Chemical Kinetics

Branch of chemistry concerned with the rates and mechanisms of chemical

reactions

Chapter 19 Reaction Rates and Equilibrium

• Rates of Reaction

• Reversible Reactions and Equilibrium

• Determining Whether a Reaction Will Occur

• Calculating Entropy and Free Energy

• The Progress of Chemical Reactions

Ch 19.1 Rates of Reaction

• Collision Theory

• Factors Affecting Reaction Rates

Collision Theory

• Particles of reactants (ions, atoms, molecules) must collide in order for reactions to occur

• An effective collision occurs when reactant particles approach each other:– at the proper angle– With enough Energy (Force)

Collision Theory

• The number of effective collisions between reactant particles that result in their changing to product in a given unit of time determines the REACTION RATE

Factors Affecting Reaction Rates

• 1) Nature of Reactants

• 2) Concentration

• 3) Temperature

• 4) Catalyst

Nature of Reactants

• Number of bonds to be broken in Reactants

• Ionic bonds break with less energy than covalent bond

Concentration of Reactants

• More particles more collisions faster reaction rate

• Homogenous Reaction = all reactants in same phase (s, l, g, aq)

• Heterogeneous Reaction = reactants in different phase – If a gas, decrease volume, increase pressure– If a solid, pulverize to increase surface area

Temperature

• Increase temperature increase KE increase collisions increases reaction rate

Catalyst

• Substance that increases the rate or speeds up a chemical reaction,with out itself being permanently altered

• Decreases the amount of energy needed for effective collisions

Endothermic Reaction

• Energy is gained, absorbed, required

• P.E. of Reactants < P.E. Products

• + Δ H = Endothermic Reactions

Exothermic Reactions

• Energy is lost, released

• P.E. of Reactants > P.E. Products

• - Δ H = Exothermic Reactions

Base your answers on the information and diagram below, which represent the changes in potential energy that occur during the given reaction. Given the reaction: A + B --> C

    a)  Does the diagram illustrate an exothermic or an

endothermic reaction? State one reason, in terms of energy, to support your answer.

   

Answer

• Endothermic, the products have more energy than the reactants.

Given the reaction: S(s) + O2(g) SO2(g) + energy 

Which diagram best represents the potential energy changes for this reaction?

Answer

• Choice 1, Energy is a product so it is exothermic

• Which statement correctly describes an endothermic chemical reaction?

        (1) The products have higher potential energy than the reactants, and the ΔH is negative.

        (2) The products have higher potential energy than the reactants, and the ΔH is positive.

       (3) The products have lower potential energy than the reactants, and the ΔH is negative.

        (4) The products have lower potential energy than the reactants, and the ΔH is positive.

Answer

• choice 2

• The potential energy diagram below represents a reaction.

• Which arrow represents the activation energy of the forward reaction? (1) A (2) B (3) C (4) D

Answer

• choice B

Ch 19.2 Reversible Reactions and Equilibrium

• Reversible Reactions

• Factors Affecting Equilibrium: Le Chatelier’s Principle

• Equilibrium Constants

Reversible Reactions

• Reactions that occur in both directions at the same time

• 2SO2(g) + O2(g) 2SO3(g)

• Equal amount of products and reactants are being produced

Chemical Equilibrium

• The forward and reverse reactions take place at the same rate

Factors Affecting Equilibrium: Le Chatelier’s Principle

• If a stress is applied to a system at equilibrium, the system shifts to release the stress.

• Stress:– Concentration of reactants– Concentration of products– Change in temperature– Change in pressure

Le Chatelier’s Principle: Concentration

• H2CO3(aq) CO2(aq) + H2O(l)

• Add CO2(aq) , shift the reaction to the left

• Remove CO2(aq) , shift the reaction to the right• Remove products, increase the yield of

products– Farmers and Chickens

Le Chatelier’s Principle: Temperature

• 2SO2(g) + O2(g) 2SO3(g) + Heat

• Add heat, shift left

• Remove heat (cool), shift right

Le Chatelier’s Principle: Pressure

• N2(g) + 3H2(g) 2NH3(g)

• Increase pressure shift right (Less moles)

• Decrease pressure shift left (More moles)

• If the number of moles on each side of the reaction are the same, change in pressure will not change equilibrium

Equilibrium Constants Keq

• Ratio of product concentrations to reactant concentrations

• Each concentration is raised to the power that equals the number of moles

• aA + bB cC + dD

• Keq = [C]c x [D]d

[A]a x [B]b

Equilibrium Constants Keq

• Keq > 1, products are favored at equilibrium

• Keq < 1, reactants are favored at equilibrium

• Write the Keq

• CS2(g) + H2 (g) CH4 (g) + S

(g)

• Write the Keq

• HCl (g) + O2(g) H2O(g) +

Cl2(g)

• Which side of the equilibrium is favored, products or reactants, for each of the following where,

•    A B.

• a)       Keq = 1.375 x 10-3

• b)       Keq = 1.375 x 10+3

• c)       Keq = 1.00 x 100

• For the following equilibrium:

• N2 (g) + H2(g) NH3 (g)

H= -386 KJ/mole

• Predict the direction the equilibrium will shift if:

• a) N2 is added?

• b) H2 is removed?

• c) NH3 is added?

• d) NH3 is removed?

Ch 19.3 Determining Whether a Reaction Will Occur

• Free Energy and Spontaneous Reactions

• Entropy

• Heat, Entropy, and Free Energy

Free Energy and Spontaneous Reactions

• Free Energy – energy available to do work– Usually does not exceed 70% efficiency– Only exists if the reaction actually occurs

Free Energy and Spontaneous Reactions

• CO2(g) C(s) + O2(g)

• This reaction is balanced, but does not occur.

• Two types of reactions: – Actual– Theoretical

Actual Reactions

• Spontaneous reactions – occur naturally and favor formation of products

• All release free energy

Theoretical Reactions

• Nonspontaneous reactions – do not favor the formation of products

Entropy

• The disorder of a system

• The Law of Disorder – processes move in the direction of maximum disorder or randomness.

Example: Your Bedroom

Example: Your Bedroom

Entropy

• Put the phases in order of disorder:

• Solid Liquid Gas

How Heat and Entropy Affect Spontaneity

Heat Entropy Spontaneous Reaction?

Decrease (Exothermic)

Increases Yes

Increase (Endothermic)

Increases Only if unfavorable heat change is offset by favorable

entropy change

Decrease (Exothermic)

Decreases Only if unfavorable entropy change is offset by favorable

heat change

Increase (Endothermic)

Decreases No

Entropy

• Symbol: S

• Units: J/K

• ΔS0 = S0(products) - S0(reactants)

Free Energy (Gibbs Free Energy)

• Symbol: ΔG

• Units: J

• ΔG = ΔH – TΔS– ΔH – change in enthalpy– T – temperature K– ΔS change in entropy

Ch 19.5 Rate Laws

• Rate Law – an expression relating the rate of a reaction to the concentration of reactants

• k = rate constant

Rate Law

• A B

• Rate = k[A]

• aA + bB cC + dD

• Rate = k [A]a [B]b

Write the rate law:

• NO(g) + O3(g) NO2(g) + O2(g)