Chemical Kinetics

Front 1

Transition vs intermediate

Back 1

Transition can not be detected intermediate can be detected and sometimes isolated

Front 2

How is rate characterised?

Back 2

Rate = -d[reactant]/dt =d[product]/dt

Front 3

Rate for a general case with stoichiometry?

Back 3

aA + bB -> cC + dD

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

Front 4

Differential rate expression?

Back 4

for A + B -> P

Rate = -d[A]/dt = d[P]/dt = k([A]^m)([B]^n)

Front 5

How is rate law determined?

Back 5

experimentally only (it cannot be deduced from the stoichimetry of the reaction)

Front 6

First order reaction? units

Back 6

Rate = -d[A]/dt = k[A]

units of k = s⁻¹

Front 7

Second order reaction? units

Back 7

Rate = k[A]² or k[A][B]

units of k = dm³ mol⁻¹ s⁻¹

Front 8

nth order rate constant units?

Back 8

1/conc^(n-1) x 1/time

Front 9

isolation method?

Back 9

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

for large [B] ≈ constant

∴k` = k[B]^n

Rate = k`[A]^m

Front 10

Initial rate method?

Back 10

measure initial rate to determine k` because [A]₀ is known

r₀ = k`[A]₀^m

plotting logr₀ vs log[A]₀ gives a straight line with slope = m

(order can be zero)

Front 11

problem with initial rates?

Back 11

can be very inaccurate and is wasteful in reactants as you need multiple runs

Front 12

Integrated rate law: zeroth order?

Back 12

-d[A]/dt = k

=> d[A] = -kdt

=>∫d[A] = - k∫dt (between [A]₀/[A] and 0/t

=>[A] - [A]₀ = -kt

=>[A] = [A]₀ - kt

Front 13

Integrated rate law: first-order?

Back 13

d[A]/dt = k[A]

=>d[A]/[A] = -k dt

=>∫1/[A] d[A] = -k∫dt

=ln[A]₀/[A] = kt

Front 14

Integrated rate law: secound order?

Back 14

-d[A]/dt = k[A]²

=> ∫1/[A]² d[A] = kt

=> 1/[A] - 1/[A]₀ = kt

Front 15

Half life for first and second order?

Back 15

constant for 1st order, increases as reaction proceeds for second order

Front 16

Arrhenius equation?

Back 16

k = A e^(-Ea/RT)

where A(pre-exponential factor) = the probabilitiy molecules meet in the correct orientation to react

e^(-Ea/RT) = probabiliy molecules have enough energy to react

Front 17

How does a catalyst work?

Back 17

gives an alternative reaction pathway with a lower activation energy

Front 18

Define molecularity, bimolecular, unimolecular and termolecular?

Back 18

-molecularity of an elementary reaction step is the number of reactant molecules involved

-bimolecular reaction is one where two reactant molecules come together

-unimolecular reaction is one where one molecule decomposes e.g. an intermediate

-termolecular reaction is one where three reactant molecules come together

Front 19

Steady state approximation?

Back 19

assume that one intermediate in the reaction mechanism is consumed as quickly as it is generated. (its concentration is constant)