Chapter 15: Chemical Kinetics

Question 1

Rate of reaction is impacted by what factors?

Answer 1

• Collision model (concentration of reactant particles) - Temperature of reactant mixture (generally occur faster with increasing temperature) - Structure and relative orientation of colliding particles

Question 2

Rate of Reaction (definition)

Answer 2

• Measure of how fast the reaction occurs - Large fraction of molecules is produced from a faster rate

Question 3

Speed Equation

Answer 3

change in distance / change in time = ∆x / ∆t

Question 4

Instantaneous Rate of Reaction: aA + bB –> cC + dD

Answer 4

Rate = -1/a (∆[A]/∆t) = -1/b (∆[B]/∆t) = +1/c (∆[C]/∆t) = +1/d (∆[D]/∆t)

Question 5

Rate Law

Answer 5

Express relationship between rate of reaction and concentration of reactant:
Rate = k[A]ⁿ

k = constant of proportionality (rate constant)
n = reaction order

Question 6

Reaction Order:

n = 0

Answer 6

• Zero order
• Rate is independent of concentration
• Concetration decreases linearly over time
• Occurs when amount of reactant available for reaction unaffected by changes in overall quantity of reactant

Question 7

Reaction Order:

n = 1

Answer 7

• First order
• Directly proportional to concentration
• Rate slows down as reaction proceeds

Question 8

Reaction Order:

n = 2

Answer 8

• Rate of reaction proportional to square concentration of reactant
• Rate is more sensitive to concentration

Question 9

How to determine order of reaction?

Answer 9

• Order of reaction is determined by experiment
• Common method is to use method of initial rates

k = rate 2 / rate 1 = k[A]ⁿ₂ / k[A]ⁿ₁

Question 10

Integrated Rate Law

Answer 10

• Relationship between concentrations of reactants and time

Question 11

First Order Integrated Rate Law

Answer 11

ln[A]_t = -kt + ln[A]₀

slope = -k

Question 12

Second Order Integrated Rate Law

Answer 12

-∆[A] / ∆t = k[A]² or

1/[A]_t = kt + 1/[A]₀

NONLINEAR

Question 13

Half-Life

Answer 13

t_1/2 = 0.693/k

Question 14

Arrhenius Equation

Answer 14

k = Ae^-Ea/RT

• Ea = activation energy
• Frequency factor (A): number of times the reactants approach activation barrier/time
• Frequency factor (Ea): Energy barrier to be surmounted by reactants
• In a graph, E of products will be contained on the right side after bump in activation energy

Question 15

Activated Complex

Answer 15

• Transition state in which it has a higher energy than either reactant or product

Question 16

Exponential Factor

Answer 16

• Number between 0 and 1 representing fraction of molecules that have enough E to make it over the activation barrier on given approach

= e^-Ea/RT

Question 17

Arrhenius Plot

Answer 17

• Plot of natural log of rate constant (lnk) versus the inverse of temperature in Kelvins (1/T)
• Creates straight line with slope -Ea/R and y-intercept lnA

Question 18

Two-Point Arrhenius

Answer 18

lnk2/k1 = Ea/R (1/T₁ -1/T₂)

Question 19

Collision Model

Answer 19

• Chemical reaction occurs after a sufficiently energetic collision between two molecules

Question 20

Orientation Factor

Answer 20

• Measure of how specific the orientation of the collision must be

Question 21

Reaction Mechanism

Answer 21

• Series of individual chemical steps by which an overall chemical reaction occurs
• Comprised of elementary steps (each step in the reaction mechanism)

Question 22

Molecularity

Answer 22

• Number of reactatnt particles involved in step; used to characterize elementary steps
• Unimolecular, bimolecular, termolecular

Question 23

Rate Determining Step

Answer 23

• Limits the overall reaction and determines the rate law for the overall reaction
• Obtain rate law from “slow step”

Question 24

Catalyst

Answer 24

• Substance that increases rate of chemical reaction but is not consumed by the reaction
• Provides alternative mechanism for reaction (in which rate-determining step has lower Ea)

Question 25

Homogenous Catalyst

Answer 25

• Catalyst exists in same state as reactants

Question 26

Heterogenous Catalyst

Answer 26

• Catalyst exists in different phase than reactants

Question 27

Hydrogenation

Answer 27

• Involves hydrogenation of double bonds of alkenes

Question 28

Catalyst Procedure

Answer 28

1) Absorption: Reactants absorbed onto metal surface
2) Diffusion: Reactants diffuse on surface until they approach each other
3) Reaction: Reactants react to form products
4) Desorption: Products desorb from surface to react

Question 29

Enzymes

Answer 29

• Biological catalysts that increase rates of biochemical reaction
• Usually large protein molecule with 3D structures
• Active site: Binds reactant molecule
• Substrate: Fits into active site (only catalyze one substrate)