LO 29 - 35

Question 1

What does the Arrhenius equation relate?

Answer 1

relates the rate constant k to temperature in kelvin

Question 2

What are the variables of the following equation?
What does it relate?
K = A · e^ (-Ea ∕ R·T)

Answer 2

K - rate constant
A - frequency factor
Ea - activation energy
R - 8.314 J/mol · K
T - kelvin
This equation relates the rate constant k to temperature

Question 3

What does (A) in the Arrhenius equation represent?

Answer 3

frequency factor which is the number of times the reactants approach the activation barrier per unit time

Question 4

What does (Ea) in the Arrhenius equation represent?

Answer 4

activation energy needed to achieve the activation complex

Question 5

The greater the activation energy needed, the ______ the reaction rate

Answer 5

slower

Question 6

What is the exponential factor in the Arrhenius equation?

Answer 6

a number between 0 and 1 that represent the fraction of molecules that have enough energy to make it past the activation barrier on a given approach

Question 7

Low activation energy + high temperature = _______ exponential factor

Answer 7

small, negative so the value approaches 1 (e^-0 = 1)

Question 8

When the exponential factor approaches 0 what does that signal about the reaction?

Answer 8

since e^-0 = 1, it means that every approach is successful, lots of products

Question 9

High activation energy + low temperature = ______ exponential factor

Answer 9

large, negative so the value approaches 0 (e^-∞ = 0)

Question 10

When the exponential factor approaches a large number what does that signal about the reaction?

Answer 10

since e^-∞ = 0, it means there are no successful approaches, no products

Question 11

The sensitivity of reaction rates to temperature depends on the ___________ of the activation barrier.

Answer 11

magnitude

Question 12

The greater the activation barrier, the ________ the temperature sensitivity of reaction rate

Answer 12

greater

Question 13

The frequency factor for most gaseous reaction tends to be _____ than the # of collisions per second.

Answer 13

Smaller

Question 14

How is the Arrhenius equation
(K = A · e^ (-Ea ∕ R·T) modified for gaseous reactions

Answer 14

K = PZ · e^ (-Ea ∕ R·T)
P - collision frequency
Z - orientation factor to represent fraction collisions with proper orientation

Question 15

An orientation factor (Z) close to _____ means that the reaction is unspecific, while Z close to ________ would be very specific

Answer 15

1 , 0

Question 16

The reaction mechanism is the …

Answer 16

series of individual, elementary steps by which a reaction occurs

Question 17

In a given elementary step, the number of reactants is represented by the ______

Answer 17

molecularity

Question 18

The ________ is much slower than other elementary steps and therefore determines the ________ for the overall reaction

Answer 18

rate determining step
rate law

Question 19

What are the conditions of the rate determining step?

Answer 19

1. the elementary steps in the mechanism must sum to the overall reaction
2. the rate law predicted must be consistent with experimentally determined results

Question 20

What happens when the first elementary step in mechanism is a “fast” step?

Answer 20

it means a subsequent step containing intermediates is the rate determining step and we thus have to express concentrations of intermediates in terms of reactants

Question 21

Reaction rates can be increased using a ________ which is NOT consumed during the reaction

Answer 21

catalyst

Question 22

Catalysts provide an ________ typically with a lower Ae for the rate determining step

Answer 22

alternative mechanism

Question 23

the catalyst exists in the same phase as the reactants

Answer 23

homogenous

Question 24

the catalyst exists in a different phase than the reactants

Answer 24

heterogenous

Question 25

What is hydrogenation?

Answer 25

the catalyzed addition of H to alkene double bonds to make single bonds

Question 26

What are the steps of catalyzation in the presence of a metal?

Answer 26

1. adsorption of reactants onto metal surface
2. diffusion of reactants across the surface until they reach each other
3. reaction
4. desorption of products off the metal surface into the gas phase

Question 27

Most of the reactions we need to survive occur too slowly at room temperature, so we rely on ______________ to speed up reactions

Answer 27

enzymes as biological catalysts

Question 28

The concentrations of reactants and products in equilibrium are described by the ____________

Answer 28

equilibrium constant K

Question 29

A large K value indicates a high concentration of __________.
A small K value indicates a high concentration of _________.

Answer 29

Products
Reactants

Question 30

_____________ for a chemical reaction is the condition in which the rate forward equals the rate of the reverse reaction

Answer 30

dynamic equilibrium

Question 31

_________ remain constant at equilibrium unless there is a change in _________

Answer 31

concentrations
temperature

Question 32

equilibrium constant K is described by the law of mass action in which ….

Answer 32

K = [C]^c · [D]^d / [A]^a · [B]^b
concentrations of products OVER reactants raised to their stoichiometric coefficients

Question 33

Given a chemical equation, if we reverse the reaction, we _____ K

Answer 33

invert
K reverse = (1/ K forward)

Question 34

Given a chemical equation, if we multiple coefficients by some factor x then we ________

Answer 34

raise K to the same factor
K^x

Question 35

Given multiple chemical equations, when adding to get the overall reaction we ________ the corresponding K values

Answer 35

multiply
K 1 · K 2 = K overall

Question 36

For gaseous reactions we express K in terms of ________

Answer 36

partial pressure

Question 37

Kc means _________
Kp means ________

Answer 37

K in terms of concentration
K in terms of partial pressure

Question 38

How do you go from Kc to Kp?

Answer 38

Kp = Kc (R · T) ^ Δn
where Δn is the mols gaseous products - mols gaseous reactants

Question 39

Is Kp the same as Kc?

Answer 39

NO! they’re only equal if Δn = 0