Chem Exam 2 B Collision Theory, Reaction Mechanisms, & Catalysis (Sects. 12.5-12.7)

Collision Theory Reaction rate is proportional to the

# of collisions (i.e. more collisions, higher changes of reaction)

Collision Theory Collisions must occur in correct orientation for

bonding of atoms found in product(s)

Collision Theory Collisions must be of sufficient energy for

new bonds to form

transition state =

usually short-lived, unstable species (a.k.a. activated complex)

activation energy (Ea) =

minimum energy necessary to form a product during a collision of reactants

enthalpy change of reaction (ΔH) =

difference in energy between reactants and products

In an endothermic reaction, heat

had to go in or be added to make it happen

In an exothermic reaction, heat

had to be taken out or subtracted to make it happen

A catalyst can be determined by

Find which reactant appears in the product

How to find intermediate

Look for substance produced then consumed in the reaction

What does the curve look like in an catalized reaction

Less activation energy than the initial uncatalyzed reaction. Same starting and ending point

What does a graph look like in an exothermic reaction?

Higher starting energy then lower ending energy point

What does a graph look like in an endothermic reaction?

Lower starting energy then higher ending energy point

Define Arrhenius equation =

relationship between the activation energy (Ea), temperature, and the rate constant (k) Incorporates all three parts of collision theory: # of collisions, orientation, & energy (temperature)

How can Ea be determined experimentally k=

Ae… (take the natural log (ln) of both side

To find activation energy equation Ea=

-R (lnk2-lnk1/ (1/t 2) - (1/ t 1))

Use collision theory to explain how physical state, temperature, and concentrations affect reaction rates

reaction rate properly oriented collisions temperature # of collisions all increase

Reaction mechanism (reaction path) =

step-by-step process by which a reaction happens

Elementary reactions depict only

reactants/products undergoing bond-breaking/-making events

reaction orders CAN be determined from the

stoichiometry of elementary reactions

Elementary Rxn Unimolecular

A —> PRODUCTS

Elementary Rxn Bimolecular

A+B—> PRODUCTS OR 2A—> PRODUCTS

Elementary Rxn Termolecular

3 MOLECULES —> PRODUCTS

Rate law Unimolecular RATE =

k[A]

Rate law Bimolecular RATE =

k[A][B] OR k[A]² OR k[A][A]

Rate law Termolecular RATE =

k[NO]²[O2]

Unimolecular EX EQUATION

O3 —> O2+ O

Bimolecular EX EQUATION

O + O3 —> 2O2

Termolecular EX EQUATION

2NO + O2 —> 2NO2

Rate-determining step =

the slowest elementary step of a reaction mechanism that determines the overall rate of reaction (and rate law)

Define elementary reactions & determine their molecularity

one step in a reaction mechanism; depicts only reactants/products undergoing bond-breaking/-making events

DEFINE Catalyst have an alternative reaction mechanism with a

lower activation energy

Catalyzed reactions may or may not involve a different number of steps (a catalyzed reaction could have the

same number of steps as the uncatalyzed reaction—in this case, one step)

Catalyzed reactions do not result in a change in the

energies or identities of reactants or products

Examples of catalysts homogenous catalyst = NO in ozone decomposition

catalyst present in the same phase as the reactants

Examples of catalysts heterogeneous catalyst = Pt-Rh in catalytic converters

catalyst present in a different phase than reactants (usu. solid)

Examples of catalysts

Enzymes

Which of the following are equal for a chemical system at equilibrium?

The rates of the forward and reverse reactions are equal.

Q indicates in which direction a reaction will proceed. Q>K

Rxn goes backwards starts high on chart

Q indicates in which direction a reaction will proceed. Q<K

Rxn goes forward on chart

K « 1

REACTANTS ARE FAVORED

K ≈ 1

NEITHER DIRECTION IS FAVORED

K»1

PRODUCTS ARE FAVORED

Does K tell us anything about how fast the reaction will reach equilibrium?

NO

When would I actually need to convert from Kp to Kc (or vice versa) or pressures to concentrations? homogenous

all reactants & products are in the same phase

When would I actually need to convert from Kp to Kc (or vice versa) or pressures to concentrations? heterogenous equilibria

reactants & products are in 2+ different phases

Complex Equilibria:

Deriving new equilibrium constants from known ones

What does it mean for a system to be at equilibrium?

rate of forward rxn = rate of reverse rxn (concentrations of reactants and products do not change)