Kinetics (II) - Topic 16

Question 1

What is zero order

Answer 1

Changing the concentration of the chemical has no effect on the rate of the reaction

Therefore, it is not included in the rate equation

Question 2

what is first order

Answer 2

The concentration of the chemical is directly proportional to the rate of reaction

rate = K[A]

Question 3

what is second order

Answer 3

The rate is directly proportional to the square of the concentration of that chemical

rate = k[A]2

Question 4

what is overall rate determined by

Answer 4

slowest step of reaction

Question 5

write the rate equation for
A (aq) + B (aq) → C (aq) + D (g)

Answer 5

Rate of reaction = k [A]m [B]n

[A] and [B] are the concentrations of the reactants
m and n are orders with respect to each reactant involved in the reaction

Question 6

what does the order of a reaction show

Answer 6

The order of a reactant shows how the concentration of a chemical, typically a reactant, affects the rate of reaction

Question 7

what does it mean if the order is a fraction

Answer 7

that the reaction involves multiple steps

Question 8

units for initial rate of reaction and the reactant

Answer 8

initial rate of reaction = moldm-3s-1
reactant = moldm-3

Question 9

draw concentration time graph for a zero order reaction

Question 10

draw concentration time graph for first order reaction

Question 11

draw concentration time graph for second order reaction

Question 12

how can you deduce order of reaction from its half life

Answer 12

For a zero-order reaction the successive half-lives decrease with time
This means that it would take less time for the concentration of reactant to halve as the reaction progresses

The half-life of a first-order reaction remains constant throughout the reaction
The amount of time required for the concentration of reactants to halve will be the same during the entire reaction
For a second-order reaction, the half-life increases with time

This means that as the reaction is taking place, it takes more time for the concentration of reactants to halve

Question 13

draw rate concentration graphs for zero order reaction

Question 14

draw rate concentration graphs for first order reaction

Question 15

draw rate concentration graphs for second order reaction

Question 16

what is the rate determining step

Answer 16

slowest step in the reaction as a chemical reaction can only go as fast as the slowest part of the reaction

Question 17

what comes up in the rate equation

Answer 17

the reactants in the rate determining step

Question 18

what type of halogenoalkanes undergo Sn1 mechanism

and what does
S
N
1
stand for

Answer 18

tertiary halogenoalkanes

S’ stands for ‘substitution’
‘N’ stands for ‘nucleophilic’
‘1’ means that the rate of the reaction depends on the concentration of only one reagent, the halogenoalkane

Question 19

describe the SN1 mechanism

Answer 19

The SN1 mechanism is a two-step reaction
In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion (this is the slow and rate-determining step)
As the rate-determining step only depends on the concentration of the halogenoalkane, the rate equation for an SN1 reaction is rate = k[halogenoalkane]

This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
In the second step, the tertiary carbocation is attacked by the nucleophile

Question 20

draw mechanism of nucleophilic substitution of 2-bromo-2-methylpropane with hydroxide

Question 21

what type of halogenoalkanes undergo Sn2 mechanism

and what does
S
N
2
stand for

Answer 21

primary halogenoalkanes

S’ stands for ‘substitution’
‘N’ stands for ‘nucleophilic’
‘2’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of both the halogenoalkane and the nucleophile ions

Question 22

describe the sn2 mechanism

Answer 22

The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond

As this is a one-step reaction, the rate-determining step depends on the concentrations of the halogenoalkane and nucleophile, the rate equation for an SN2 reaction is rate = k[halogenoalkane][nucleophile]

An SN2 reaction is bimolecular as there are two molecules involved in the rate-determining step

At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond
The halogen leaves the halogenoalkane as an X- ion

Question 23

draw the Sn2 mechanism of bromoethane with hydroxide

Question 24

when does the constant k change

Answer 24

If the temperature is changed or a catalyst is used or changed

Question 25

what is the Arrhenius equation

Answer 25

k = A e ^− E a R T

Lnk = lnA - (Ea/RT)

Question 26

what does
A stand for in the Arrhenius equation

Answer 26

arrhenius constant related to collision frequency and orientation of the molecules

Question 27

what does the Arrhenius equation show

Answer 27

the effect that a change in temperature has on the rate constant, k, and thus on the overall rate of the reaction

An increase in temperature (higher value of T) gives a greater value of ln k and therefore a higher value of k

Since the rate of the reaction depends on the rate constant, k, an increase in k also means an increased rate of reaction

effect of increasing the activation energy on the value of the rate constant, k
An increase in the activation energy, Ea, means that the proportion of molecules which possess at least the activation energy is less

This means that the rate of the reaction, and therefore the value of k, will decrease

Question 28

when can you work out the gradient -Ea/R from a graph

Answer 28

The graph is of ln k against 1/T

Question 29

EXAM QUESTION
Deduce the rate equation for the reaction of NH3 when it is 0 order and justify ur answer

Answer 29

Rate = k[NH3]
because the rate is independent of the partial pressure of ammonia / rate is constant
or
because the graph is a straight line / (1)
linear
gradient is constant

Question 30

how to work our rate constant from graph / K

Answer 30

gradient

Question 31

describe the stages of catalytic decomposition of ammonia by tungsten

Answer 31

• adsorption of ammonia / reactant onto surface of tungsten / catalyst
  (1)
• breaking bonds in ammonia

desorption of nitrogen and hydrogen / products / gases from surface of tungsten / catalyst