IB Chemistry on Dynamic Equilibrium and Homogeneous Equilibrium Constant

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Prepared by Lawrence Kok

Tutorial on Dynamic Equilibrium, Homogeneous, Heterogeneous Equilibrium and Equilibrium Constant .

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Dynamic Equilibrium

Reaction that goes to completion (irreversible) Mg + 2HCI → MgCI2 + H2

• Ea is low, energetically/kinetically favourable with ΔH -ve• Reaction in one direction reactants to products → • Reactants used up and becomes zero.

Reaction that will not go to completion (reversible) H2(g) + I2(g) ↔ 2HI(g)

• Reaction in both directions, reversible ↔• Forward reaction, to right forming products• Reverse reaction, to left forming reactants• Products dissociate to form reactants (Reactants will not be zero)• Must be in a closed system

Dynamic equilibrium• Both forward and reverse reactions continue to happen after achieving chemical equilibrium• Process/movement of particles between both sides is going on• Concentration of reactants and products remain constant because rate of forward = rate of reverse• Formation and decomposition continues

Static Equilibrium • Process /movement of particles between both sides stop• Formation and decomposition stops

Photo: http://declanfleming.com/man-vs-escalator-equilibrium-model/

http://chemistry.tutorvista.com/physical-chemistry/reversible-reaction-and-irreversibility.html

N2O4 (g) ↔ 2NO2

(g)

Rf (rate forward) = Rr (rate reverse) kf [N2O4] = kr [NO2]

2

Kc = (NO2)2

(N2O4)

When a reversible reaction achieved dynamic equilibrium - aA + bB ↔ cC and dD• Equilibrium constant Kc = ratio of molar conc of product (raised to power of their respective stoichiometry coefficient) to

molar conc of reactants (raised to power of their respective stoichiometry coefficient) • Kc is constant at constant temperature

Equilibrium Law

Kc = (C)c(D)d

(A)a(B)b

At equilibrium:

N2O4 (g) ↔ 2NO2

(g)


2

kf/kr = (NO2)2

(N2O4)

aA + bB ↔ cC and dD

N2O4 (g) ↔ 2NO2

(g)


2

Kc = (NO2)2

(N2O4)



Equilibrium Law

Kc = (C)c(D)d

(A)a(B)b

At equilibrium:

N2O4 (g) ↔ 2NO2

(g)


2

kf/kr = (NO2)2

(N2O4)

Kc = kf / kr


N2O4 (g) ↔ 2NO2

(g)


2

Kc = (NO2)2

(N2O4)

• Kc is a ratio of rate constant and is temperature dependent• Kc is a ratio of products conc to reactants conc• Magnitude of Kc indicate how far/extend of the reaction proceeds towards a product at a given temperature



Equilibrium Law

Kc = (C)c(D)d

(A)a(B)b

At equilibrium:

N2O4 (g) ↔ 2NO2

(g)


2

kf/kr = (NO2)2

(N2O4)

Kc = kf / kr


N2O4 (g) ↔ 2NO2

(g)


2

Kc = (NO2)2

(N2O4)

• Kc is a ratio of rate constant and is temperature dependent• Kc is a ratio of products conc to reactants conc• Magnitude of Kc indicate how far/extend of the reaction proceeds towards a product at a given temperature

Small Kc : N2(g) + O2(g) ↔ 2NO(g) Kc = 1 x 10 -30 small ↓ Kc = (NO)2

(N2)1(O2)

1 Kc small → low product ↓, more reactants ↑, close to no reaction at all

Large Kc : 2COg) + O2 ↔ 2CO2(g) Kc = 2.2 x 10 22 high ↑ Kc = (CO2)2

(CO)2(O2)1 Kc large → high product ↑, small reactants ↓ , close to completion

Intermediate Kc : 2HI(g) ↔H2(g) + I2(g) Kc = 0.02 Kc = (H2)(I2) (HI)2 Kc intermediate → significant amount of reactants and products



Equilibrium Law

Kc = (C)c(D)d

(A)a(B)b

At equilibrium:

N2O4 (g) ↔ 2NO2

(g)


2

kf/kr = (NO2)2

(N2O4)

Kc = kf / kr


How dynamic equilibrium is achieved in a closed system?

As reaction proceedsConc of N2O4 decrease ↓over time

As reaction proceedsConc of NO2 increase ↑over time

N2O4 (g) ↔ 2NO2 (g)




Rate of forward rxn, Rf decrease ↓ over time Rate of reverse rxn, Rr increase ↑over time

N2O4 (g) ↔ 2NO2 (g)




Rate of forward rxn, Rf decrease ↓ over time Rate of reverse rxn, Rr increase ↑over time

At dynamic equilibrium• Forward Rate = Reverse Rate • Conc of reactant/product remain constant

N2O4 (g) ↔ 2NO2 (g)

Time, t = 0


Time, t = 0


Rate reverse = 2 (2R formed)

Rate forward = 4 (4R breakdown)

Time, t = 0

Time, t = 1




Net R breakdown = 4 -2 = 2R Net P formed = 2P

Time, t = 0

Time, t = 1







Time, t = 0

Time, t = 1


Time, t = 2







Time, t = 0

Time, t = 1


Time, t = 2










Dynamic Equilibrium Rate forward = Rate reverseConc Reactant/Product remain constant

Equilibrium Law for heterogeneous closed system• reactants and products are in the different phase• solid and liquid in equilibrium• solid and gas in equilibrium• liquid and gas in equilibrium• solute and solution in equilibrium

Macroscopic level (observable level)• No change in colour, density, concentration of solution, amount/mass of undissolved solid

Microscopic level (molecular level)• Particles are undergoing dynamic equilibrium where forward and reverse reaction are going on

