6 REACTION KINETICS

Question 1

rate of reaction

Answer 1

the change in concentration of a reactant or product per unit time

Question 2

unit for rate

Answer 2

moldm-3s-1

Question 3

what can be determined through experiments

Answer 3

rate equation, order of reaction and rate constant k

Question 4

at a given temperature, rate constant k is

Answer 4

constant

Question 5

t1/2 =

Answer 5

ln2/k

Question 6

conc-time graph: straight line

Answer 6

rate is constant and is independent of [R]. The reaction is zero order w.r.t. [R]

Question 7

conc-time graph: curve with constant t1/2

Answer 7

the reaction is 1st order w.r.t. [R]

Question 8

conc-time graph: curve with varying t1/2

find rate

Answer 8

order of reaction wrt [R] can be determined from the gradient. Plot rate-conc to find.

Question 9

t1/2 of [P]-time: take t1/2 at

Answer 9

[P]= 1/2 & [P]=3/4

Question 10

rate-conc graph: 0 order

Answer 10

The graph is a horizontal line. Hence, rate is independent of [R], The reaction is zero order wrt [R]

Question 11

rate-conc graph: 1st order

Answer 11

The graph is a straight line passing through the origin. Hence, rate is directly proportional to [R], The reaction is first order wrt [R]

Question 12

rate-conc graph: >1st order

then how to find order of reaction?

Answer 12

The graph is a curve. The reaction is not first order wrt [R]
eg rate=k[R]^2 plot rate against [R]^2
OR lg rate= lg k+ 2lg[R]

Question 13

rate-time graph: 0 order

Answer 13

horizontal line

Question 14

rate-time graph: 1st order

Answer 14

decreases exponentially with time

Question 15

rate-time graph: >1st order

Answer 15

decreases exponentially with time with a steeper gradient

Question 16

initial rate

Answer 16

[I2]/t when [R] is small

rate ∝1/t when the total volume of the mixture is kept constant

Question 17

activation energy Ea

Answer 17

the energy barrier the reactants must overcome in order to become the products

Question 18

energy profile diagram

Answer 18

y-axis: energy. x-axis: reaction progress

Question 19

2 factors affecting rates of reaction

Answer 19

1 frequency of effective collision

2 proportion of particles with KE≥Ea

Question 20

factors affecting rates of reaction: conc or pressure

Answer 20

as conc↑, ↑particles per unit volume, particles are closer, frequency of effective collision increases, so rate increases

Question 21

factors affecting rates of reaction: temperature

Answer 21

as T↑, ↑particles with average KE≥Ea↑, frequency of effective collision increases, so rate increases

Question 22

catalyst

Answer 22

a substance that speed up a chemical reaction by providing an alternative reaction pathway of lower activation energy, while remaining chemically unchanged at the end of the reaction

Question 23

heterogeneous catalyst: Haber process: adsorption

N2+ 3H2⇌ 2NH3

Answer 23

1 The gaseous molecules are adsorbed onto the active sites by the formation of weak attraction forces.
2 This process brings reactant molecules closer together, weaken the covalent bonds in the reactant molecules,
thus lowering the Ea and
4 orientates reactant molecules correctly.

Question 24

heterogeneous catalyst: Haber process: desorption

N2+ 3H2⇌ 2NH3

Answer 24

product NH3 desorbs from the catalyst surface so that active sites are exposed for further reaction

Question 25

heterogeneous catalyst: Haber process: order of reaction wrt [R] is zero when

Answer 25

the catalyst is saturated (active sites are fully occupied)

Question 26

autocatalysis

Answer 26

reaction is catalysed by one of its products

Question 27

enzymes as catalyst, they are h___geneous catalyst

Answer 27

homogeneous

Question 28

2 characteristics of enzymes as catalysts

Answer 28

1 highly specific

2 highly efficient

Question 29

relationship of [substrate] and rate in an enzyme-catalysed reaction

Answer 29

at low [substrate]: reaction is 1st order wrt [substrate]

at high [substrate]: reaction is zero order wrt [substrate], [enzyme] then matters

Question 30

2 factors affecting enzyme activities

state & draw

Answer 30

need optimal T & pH

graph of T is skewed

Question 31

At high pressure, the rate of decomposition of ammonia is independent on the partial pressure of ammonia. But at low pressure, the rate is ∝ partial pressure of ammonia. why?

Answer 31

when [NH3] is low,
1 the active sites on the surface of the catalyst are not saturated,
2 more NH3 can be adsorbed onto the site as P NH3↑,
3 so rate∝ partial pressure of NH3

when [NH3] is high,
1 the active sites on the catalyst are saturated,
2 H2 & N2 must desorb from the active site b4 any NH3 molecules are adsorbed,
3 so ↑ P NH3 has no effect on the rate