Chemical Kinetics and Equilibrium

Reaction Rates• How fast or slow the reaction

occurs

Collision Theory

• 2 conditions must be satisfied for a chemical reaction to occur

Particles of reactants must collide with one another; and

Colliding particles must have sufficient energy

Activation Energy• Minimum amount of energy needed for a reaction to

occur

F> Ea Ea

F < Ea Ea

Energy Diagram for Exothermic reaction

increasing Average energy energy of reactants

Energy difference bet. Average energy reactants and products of products

Energy Diagram for Endothermic reaction

increasing energy

energy of the products energy of reactants energy difference

Factors Affecting Rate of reaction

• Surface Area• Concentration of reactants• Temperature• Presence of Catalyst

Surface area of reactants

• Consider the block of wood or wood shavings

The smaller the particle size of wood the greater the surface area exposed to oxygen thus the rate of reaction increases.

Concentration of Reactants

• An increase in concentration of the reactants causes an increase in the rate of chemical reaction

Temperature

• Higher temperature increases reaction rates

• Ex: ripe fruits are placed in the refrigerator to slow down the ripening process.

Presence of Catalyst

• CatalystSubstance that speeds up chemical

reaction without itself undergoing a chemical change

Inhibitor--->substances that slow down reaction

Activation energy of uncatalized reaction

• catalyzed activation energy

Uncatalyzed• activation energy • Note: Catalysts increases• reaction rates by• providing alternative• reaction pathways • with lower activation• energies

Lower activation energy

• Means that a lower amount of energy is needed to surpass the energy barrier.

Chemical Equibrium

• It is always assumed that chemical reaction go to completion

• However, only few chemical reactions proceed to completion

• Reversible reaction N2O4(g) 2NO

Reversible Reaction

• A chemical reaction in which the products can regenerate the original reactants

N2O4(g) 2NO2

colorless brownN2O4(g) -------> 2NO2(g) ( forward Reaction)

2NO2 (g)-------> N2O4(g) (backward Reaction)

Chemical Equilibrium • The rate of forward reaction and backward

reactions are equal• Is dynamic The reactants constantly forms the products

while the products constantly forms the reactants

Note:In equilibrium, concentration of reactants and

products do not change

Ex: dynamic Equilibrium

• Carbonated Drinks• CO2 + H20 ---> H2CO3

carbonic acidWhen the bottle is opened the pressure is released, carbon

dioxide evolved from the decomposition of carbonic acid

H2CO3 ------> CO2 + H2O

• CO2 + H20 H2CO3

Equilibrium Constants(K)Can be obtained in any equilibrium reactionEqual to the Ratio of the equilibrium

concentration of the products to the equilibrium concentration of the reactants

aA + bB ----->cC + dDWhere:a,b,c & d -- are the coefficients A,B,C & D– are the chemical species

Equilibrium Constant(K)

aA + bB ----->cC + dD• K = [C]c [D]d = [ products ] [A]a [B]b [reactants]--> Experimentally at equilibrium, Concentration of the

products raised to a certain power divided the concentration of the reactants also raised to a certain power is constant at( fixed temperature)

Is also called equilibrium expression

Note:• K = [ products] [reactants]• is the concentration of products over that of

the reactants• If the K value is small, it means that the

equilibrium concentration of the products is small while the reactants are large. This indicates that at equilibrium, the system consist mostly of reactants

• If the K value is great , it means that the equilibrium system consist mostly of the products

Writing equilibrium expression

• If a pure liquid or solid is involved in a reaction, its concentration is omitted in the equilibrium expression because it has constant concentration in mol/L at constant Temperature

Ex:

• Write the equilibrium expression for the reaction below

• H2(g) + F2(g) 2HF(g)

• K = [ HF]2

• [H2] [F2]

Ex:2

• Write the equilibrium expression for the reaction below

• N2(g) + 3H2(g) 2NH3(g)

• K = [NH3]2

[N2][H2]3

Ex:3

• Write the equilibrium expression for the reaction below

• CaCO3(s) CaO(s) +CO2(g)

• K = [ CO2]

Ex:4

• Write the equilibrium expression for the reaction below

• 2Hg(l) + O2(g) 2HgO(s)

• K = 1 since solid and liquid is omitted

• [O2]