During Dynamic Equilibrium

• Sugar solution is saturated with sugar crystals• Excess sugar will not dissolve• Adding addition sugar will not increase the sugar concentration/its sweetness• Dynamic equilibrium, Kc between sugar solid and solution remain the same• Macroscopic level – conc/sweetness and amount of undissolved crystals remain constant• Microscopic level – sugar crystal and its solution are in dynamic equilibrium ( Rate of Dissolving = Rate of Crystallization )

Photo :http://fphoto.photoshelter.com/image/I00002XO18fsUoF0

Solute-solution equilibrium Sugar(s) ↔ Sugar (aq)

Equilibrium Law for heterogeneous closed system• reactants and products are in the different phase• solid and liquid in equilibrium• solid and gas in equilibrium• liquid and gas in equilibrium• solute and solution in equilibrium

Macroscopic level (observable level)• No change in colour, density, concentration of solution, amount/mass of undissolved solid

Microscopic level (molecular level)• Particles are undergoing dynamic equilibrium where forward and reverse reaction are going on

During Dynamic Equilibrium

• Sugar solution is saturated with sugar crystals• Excess sugar will not dissolve• Adding addition sugar will not increase the sugar concentration/its sweetness• Dynamic equilibrium, Kc between sugar solid and solution remain the same• Macroscopic level – conc/sweetness and amount of undissolved crystals remain constant• Microscopic level – sugar crystal and its solution are in dynamic equilibrium ( Rate of Dissolving = Rate of Crystallization )

Photo :http://fphoto.photoshelter.com/image/I00002XO18fsUoF0

Solute-solution equilibrium Sugar(s) ↔ Sugar (aq)

Equilibrium constant Kc depends on the change in conc of reactants/productsPure solid sugar – conc is constant/fixedAdding pure solid sugar – will not affect the Kc – Conc of sugar in solution remains same( Rate of Dissolving = Rate of Crystallization )Adding more water – will affect Kc – Conc sugar in solution changes( Rate of Dissolving > Rate of Crystallization )

• Pure liquid and gaseous bromine are in dynamic equilibrium• Adding more liquid bromine will only increase its liquid mass but not its concentration• Dynamic equilibrium, Kc between liquid and gaseous bromine remain the same• Macroscopic level – colour/intensity and conc of liquid/gaseous bromine remain constant• Microscopic level – liquid /gaseous bromine are in dynamic equilibrium( Rate of Vapourization = Rate of Condensation)

Photo: http://equilibriumstudyguide.blogspot.com/2009/05/types-of-equilibrium.html

Liquid -Vapour equilibrium Br2(l) ↔Br2(g)

Equilibrium Law for heterogeneous closed system

Equilibrium constant Kc depends on the change in conc of reactants/productsPure liquid bromine – conc is constant/fixedAdding pure liquid bromine – won’t affect the Kc – Conc of gaseous bromine remains same( Rate of Vapourization = Rate of Condensation)Removing gaseous bromine – will affect Kc – Conc gaseous bromine changes( Rate of Vapourization > Rate of Condensation)

CaCO3 (s) → CaO(s) + CO2(g)

Kc = (CaO)(CO2 ) Kc involve a change of conc of reactant or products (CaCO3)

Conc = Amount/ Volume or Density = Mass/VolumeConc = Density

Density for solid/ liquid always constant

Concentration solid/ liquid always constant

Pure solid /liquid, their conc is EXCLUDED in Kc, there is no change in conc

Solid - Vapour equilibrium CaCO3 (s) → CaO(s) + CO2(g)

Kc = (CaO)(CO2 ) → Kc = (CO2 ) (CaCO3)

Equilibrium Law for homogeneous closed system• reactants and products are in the same phase

Equilibrium Law for homogeneous system• H2(g) + I2(g) ↔ 2HI(g)

• CH3COOH(l) + C2H5OH(l) ↔ CH3COOC2H5(l) + H2O(l)

• PCI5(g) ↔ PCI3(g) + CI2(g)

• N2O4(g) ↔ 2NO2(g)

• N2(g) + 3H2(g) ↔ 2NH3(g)

Kc for homogeneous/heterogeneous system

4NH3(g) + 5O2(g) ↔ 4NO(g) + 6H2O(g) N2(g) + 3H2(g) ↔ 2NH3(g)

NH4CI (s) ↔ NH3(g) + HCI(g) 2SO2(g) + O2(g) ↔ 2SO3(g)

Equilibrium Law for homogeneous closed system• reactants and products are in the same phase

Equilibrium Law for homogeneous system• H2(g) + I2(g) ↔ 2HI(g)

• CH3COOH(l) + C2H5OH(l) ↔ CH3COOC2H5(l) + H2O(l)

• PCI5(g) ↔ PCI3(g) + CI2(g)

• N2O4(g) ↔ 2NO2(g)

• N2(g) + 3H2(g) ↔ 2NH3(g)

Kc = (NH3)1(HCI)1

(NH4CI)Kc = (SO3)2

(SO2)2(O2)

1

Kc for homogeneous/heterogeneous system

4NH3(g) + 5O2(g) ↔ 4NO(g) + 6H2O(g) N2(g) + 3H2(g) ↔ 2NH3(g)

NH4CI (s) ↔ NH3(g) + HCI(g) 2SO2(g) + O2(g) ↔ 2SO3(g)

Kc = (NH3)2

(N2)1(H2)

3

Kc = (NO)4 (H2O)6

(NH3)4(O2)5

Kc = (NH3)1(HCI)1

Conc of solid is constant

Acknowledgements

Thanks to source of pictures and video used in this presentation

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Prepared by Lawrence Kok

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IB Chemistry on Dynamic Equilibrium and Homogeneous Equilibrium Constant

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