Block 3 - Kinetics

Question 1

kinetics

Answer 1

study of chemical reactions and the factors that affect them

Question 2

use of kinetics

Answer 2

understand reactions mechanism (what happens at molecular level) and control it

Question 3

reaction rate

Answer 3

• no. times reaction happens per second (always positive)
• how fast reactant decreases
• how fast product increases

Question 4

methods of measuring reaction rate experimentally

Answer 4

• pH changes
• colour changes (could be affected by conjugation)
• mass of solid
• capture gas escaping (easiest way)

Question 5

instantaneous rate

Answer 5

+-1/a.d[A]/dt

Question 6

instantaneous rate on graph

Answer 6

conc. vs. time graph:

- tangent

Question 7

average rate

Answer 7

total change in conc. over total time

Question 8

factors affecting rate

Answer 8

• nature of reaction
• temp.
• action of light (may depend on wavelength)
• state (surface area)
• conc.
• catalyst

Question 9

rate laws can allow

Answer 9

prediction of reaction rate from conc. of reactant [A]

- rate calculated from rate law and rate constant

Question 10

order in/with respect to A

Answer 10

• zero order: A: increase/decrease, rate: no change
• first order: A and rate change by same amount
• second order: A: doubled, rate: 4x

Question 11

determining rate law experimentally

Answer 11

• change conc. of one reactant while keeping everything else constant
• investigate how change affects (initial) reaction rate to find order in that reactant
• repeat for other reactants
• calculate rate constant using one of the experiments
  (realistically slightly different values for each–experimental error– but could average or use differences to determine confidence)

Question 12

rate constant

Answer 12

contains info about nature of reaction and temperature

Question 13

integrated rate law

Answer 13

instead of carrying out multiple experiments, measure conc. at various times after reaction starts to determine slope of graph

Question 14

integrated first order rate law

Answer 14

• exponential decrease

- straight line in ln(conc.) vs. time graph

Question 15

integrated second order rate law scenarios

Answer 15

• single second order reactant

- two first order reactants

Question 16

single second order reactant

Answer 16

straight line in 1/[R] vs. time graph

Question 17

two first order reactants

Answer 17

since initial conc. may be different and they may change at different rates, make conc. of one reactant very large => changes negligible => first order problem
- PSEUDO-FIRST ORDER

Question 18

half-life

Answer 18

amount of time (usually in sec) taken for amount of reactant to decrease to half (t1/2)

Question 19

radioactive decay is a

Answer 19

first order process

Question 20

collision theory

Answer 20

reactions happen when molecules effectively collide

- more collisions = more chances of reaction (high conc)

Question 21

effective collisions criteria

Answer 21

• enough energy to surpass Ea (high Ea = slow reaction): temp. matters
• correctly aligned/have correct orientation: nature of reaction, temp. doesn’t matter

Question 22

absolute temp. scale

Answer 22

0 K = -273.15ºC

Question 23

if E of reactants at r.t higher than Ea?

Answer 23

still need enough reactants and correct orientation for reaction to occur but will likely cause faster reaction if it works

Question 24

unimolecular reaction

Answer 24

single molecule reaction

- molecules collide = energy transplanted = too much energy = get rid of by falling apart

Question 25

elementary reactions

Answer 25

• single collision

- rate dependent on rate of collisions (conc. and temp.)

Question 26

elementary reaction naming

Answer 26

unimolecular, bimolecular, termolecular

Question 27

overall reaction rate depends on

Answer 27

slowest step (rate determining step)

Question 28

determining the slow step

Answer 28

• given
• graph (highest Ea = slow step)
• chemical intuition (slow to produce radicals)

Question 29

fast pre-equilibrium

Answer 29

use equilibrium constant equation of first fast step to replace intermediate in rate law deducted from second slow step

Question 30

if experimental rate law correlates with that of a potential mechanism

Answer 30

good evidence but would need to confirm by directly detecting intermediate by some sensitive means

Question 31

catalysts

Answer 31

substance that changes rate of reaction without being used up overall

• usually react in early step and regenerated/reformed in later step
• do not appear in overall balanced chemical equation
• changes elementary steps in reaction

Question 32

catalysts method of action

Answer 32

• creates new pathway where Ea is lower

- molecules aligned by catalyst

Question 33

catalysts in competing reactions

Answer 33

product distribution depends on relative Ea of competing reactions and a catalyst can provide a pathway for one reaction but not the other which alters the outcome of reactions (different major products)

Question 34

types of catalysts

Answer 34

homogenous, heterogenous

Question 35

homogenous catalysts

Answer 35

catalyst in same phase as reactants

• usually appears in rate law
• liquid catalyst to soln. or gas catalyst with gaseous reactants

Question 36

heterogenous catalysts

Answer 36

catalyst in different phase as reactants

• solid cat. + gas react.
• solid cat. + solution
• immiscible liquid phases