Kinetics

Question 1

rates of reaction can be determined using the two formulas

Answer 1

A->B
rate of production of B = Δ[B]/Δt
rate of consumption of A = -Δ[A]/Δt

Question 2

rate =

Answer 2

change in concentration/change in time

Question 3

The General Reaction Rate

Answer 3

for a balanced reaction:

aA -> bB + cC

rate = -1/a(Δ[A]/Δt) = 1/b(Δ[B]/Δt) = 1/c*(Δ[C]/Δt)

Question 4

Initial instantaneous rate vs instantaneous rate

Answer 4

initial instantaneous rate is at t=0.
instantaneous rate can be at any point on the graph

Question 5

describe the rate law in words

Answer 5

an equation that shows how the reaction rate depends on the concentration of each reactant

Question 6

Rate Law

Answer 6

aA + bB -> cC + dD

rate = k[A]^m[B]^n

[A], [B] - concentration in M
k - rate constant
m - reaction order in A
n - reaction order in B

Question 7

what si the overall order of a reaction?

Answer 7

the sum of the individual orders

Question 8

units of rate

Answer 8

always M/s

Question 9

units of k

Answer 9

depends on the rate law expression

Question 10

units of k if:
rate = k
rate = k[A]
rate = k[A][B]
rate = k[A][B]^2

Answer 10

M/s
1/s
1/(Ms)
1/(M^2s)

Question 11

how are the order of reactions (m, n) and the rate constant (k) determined?

Answer 11

experimentally

Question 12

give 2 ways of determining order of reaction and rate constant

Answer 12

1. method of initial rates:
   - vary initial concentration of one reactant at a time
   - measure the initial reaction rate for each reaction
   - solve a system of rate law equations to determine the order
2. graphical method/integrated rate law
   - monitor the course of a reaction over time
   - plot data
   - the shape of the curve reveals reaction order

Question 13

0th order reaction

Answer 13

A –(k)–> product

[A]t = -kt + [A]0
[A]0 is the initial concentration
[A]t is the concentration at time t

• plot the experimental [A] vs time
• if the graph is linear, the reaction is zero-order
• rate constant k = (-slope)

Question 14

half life

Answer 14

the time required for reactant concentration to reach half of its original value

Question 15

half life for a 0th order reaction

Answer 15

t(1/2) = [A]0/2k
- dependent on the (initial) concentration
- gets shorter over the course of the reaction (each successive half-life is half as long)

Question 16

how are 0th order reactions possible?

Answer 16

in reactions whether the kinetics are governed by the availability of a catalyst

Question 17

1st order reaction

Answer 17

A –(k)–> products

[A]t = [A]0e^-kt
ln[A]t - ln[A]0 = -kt

• plot the experimental ln[A] vs time
• if the graph is linear, the reaction is first-order
• the rate constant k = (-slope)

Question 18

half life of 1st order reaction

Answer 18

t(1/2) = ln(2)/k
- t(1/2) is independent of the concentration
- every half-life is an equal period of time

Question 19

2nd order reaction

Answer 19

A –(k)–> products

1/[A]t = kt + 1/[A]0

• plot the experimental 1/[A] vs. time
• if the graph is linear, the reaction is second-order
• the rate constant k = slope

Question 20

half life of 2nd order reaction

Answer 20

t(1/2) = 1/k[A]0
- t(1/2) is dependent on the (initial) concentration
- t(1/2) gets longer over the course of the reaction, so each successive t(1/2) doubles in length

Question 21

draw graphs for zeroth order, first order, and second order. also draw straight-line plots used to determine rate constant

Question 22

how can we find the overall reaction from the elementary steps?

Answer 22

the elementary steps in the true reaction mechanism must sum to give the overall (stoichiometric) reaction

Question 23

reaction intermediate

Answer 23

a species formed in one step of a reaction mechanism and consumed in a later step

Question 24

molecularity

Answer 24

a classification of an elementary reaction based on the number of molecules (or atoms) on the reactant side of the chemical equation

Question 25

unimolecular reaction

Answer 25

an elementary reaction that involves a single reactant molecule eg rate = k[O3]

Question 26

bimolecular reaction

Answer 26

an elementary reaction that results from an energetic collision of two reactant molecules eg rate = k[O3][O]

Question 27

termolecular reaction

Answer 27

an elementary reaction that involves three atoms or molecules (rare) eg rate = k[O]^2[M]

Question 28

how can you find a plausible mechanism for a reaction?

Answer 28

- elementary steps must add up to the overall reaction - mechanism must correlate with the experimental rate law

Question 29

the rate law of the overall reaction must be the rate law of

Answer 29

the rate limiting step

Question 30

how do you determine the rate law for a reaction where the rate limiting step is the second step?

Question 31

factors affecting reaction rates

Answer 31

1. concentration; increased concentration of reactants = increased rate - moving molecules are closer together - increased likelihood that reactants will collide 2. temperature: observed increase in rate wen there is an increase in temperature - due to collision and transition state theory - molecules collide with energy greater than Ea more often 3. catalyst: a catalyst provides an alternative reaction mechanism that proceeds faster than the original (uncatalysed) mechanism

Question 32

collision theory

Answer 32

for a bimolecular reaction to take place, reactants A and B must collide with proper orientation, and an energy greater than the activation energy, Ea

Question 33

k =

Answer 33

Zpf Z = volumetric collision frequency p = fraction with correct orientation f = fraction with sufficient energy

Question 34

k depends strongly on temperature - so which of Z, p, and f depend on temperature?

Answer 34

collision frequency (Z) - increase in temperature -> reactants collide more often (but not that more often) fraction with correct orientation (p) - increase T -> no change fraction with sufficient energy (f) - increase T -> reactants collide with greater kinetic energy

Question 35

transition state

Answer 35

the unstable group of atoms that are the highest-energy species along the pathway from reactants to products

Question 36

activation energy

Answer 36

the minimum energy required for a successful reaction

Question 37

f =

Answer 37

e^(-Ea/RT)

Question 38

as T increases, how does f change?

Answer 38

increases exponentially

Question 39

draw a graph of collision energy vs fraction of collisions for two temperatures

Question 40

Arrhenius equation

Answer 40

k = Ae^(-Ea/RT) A = zp : the frequency factor (aka pre-exponential factor) ln(k) = -Ea/R (1/T) + lnA ln(k) = y -Ea/R = m 1/T = x lnA = b

Question 41

Arrhenius plot

Answer 41

y intercept = ln(A) slope = -Ea/R

Question 42

catalysts

Answer 42

a substance that provides an alternative reaction mechanism that has a lower activation energy than the uncatalyzed mechanism -> faster reaction - usually involved in the rate-limiting step of the new pathway - often appears in the rate law of the catalysed reaction - react early in a multi-step mechanism; regenerated in a later step, so not consumed - do not show up in balanced, overall reaction - catalysed reaction has the same endo/exothermicity as the uncatalysed reaction

Question 43

Michaelis-Menten kinetics

Answer 43

enzymes are catalysts of biological organisms, typically protein molecules with large molecular weights at low substrate concentration - first order behaviour: rate increases linearly with [S] at high substrate concentration - zeroth order behaviour: once all the enzyme is complexed, rate of reaction saturates