TOPIC 7: CHEMICAL EQUILIBRIA

Question 1

[Definition] Reversible Reaction

Answer 1

A reversible reaction is a reaction where the reactants form products that can react with each other to re-form the reactants.

Question 2

What reactions are reversible and what are irreversible?

Answer 2

In principle, all reactions are reversible in a CLOSED system and irreversible in an OPEN system

Question 3

What is position of equilibrium?

Answer 3

The composition (aka concentration) of reactants and products at a particular state of equilibrium

Question 4

[Definition] Dynamic Equilibrium

Answer 4

Dynamic equilibrium refers to a reversible process at equilbrium in which the rate of forward reaction equals to the rate of backward reaction and the concentrations of reactants and products do not change.

Question 5

[Formula] Equilibrium Constant, Kc

Answer 5

aA + bB ⇌ cC + dD

Kc = ( [C]ᶜ[D]ᵈ ) / ( [A]ᵃ[B]ᵇ )

Question 6

Difference between Kc and Q (reaction quotient)

Answer 6

Q is ratio of concentration of products to reactants at ANY INSTANT

So, at equilbrium, Q = Kc

Question 7

Purpose of Kc

Answer 7

To know the ratio of products to reactants to determine the position of equilibrium (ie the extent of the reaction at equilibrium)

ONLY tells how FAR the reaction is, NOT how FAST the reaction is

Kc&raquo_space; 1 (position of eqm lies to far right, rxn alm complete)
Kc &laquo_space;1 (position of eqm lies to far left, rxn barely started)

Question 8

Homogenous vs Heterogenous Equlibrium

Answer 8

Homogenous: All substances are in the same phase
Heterogenous: Substances in different phases

Question 9

How many phases are there for oil on water in a glass container?

Answer 9

4 - air, oil, water, glass

Question 10

What is omitted from a Kc expression?

Answer 10

H2O if it is the solvent (ie other substances are aqueous)

Solids and liquids becausae both are constants

Question 11

Calculation of Kc

Answer 11

Initial amt / conc
Chg in amt / conc
Eqm amt / **EQM CONC

**Must have for calculation.
The rest are amt / conc cos it depends on what the qn gives

Question 12

[Formula] Equilibrium Constant, Kp

Answer 12

aA + bB ⇌ cC + dD

Kc = ( [p of c]ᶜ[p of d]ᵈ ) / ( [p of a]ᵃ[p of b]ᵇ )

Question 13

Possible units of Kp

Answer 13

Pa (most commonly used)
atm
Nm⁻²

Question 14

What is omitted from a Kp expression

Answer 14

Solids and liquids cos it contributes little to no pressure to the system

Question 14

What is omitted from a Kp expression

Answer 14

Solids and liquids cos it contributes little to no pressure to the system

Question 15

Factors affecting Kc / Kp

Answer 15

ONLY temperature !!!

NO concentration / partial pressure / pressure / catalyst / reaction mechanism

Question 16

How to determine Kc experimentally?

Answer 16

aA + bB ⇌ cC + dD

1. Mix known amount of A and B
2. Stand for some time in a thermodynamically-controlled water bath
3. When eqm established, quickly cool
4. Titrate rapidly to determine conc
5. Repeated at same temp usint diff initial amts

Question 17

Interpretation of ΔG and K

Answer 17

ΔG &laquo_space;0 & K&raquo_space; 1 : Position of eqm lies to far right

ΔG&raquo_space; 0 & K &laquo_space;1 : Position of eqm lies to far left

Question 18

[Definition] Le Chatelier’s Principle

Answer 18

Le Chatelier’s Principle states that if a system in equilibrium is subjected to a change which disturbs the equilibrium, the system responds in such a way to counteract the effect of the change imposed, in order to re-establish the equilibrium of the system.

Question 19

Factors affecting chemical equilibria

Answer 19

1. Concentration / Partial Pressure
2. Pressure / Volume
3. Temperature
4. Catalyst

Question 20

[Phrasing] Chg in conc / partial pressure

Answer 20

When XX is added / removed,
the position of equilibrium shifts to the left / right
to remove / form some of the XX that was added / removed