Equilibrium

Question 1

define chemical equilibrium

Answer 1

the state reached when the concentrations of reactants and products remain constant over time
- forward and reverse reactions occur at the same rate
- reactions are still occurring, but there is no net reaction
- chemical equilibria are dynamic and reversible

Question 2

Kc =

Answer 2

aA + bB <–> cC + dD

Kc = [C]^c[D]^d/[A]^a[B]^b
concentrations at equilibrium

Question 3

Kc when you reverse the reaction

Answer 3

1/Kc

Question 4

Kc when you multiply the reaction by 2

Answer 4

Kc^2

Question 5

Kc when you add two reactions

Answer 5

Kc1 x Kc2

Question 6

units of the equilibrium constant

Answer 6

equilibrium constants are defined as unites quantities

Question 7

homogeneous equilibria

Answer 7

all species are present in the same phase.

Question 8

heterogeneous equilibria

Answer 8

system’s state of equilibrium contains components from multiple phases

Question 9

why are pure solids and liquids excluded from equilibrium expressions?

Answer 9

they are always in their standard state, where a=1: their concentrations don’t change

Question 10

equilibrium constant Kp

Answer 10

for gases, we can use pressures instead of concentrations

Kp = Kc(RT)^Δn
Kp = Pc^c x Pd^d/Pa^a x Pb^b

Δn is the number of moles of gaseous products minus the number of moles of gaseous reactants

Question 11

Kc > 10^3

Answer 11

reaction proceeds nearly to completion, products favoured

Question 12

Kc < 10^-3

Answer 12

reaction proceeds hardly at all, reactants favoured

Question 13

10^-3<Kc<10^3

Answer 13

appreciable concentrations of both reactants and products

Question 14

reaction quotient, Qc

Answer 14

defined similarly to an equilibrium constant Kc except that the []’s in Qc can have ant values, not necessarily equilibrium values

Question 15

if Qc = Acc

Answer 15

no net reaction occurs, already at equilibrium

Question 16

if Qc < Kc

Answer 16

net reaction goes from left to right

Question 17

if Qc > Kc

Answer 17

net reaction goes from right to left

Question 18

le Chatelier’s principle

Answer 18

if a stress is applied to a reaction mixture at equilibrium, a net reaction then occurs in the direction that relieves the stress

Question 19

in general, when an equilibrium is disturbed by the addition or removal of any reactant or product, Le Chateliers principle predicts that

Answer 19

• the concentration stress of an added reactant or product is relieved by net reaction in the direction that consumes the added substance
• the concentration stress of a removed reactant or product is relieved by net reaction in the direction that replenishes the removed substance

Question 20

in general, when an equilibrium is disturbed by a change in volume that results in a corresponding change in pressure, Le Chateliers principle predicts that

Answer 20

• reduction in volume increases the partial pressures of all gases, which provokes a net reaction in the direction that decreases the total moles of gas
• enlargement in volume decreases the partial pressures of all gases, which provokes a net reaction in the direction that increases the total moles of gas

Question 21

adding inert gas

Answer 21

no effect, because the partial pressures (molar concentrations) don’t change

Question 22

temperature can alter the equilibrium concentrations, but for a different reason:

Answer 22

it changes the value of Kc

Question 23

in general, when an equilibrium is disturbed by a change in temperature,

Answer 23

Kc for an exothermic reaction (-ΔH) decreases as the temperature increases
Kc for an endothermic reaction (+ΔH) increases as the temperature increases

Question 24

linking chemical equilibrium and kinetics

Answer 24

A + B <-(Kf)-(Kr)-> C + D

rate forward = Kf[A][B]
rate backward = Kr[C][D]
at equilibrium: Kf[A][B]=Kr[C][D]

Kf/Kr = [C][D]/[A][B] = Kc
Kc = kf/kr

Question 25

if Kc is really large
if Kc is nearly unity
if Kc is really small

Answer 25

Kf&raquo_space; Kr and the reaction goes almost to completion
Kf = Kr and both reactants and products exist at equilibrium
Kr&raquo_space; Kf and the reaction mixture consists mostly of reactants

Question 26

effect of a catalyst on equilibrium

Answer 26

• because the forward and reverse reactions pass through the same transition state, the catalyst lowers the activation energy barrier by the same amount
• the catalyst accelerates the forward and reverse reactions by the same factor, so the composition of the equilibrium mixture is unchanged
• Kc is not affected by the catalyst
• we reach the same equilibrium, only faster

Question 27

Haber proces

Answer 27

N2 + 3H2 <-> 2NH3
exothermic

reaction conditions:
- catalyst: iron mixed with metal oxides increases the rate
- temperature: 400-500’C
- pressure: yield improved by running the reaction at high pressures (130-300atm)