Transcript Chapter 19 Reaction Rates And Equilibrium

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)
Increase
(Endothermic)
Increases
Yes
Decrease
(Exothermic)
Increase
(Endothermic)
Increases
Only if unfavorable heat
change is offset by favorable
entropy change
Decreases Only if unfavorable entropy
change is offset by favorable
heat change
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)