Chapter 16: Kinetics

Question 1

Chemical kinetics: the study of?

Answer 1

The study of how fast a chemical reaction occurs: deals with speed of a reaction and its mechanism

Question 2

Reaction Rate (definition)

Answer 2

changes in concentration of reactants or products as a function of time

Question 3

Reaction Rate and conditions?

Answer 3

Under any given set of conditions, a rate is determined by nature of reactants and has a characteristic rate. For any given reaction, it has a different rate under different conditions.

Question 4

Four factors that affect reaction rate

Answer 4

1. Concentration of reactants 2. Physical state of reactants 3. Temperature of reaction 4. Catalyst

Question 5

Concentration on Reaction Rate?

Answer 5

Molecules must collide to react: more molecules = more frequent collisions = more often reactions –> reaction rate is proportional to the number of collisions, which depends on the concentration of reactants. rate α collision frequency α concentration

Question 6

Physical State of Reaction Rate?

Answer 6

Molecules must mix to collide: the more finely divided a solid/liquid reactant, the greater its surface area, the more contact it makes with other reactant, the faster the reaction occurs.

Question 7

How reactants mix in same vs. different phases?

Answer 7

Same phase: random thermal motion, gentle stirring mixes them further - eg. aqueous solution Different: contact only at interface between phases, vigorous stirring needed

Question 8

Temperature on Reaction Rate (2)

Answer 8

Molecules must collide with enough energy: temperature increases frequency and energy of collisions

Question 9

Temperature on Frequency of Collisions

Answer 9

molecules in gas sample have a range of speeds, most probably speed is a function of temperature: higher temperature = more frequency collisions. Rate α Collision Frequency α Temperature

Question 10

Temperature on Collision Energy

Answer 10

temperature affects kinetic energy: higher temperature = more sufficiently energetic collisions occur. Rate α Collision Energy α Temperature

Question 11

Rate of Reaction: what changes?

Answer 11

reactant concentrations decrease, product concentration increases

Question 12

Formula for Rate

Answer 12

-Δ[A]/Δt [A] = concentration in mol/L of reactant

Question 13

Why a negative sign in rate formula?

Answer 13

reaction rate is a positive number –> reactant concentration decreases so (final-initial) is negative, extra negative sign balances it out

Question 14

Units for Rate

Answer 14

Moles per liter per second: mol/L•s

Question 15

What if for Rxn Rate we measure product concentration?

Answer 15

final conc is higher than initial, so change in product concentration is positive Rate = Δ[B]/Δt

Question 16

The Three Types of Reaction Rate?

Answer 16

• > in most cases, rate varies as reaction proceeds (curved line)
  1. Average Rate
  2. Instantaenous Rate
  3. Initial Rate

Question 17

Why does rate decrease over course of reaction?

Answer 17

as reactant molecules react, fewer are present to collide with other reactants, so the change in the concentration of the reactants over time (aka rate) decreases

Question 18

Average Rate

Answer 18

slope of line joining two points along curve: total change in concentration divided by total change in time

Question 19

Instantaneous Rate

Answer 19

rate at a particular instant during the reaction: slope of line tangent to curve at any point

Question 20

Initial Rate

Answer 20

instantaneous rate at the moment the reactants are mixed, aka at t = 0

Question 21

Rate formula for non 1:1 Reactions

Answer 21

aA + bB → pP + qQ

lowercase = stoichiometric coefficients, capital letters = reactants and products

Question 22

Rate Law (Rate Equation)

Answer 22

expresses rate as a function of concentrations and temperatures -> we usually only consider reactions where products don’t appear in the law, so rate only depends on reactant concentrations and temperature

Question 23

Rate Law Formula

Answer 23

For a generic reaction aA + bB → C

Question 24

Meaning of k, x and y in Rate Law

Answer 24

k = rate coefficient or rate constant of the reaction -> specific to a given reaction at a given temperautre, but doesn’t change as the reaction proceeds

x and y = reaction orders: define how the rate is affected by reactant concentrations

Question 25

Two key points of Rate Law

Answer 25

1) balancing coefficients a and b in reaction equation aren’t necessarily related to reaction orders
2) components of rate law (rate, reaction orders, rate constant) must be found by experiment

Question 26

Lab Methods to Determine Initial Rate (3)

Answer 26

1. Spectrometric methods
2. Conductometric methods
3. Manometric methods

Question 27

Spectrometric methods to determine Initial Rate

Answer 27

measures concentration of a component that absorbs/emits characterstic wavelengths of light

ex. colorless NO and O₃ -> brown NO₂: spectrometer measures wavelength and intensity of brown color, rate of NO₂ formation is proportinal to inccrease in intensity

Question 28

Conductometric methods to Determine Initial Rate

Answer 28

change in electrical conductivity of reaction solution when nonionic reactants from ionic products (or vice versa)

Question 29

Manometric Methods

Answer 29

manometer attached to reaction vessel of fixed volume and temperature: measures pressure over time of a reaction that involves a change in the number of moles of gas -> rate is proportional to increase in product gas pressure

Question 30

Reaction Order: two types

Answer 30

individual order with respect to each reaction, and overall order which is the sum of individual orders.

Question 31

Reaction Order - Def

Answer 31

the expontents in the rate law - define how rate is affected by reactant concentrations

Question 32

Reaction order: two key points

Answer 32

1. balancing coefficients in reaction equation not necessarily related to reaction orders. 2. components of rate law must be found by experiment

Question 33

Reaction Orders - from equations?

Answer 33

NO - reaction orders cannot be deduced from balanced equqtion, must be determined from experimental data