Reaction Kinetics

Question 1

Defn of rate of rctn
units
eqn

Answer 1

change in conc of a particular prod/rct per unit time
mol dm^-3 s^-1
rate of eqn = change in conc / time taken

Question 2

Defn of rate eqn

eqn

Answer 2

Experimentally determined mathematical exp b/w the rctn rate and the conc of rcts
Rate = k [A]^m [b]^n

Question 3

Defn of order of rctn

Answer 3

the order of rctn wrt a rct is the power in its conc term in the rate eqn and is experimentally determined

Question 4

Defn of rate const

Answer 4

Is the proportionality const in the exp-determined rate eqn. It is a const at a given temp

Question 5

Defn of instantaneous & initial & ave rate

Answer 5

Instantaneous rate - is the rate at a particular time
initial rate - the instantaneous rate at time=0
Ave rate - is the change in conc of a rct or a product over that time interval

Question 6

In prac, why may the initial rate ≈ ave rate provided（2）

Answer 6

1. time interval is small enough

2. time interval starts at t=0

Question 7

Interpreting rate eqn
units
formula
what order rctn

Answer 7

When
rate = k ——– zero-order rctn
units of k: mol dm^-3 s^-1

rate = k[A] ——- first-order rctn
units of k: s^-1

rate = k[A][B] &. =k[A]^2 ——– second-order rctn
units of k: mol^-1 dm^3 s^-1

Question 8

Defn of first order rctn & eqn & graph

Answer 8

One in which the rctn rate is directly prop to the conc of a single rct
rate = k[A]

Question 9

Defn of 2nd order rctn & eqn & graph

Answer 9

One in which the rctn rate is directly prop to the sq of the conc of a single rct
rate = k[A][B] & =k[A]^2

Question 10

Defn of zero-order rctn & eqn & graph

Answer 10

One in which the rctn rate is independent of the conc of a single rct
rate = k

Question 11

Defn of half life
how to find it (2)
& working on graph
The proving statement too

Answer 11

Time taken for the conc of a rct to decrease to half its initial value
From the graph,
1st t1/2 = time taken for […] to decrease from …. to …. = t1
2nd t1/2 = time taken for […] to decrease from …. to …. = t2
Since t1 = 12, t1/2 is const

1st order wrt [A]

On graph draw the &
in ans: Since the half-life is approximately constant (averaget1/2 = 15.5min), the reaction is first order with respect to N2O3

ALSO t1/2 = ln2/k

Question 12

Pseudo-order rctns (3)

and the way of writing the k’ eqn

Answer 12

Presence of a large excess of a rcnt
Rcnt is also the solvent
Presence of a catalyst

eg rate = k[S2O82−][I−] = k’[I-] where k’ = k[S2O8 2-] = (0.2)(0.5)

Question 13

Finding order of rctn (3)

Answer 13

By inspection method
By Calc
By conc-time graph

Question 14

Find k

Answer 14

sub values into rate eqn for a certain exp (any exp will do)

Question 15

Why is sth used in large excess

Answer 15

Ensure that [Q] stays effectively const throughout the rctns so that the effect of changing [P] on the rate of the rctn can be studied

Question 16

Exp-tech

Physical methods

• gas (1)
• colour intensity (3)

Chem methods

• sampling followed by quenching then titrimetric method (3)
• ‘clock rctns’ (2)

Answer 16

look at notes

Question 17

Factors affecting rate of rctn (4)

Answer 17

?

Question 18

‘Clock rctns’

why must the beaker be the same
why must the total vol be const
why is initial value inversely prop to the time taken for a___ solid/colour to appear

Answer 18

ensure depth of rctn mixture remains the same, same amt of __ needs to be precipitated before the cross is obscured

ensures initial conc of each reactant in the rctn mixture is directly prop to its vol used

time interval is small and measured from 0

Question 19

Defn of rctn mechanism

Answer 19

collection of elementary steps in the proper sequence showing how rcnt particles are converted into products. It is the explanation of how a rctn takes place

Question 20

Defn of elementary step

Answer 20

distinct step in a rctn mechanism which describes a single molecular event that involves breaking or making bonds

Question 21

Defn of rate-determining step

Answer 21

Slowest step and highest Ea (find actual defn from tut?)

Question 22

What is the molecularity of an elementary step

eg unimolecular step, bimolecular step

Answer 22

No. of rct particles taking part in that step

Question 23

Defn of an intermediate

Answer 23

a species that is formed in 1 step of a rctn mechanism and consumed in a subsequent step

Question 24

Defn of single/multi-step rates
How to find it from stoic eqn
deducing rate eqn

Answer 24

Single step rctns is one that takes place in a single step and is termed an elementary rctn

Question 25

Maxwell-Boltzmann distribution curve

Answer 25

-

Question 26

Defn of catalyst &
types of catalyst (2)
- 4 eg
- 3 eg

Answer 26

Catalyst is a substance which increases the rate of rctn without undergoing any permanent chemical change

Homogenous Catalysis - catalyst and rcts are in same phase (state)
* Fe2+ / Fe3+ rctn b.w peroxodisulfate ions and iodide ions

• No2 catalytic oxidation of atmospheric sulfur dioxide

Mn2+ autocatalytic rctn

Heterogeneous catalysis - catalyst and rcts are in diff phase (state)
Ni catalytic reduction / hydrogenation of alkenes to form alkanes

*Fe haber process

V2O5 contact process

Question 27

Defn of autocatalysis
graph
explain graph
test for autocatalyst is to..

Answer 27

-

Question 28

Defn of Enzymes

Answer 28

Are proteins which catalyse chemical rctns in living systems. They are often called biological catalysts or biocatalysts.

Question 29

Definition of inhibitor

Answer 29

is a substance which decreases the rate of a chemical rctn

Question 30

Factors affecting rate of enzyme-catalysed rctn (3)

& graph(s) for one of the factors

Answer 30

temp
pH
conc of enzyme

Question 31

Enzymes - models (2)

Answer 31

lock and key model

induced-fit model

Question 32

5 properties of enzymes

Answer 32

Nature and size
efficiency
specificity
temp
sensitivity to pH

Question 33

k is affected by

Answer 33

temp or [catalyst] / lower Ea basically

Question 34

Process of catalytic reduction/hydrogenation of ethene (5)

Answer 34

Diffusion - diffuse towards the catalyst surface

Adsorption - rct molecules become adsorbed onto the active sites of the catalyst surface. This increase in conc of rcts at the catalytic surface and also weakens the covalent bonds in the molecules

Rctn - Adjacent molecules react to form products. This rctn has lower Ea than the uncatalysed rctn

Desorption - the reverse of adsorption. The prod molecule eventually breaks free from the catalyst surface

Diffusion - product molecules diffuse away from the surface. The vacant active sites are now available for adsorbing other rct molecules

Question 35

Eg outline how the rate of each of the following rctns may be followed experimentally

general strategy?

Answer 35

1. Identify what can be monitered over time, then identify method to use:
   - change in [rct] or [prod] with time can be monitored by diff methods eg calorimetry
2. Perform the exp and collect data over time
3. Plot a graph of data against time - (3 to 5 data pts)
4. Instantaneous rate may be found by drawing a tangent to the curve at the particular time and finding its gradient g1
   - rate = -g1 if data ∝ [rct] while rate = g1 if data∝ [prod]