Exercises

• Write the expression of the followingReactionsa) 2H2S(g) + 3O2(g) 2H2O(g) + 2SO2(g)

b) CO2(g) + H2(g) ---> CO(g) + H2O(l)

c) FeO(s) + H2(g) --->

Le Chatelier’s Principle

• States that if a change in conditions is imposed on a system at equilibrium, the equilibrium position will shift in the direction that tends to reduce the effects of that change

-->The effect of change in concentration

The effect of change in Volume The effect of change in temperature

Effects of Change in Concentration on reaction Equilibrium

• Consider the equation belowN2 + 3H2 2NH3(g)This equation shows equilibrium because of the two arrows of the

same length that is rate of forward reaction is equal to the rate of backward reaction.

What will happen if you increase the concentration of N2?increasing the concentration of the reactants means more collision

between them that increases the rate of forward reaction as shown below

N2 + 3H2 2NH3(g)Notice that the arrow pointing to the right has a greater length which

indicates forward reaction increases meaning more production of NH3

What happens if you increase the concentration of NH3 in the previous example?

N2 + 3H2 2NH3(g)

Increasing the concentration of NH3 will shift the reaction to the left.

Note: If a reactant or product is added to the system, the

system shifts away from the added componentIf a reactant or product is removed, the system shifts

toward the removed component

Exercise• Suppose the reaction system

2SO2(g) + O2(g) 2SO3 (g)

Has reached equilibrium. Predict the effect of each of the following changes on the position of the equilibrium

a) SO2(g) is added to the system

Ans. Shift to the right( forward)b) The SO3(g) present is liquefied and removed from the

systemAns. Shift to the right(forward)b)Some of the O2 gas is removed from the system

Ans. Shift to the left ( backward)

The effect of Change in Volume and pressure Recall : Boyle’s law : Pressure is inversely proportional to Volume of gases The smaller the volume, the higher the pressure between molecules Avogadro’s Law : V is directly proportional to the # of moles at constant T

• Consider the equation below: suppose the gases below are mixed in a vessel at equilibrium 2 NOCl(g) 2NO(g) + Cl2(g)What will happen to the equilibrium position if we reduce the volume? Reducing volume means increasing the pressure between the molecules of

the gases, the system moves in the direction that lowers its pressure. In the equation above, the direction will shift to the left because the products has more molecules (3 molecules) than the reactant (2molecules).

In other words, the equilibrium position will shift toward the side of the reaction that involves the smaller number of gaseous molecules in the balanced equation

2 NOCl(g) 2NO(g) + Cl2(g)

What happens when volume is increased in the previous example?

2 NOCl(g) 2NO(g) + Cl2(g)

• When the volume is increased, it lowers the pressure of the system, thus the direction will shift to the right to increase the total number of gaseous molecules present

Predict the shift in equilibrium of the reaction below when volume is reduced

a) P4(s) + 6Cl2(g) 4PCl3(l) 6 gaseous molecules 0 gaseous moleculesThe direction will shift toward the right since it has 0 gaseous molecules

b) PCl3(g) + Cl2(g) PCl5(g)

2 gaseous molecules 1 gaseous moleculeThe direction will shift to the right since it has lesser number of gaseous

moleculesc) PCl3(g) + 3NH3(g) P(NH2)3(g) + 3HCl(g)

4 molecules 4 moleculesSince the # of gaseous molecules in both sides are equal, a change in

volume will have no effect on the equilibrium position

Practice:• For each of the following reaction predict the direction in which

the equilibrium will shift when volume in the container is increased

a) 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

b) FeO(s) + H2(g) Fe(s) + H2O(g)

The effect of change in Temperature

The change in concentration and volume in the system alter the equilibrium position but not the equilibrium constant (K)

The effect of temperature on equilibrium is different because the value of equilibrium constant (K) is change

If the reaction is exothermic that means it releases heat the direction will shift to the left since heat is is one of the products

A) C(s) + O2(g) <-------> CO2(g) + heat direction will shift away from heat( to the left) If the reaction is endothermic, energy is one of the

reactants so the direction will shift to the right

Exercise:

• Predict the shift would each of the changes below have on the reaction

O2(g) + 2CO(g) 2CO2(g) + heat

a)Increasing [CO]b)Increasing [CO2]

c) Increasing Temperature

d) Decreasing volume