3.8 equilibrium constants

Question 1

what is Kc?

Answer 1

• the equilibrium constant in terms of concentration
• aA (aq) + bB (aq) ⇌ cC (aq) + dD (aq)
• Kc = [C]^c[D]^d / [A]^a[B]^b

Question 2

what is Kp?

Answer 2

• the equilibrium constant in terms of partial pressures
• only useful for gas mixtures
• Kp= pC^c pD^d / pA^a pB^b

Question 3

give the equilibrium constant of Sn4+ + 2Fe2+ ⇌ 2Fe3+ + Sn2+

Answer 3

Kc= [Fe3+]^2 [Sn2+] / [Sn4+] [Fe2+]^2

Question 4

give the equilibrium constant of N2O4 (g) ⇌ 2NO2 (g)

Answer 4

Kp= p(NO2)^2 / pN2O4

Question 5

what are the units for equilibrium constant?

Answer 5

• some have units and some dont
• if there are the same number of concentration terms on both the top and bottom lines of the end constant, these units cancel out and there are no units

Question 6

A + B —> 2C + D
- if an equimolar solution of A and B where the concentration of each is 0.5moldm^-3 is allowed to reach equilibrium then the equilibrium mixture contains 0.2moldm^-3 of D. calculate the value of Kc

Answer 6

[A] [B] [C] [D]
conc at start: 0.5 0.5 0 0
equilibrium: 0.3 0.3 0.4 0.2
so Kc= [C]^2 [D] / [A] [B]
= 0.4^2 x 0.2 / 0.3 x 0.3
= 0.355 moldm^-3

Question 7

an equilibrium that has similar amounts of starting materials and products would have a Kc value around ___?

Answer 7

1

Question 8

if Kc < 1, equilibrium lies to the (right/left) hand side?

Answer 8

LHS where very little products are formed

(and ∆G is +ve, so the reaction doesnt occur spontaneously)

Question 9

if Kc>1, equilibrium lies to the (right/left) hand side?

Answer 9

RHS
- most of the reactants have been converted into products
- this is the case when ∆G is -ve, as the reaction will occur spontaneously

Question 10

what is the only factor that will change the value of Kc or Kp?

Answer 10

temperature

(changing pressure, adding a catalyst or making any other change doesnt change the values of Kc and Kp)

Question 11

if we change the temperature, how can we work out the effect on the equilibrium position?

Answer 11

using le chatelier’s principle

Question 12

what happens to the equilibrium constant if you increase the temperature in an exothermic reaction?

Answer 12

• if reaction is exothermic, increasing temperature will shift the equilibrium to the left as this is the endothermic direction
• this decreases the products and increases the reactants
• as equilibrium constant is products/reactants, equilibrium constant decreases

Question 13

what happens to the equilibrium constant if you increase the temperature in an endothermic reaction?

Answer 13

• if the reaction is endothermic, increasing the temperature will shift the equilibrium to the right as this is the endothermic direction
• this increases the products and decreases the reactants
• which makes the equilibrium constant larger

Question 14

what information does reaction rates give us?

Answer 14

• gives us information about the changes that occur between the reactants and transition states
• this allows is to deduce what is happening during the reaction, giving the reaction mechanism

Question 15

• companies consider kinetic, energetic and equilibrium data when planning any process and any industrial reaction aims to produce the maximum amount of product as quickly as possible using the least energy
• often a compromise is needed which gets each of these as close to each each ideal value as possible

Answer 15

• equilibrium : the equilibrium yield of product can be changed by concentration, pressure or temperature
• rates : made as fast as possible by increasing temperature, increasing pressure or adding a catalyst
• energetics: will identify how much energy needs to be input into the system for a reaction to occur, avoiding the input of excess energy

Question 16

le chatelier’s principle:

Answer 16

• if a chemical system at equilibrium experiences a change in concentration, pressure or temperature, the equilibrium will shift to minimise this change

Question 17

if Kc=1 …

Answer 17

then the equilibrium has equal amounts of reactants and products

Question 18

dynamic equilibrium definition

Answer 18

• forwards rate of reaction is equal to the backwards rate of reaction

Question 19

calculate the units for Kc where Kc= [D][E]^4 / [A]^2[B]^3

Answer 19

no units

Question 20

why does Kc change when the temperature of a reversible reaction in a closed system is changed?

Answer 20

• the Kc value is only valid for a certain temperature
• when the temperature changes the position of equilibrium shifts and so the equilibrium concentrations of the products and reactants changes
• this leads to a change in Kc

Question 21

explain why a compromise temperature of 450°C is used in the Haber process:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
∆H = -46.2kJmol^-1

Answer 21

• since ∆H is negative, forwards reaction is exothermic
• so a low temperature is required to shift equilibrium towards the products, and increase the yield of products
• but the temperature cannot be too low as this will lead to a very slow rate of reaction
• so a compromised temperature is used so that the forwards reaction is favoured but the rate of reaction is also relatively fast

Question 22

how does changing the concentration of a reactant or product affect the value of Kc?

Answer 22

changing the concentration of a reactant or product has no effect on Kc

Question 23

how does a catalysts affect the value of Kc?

Answer 23

• a catalyst has no effect on Kc
• the catalyst will speed up the forwards and backwards reactions at the same rate so the ratio of products to reactants problem will remain the same

Question 24

define partial pressure and total pressure in relation to a mixture of gas:

Answer 24

• partial pressure = the pressure exerted by an individual gas in a mixture
• total pressure = the sum of all the partial pressures of the individual gases in the mixture

Question 25

what are the units of partial pressure?

Answer 25

kPa

Question 26

what is the significance of the magnitude of an equilibrium constant?

Answer 26

- the magnitude of the equilibrium constant indicates where the position of equilibrium lies - the larger the value of Kp or Kc, the further towards the right the position of equilibrium lies