Kinetics (II) - Topic 16 Flashcards
What is zero order
Changing the concentration of the chemical has no effect on the rate of the reaction
Therefore, it is not included in the rate equation
what is first order
The concentration of the chemical is directly proportional to the rate of reaction
rate = K[A]
what is second order
The rate is directly proportional to the square of the concentration of that chemical
rate = k[A]2
what is overall rate determined by
slowest step of reaction
write the rate equation for
A (aq) + B (aq) → C (aq) + D (g)
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
what does the order of a reaction show
The order of a reactant shows how the concentration of a chemical, typically a reactant, affects the rate of reaction
what does it mean if the order is a fraction
that the reaction involves multiple steps
units for initial rate of reaction and the reactant
initial rate of reaction = moldm-3s-1
reactant = moldm-3
draw concentration time graph for a zero order reaction
draw concentration time graph for first order reaction
draw concentration time graph for second order reaction
how can you deduce order of reaction from its half life
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
draw rate concentration graphs for zero order reaction
draw rate concentration graphs for first order reaction
draw rate concentration graphs for second order reaction
what is the rate determining step
slowest step in the reaction as a chemical reaction can only go as fast as the slowest part of the reaction
what comes up in the rate equation
the reactants in the rate determining step
what type of halogenoalkanes undergo Sn1 mechanism
and what does
S
N
1
stand for
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
describe the SN1 mechanism
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
draw mechanism of nucleophilic substitution of 2-bromo-2-methylpropane with hydroxide
what type of halogenoalkanes undergo Sn2 mechanism
and what does
S
N
2
stand for
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
describe the sn2 mechanism
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
draw the Sn2 mechanism of bromoethane with hydroxide
when does the constant k change
If the temperature is changed or a catalyst is used or changed
what is the Arrhenius equation
k = A e ^− E a R T
Lnk = lnA - (Ea/RT)
what does
A stand for in the Arrhenius equation
arrhenius constant related to collision frequency and orientation of the molecules
what does the Arrhenius equation show
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
when can you work out the gradient -Ea/R from a graph
The graph is of ln k against 1/T
EXAM QUESTION
Deduce the rate equation for the reaction of NH3 when it is 0 order and justify ur answer
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
how to work our rate constant from graph / K
gradient
describe the stages of catalytic decomposition of ammonia by tungsten
- 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
