore equilibrium position shifts to left so more products are used up.

What does it mean if Q

reaction mixture contains more reactants, therefore equilibrium position shifts to right so more products are made.

What does it mean if Q=Kc?

reaction is at equilibrium

What direction does a reaction proceed in regarding Gibbs Free Energy?

a reaction proceeds in the direction that involves a decrease in Gibbs free energy

Why does a reaction reach equilibrium?

At equilibrium Gibbs free energy is at its lowest

Why is movement away from equilibrium unfavourable?

it will involve an increase in Gibbs free energy

Write the equation that relates Gibbs free energy with equilibrium constant.

ΔG° = -RTlnK