Ch. 17/18
Reaction Kinetics
&
Chemical Equilibrium
Reaction Mechanisms
• Reaction Mechanism: the step-by-step
sequence of reactions by which the overall
chemical change occurs.
• Intermediates: species that appear in some
steps but not in the net equation.
• Homogeneous Rxn: a rxn whose reactants
and products exist in a single phase.
17-2
Collision Theory
• Collision Theory: the set of assumptions
regarding collisions and reactions.
* Molecules must collide with sufficient force
and correct orientation to produce a rxn.
17-3
Activation Energy
• Activation Energy (Ea): the minimum energy
required to transform the reactants into an
activated complex.
• Activated Complex: a transitional structure that
results from an effective collision and that persists
while old bonds are breaking and new bonds are
forming.
• ΔE (delta E) or change in energy: for the forward
reaction, energy change is E of products – E of
reactants.
17-4
Activation Energy
17-4
a) Ea: energy difference between activated complex and reactants.
b) Ea’: energy difference between activated complex and products.
c) ΔE: energy difference between reactants and products ΔE=Ep-Er
Reaction Rate
• Reaction Rate: the change in concentration
of reactants per unit time as a reaction
proceeds.
• Chemical Kinetics: the area of chemistry
that is concerned with reaction rates and
reaction mechanisms.
17-6
Rate-Influencing Factors
• The Nature of the Reactants.
• Surface Area.
• Temperature (temp increase favors
endothermic rxns)
• Concentration.
• Presence of Catalysts.
* # of collisions, which is affected by the
above.
17-7
Reaction Rates
• Heterogeneous Reactions: involve reactants in
two different phases.
• Catalyst: a substance that changes the rate of a
chemical rxn without itself being permanently
consumed.
• Homogeneous Catalyst: a catalyst in the same
phase as the reactants and products in a rxn
system.
• Heterogeneous Catalyst: a catalyst in a different
phase from the reactants.
17-8
Rate Laws
• Rate Law: an equation that relates reaction
rate and concentrations of reactants.
R = kAnBm
R = rate
(lower case) k = specific rate constant
A = molar concentration of reactant A
n = the power to which the concentration of A
is raised.
17-9
Reaction Rates
• Rate Determining Step: the slowest rate step.
• Exothermic Rxn: Releases energy. (feels hot)
• Endothermic Rxn: Absorbs energy. (feels cold)
Equilibrium
• Reversible Rxn: a chemical rxn in which the
products can react to re-form the reactants.
• Chemical Equilibrium: when the rate of the
forward rxn equals the rate of the reverse rxn and
the concentrations of the products and reactants
remain unchanged.
17-11
Equilibrium
A + B →← C + D
Initially, concentrations of C and D are zero, A and B are
maximum. As the rxns progress, A and B are consumed, C
and D are formed until eq. is established.
17-12
Equilibrium
nA + mB →← xC + yD
Eq. constant (capital K) is equal to:
K = Cx Dy
An Bm
With C being the concentration in mol/L.
Equilibrium Constant: the ratio of the mathematical product
of the concentration of substances formed at eq. to the
mathematical product of the concentrations of reacting
substances. Each concentration is raised to a power equal
to the coefficient of that substance. (Pure solids and
liquids can not change concentration and are not a factor in
K)
Shifting Equilibrium
• Le Chatelier’s Principle: if a system at eq.
is subjected to a stress, the eq. is shifted in
the direction that tends to relieve that stress.
.
17-14
Stresses to an Equilibrium
1.
Changes in Pressure: the rxn will shift to the side that
has less number of particles.
Ex. N2(g) + 3H2(g) ↔ 2NH3(g)
2.
Changes in Concentration: A + B ↔ C + D an increase
in the concentration of A or B will push the rxn towards
product, increase C or D and the reactants will form
faster.
3.
Changes in Temperature: an increase in temp. will shift
eq. toward the endothermic rxn, a decrease in temp. will
shift toward the exothermic rxn.
Reactions that go to Completion
• Reactions run to completion when reacting ions are almost
completely removed from solution.
1. Formation of a gas: rxns that form a gas as a product go to
completion, gases are frequently released during a rxn.
2. Formation of a percipitate (PPT): product is usually an
insoluble solid that can not reform the reactants.
3. Formation of a slightly ionized product: when molecules
are the product, ions are taken out of solution.
• Spectator Ions: don’t take part in a chemical rxn, are
present as a reactant and a product.
Ch. 17/18
The End
17-18