Reaction kinetics

Question 1

What is activation energy?

Answer 1

Minimum amt of energy, which the reactant particle must posses to overcome energy barrier in order for effective collisions to result in chem reactions

Question 2

What does the maxwell-boltzmann curve show?

Answer 2

amt of KE possessed by particular fraction of particles

total area under graph represents total number of particles

highest peak gives most probable kinetic energy have at a given temp

Question 3

How does temp affect ROR?

Answer 3

higher temp, total no. of particles remain the same but average KE increases, number of reactant particles with energy more than Ea increases. *

Question 4

Define catalyst

Answer 4

Substance that increases the rate of reaction by providing an alternative pathway with lower activation energy, remains chemically unchanged at end of reaction

Question 5

How does catalyst affect ROR

Answer 5

activation energy is lowered, number of reactant particles with energy more than Ea increases. *

Question 6

How does concentration affect ROR?

Answer 6

number of reactant particles per unit volume increases
*

Question 7

How does ROR change?

Answer 7

Frequency of effective collisions increase/decreases since rate of reaction is proportional to frequency of effective collisions ROR increases or decreases

Question 8

How does physical state affect ROR?

Answer 8

if finely divided there is greater surface area of contact with liquid and gaseous reactants. *

Question 9

How does light affect ROR?

Answer 9

average KE of reactant particles increases upon absorbing light energy leading to larger proportion of particles with sufficient energy to overcome Ea barrier

Question 10

Define rate of reaction

Answer 10

change in concentration of reactant or product per unit time

Question 11

How to determine the diff types of ROR?

Answer 11

instantaneous: gradient at a specific time

initial: gradient of tangent at t=0

average: change in concentration of reactant or product over specified time interval

Question 12

Define Rate equation

Answer 12

relates the rate of reaction to concentration of reactant raises to appropriate powers. Tate laws are experimentally determined.

Question 13

Define order of reaction

Answer 13

power to which concentration of reactant is raised to in rate equation

overall order of reaction is sum of individual order with respect to each reactant

Question 14

Define rate constant,k

Answer 14

proportionality constant in rate equation that relates concentrations of reactant to rate of reaction, no fixed units and experimentally determined

Question 15

Define zero order reaction and graphs

Answer 15

rate of reaction is independent of [A], rate is constant.
units: mol dm^-3 s^-1
straight line rate over [A] graph

slope down straight [A] over time graph

slope up straight [product] over time graph

Question 16

Define first order reaction

Answer 16

rate = k[A]
units: s^-1

graph of rate against [A] is slope up
graph of [A] against time is curve down
graph of [product] against time is curve up

constant half life (time taken for concentration of reactant, A to decrease to half its original value) ln2/k

Question 17

Define second-order reaction

Answer 17

rate of reaction is directly proportional to [A]^2

units: mol^-1 dm^-3 s^-1
graph of [A] against time curve downwards
rate against [A] curves upwards
rate against [A]^2

half life not constant

Question 18

Define pseudo-order reaction

Answer 18

one of reactants is present in large excess concentration relative to other reactants.

since A is solvent of the reaction, it will be present in large excess as compared to B. Amount of A used up would be small fraction of A present initially (negligible)
A is essentially constant throughout.
rate= k’[B] where k’ = [A]
ln2/k’

Question 19

What are the two types of kinetics experiments?

Answer 19

Continuous: one experiment is conducted and change in concentration of [A] is measured
constant temp low [A] while excess [B]

Discontinuous: several experiments are conducted, varying initial concentration of reactant while keep all other variables constant

Question 20

What is discontinuous experiment?

Answer 20

several experiments, varying initial concentrations of reactant while keeping all other variable constant. Time taken for small and fixed amt of product measured.
constant temp

for reactions in aq medium, water is added to maintain constant total volume. enabling initial concentration of reactant in mixture to be directly proportional to volume

Question 21

What are the two gaseous physical methods?

Answer 21

Change in gas volume: volume of product gas at regular time intervals, determine order of reaction using half life

Change in gas pressure: used for gaseous reaction which no. of moles of gaseous reactants is diff from products

Question 22

What are the two physical methods?

Answer 22

Change in electrical conductivity: use for reactions, when the exchange in number of ions in solution, and thus a change in electrical conductivity of solution

Change in colour intensity : reactions, involving coloured substance colour, intensity can measure at regular time intervals using colorimeter
Weight of reaction is directly proportional to the rate of change in colour, intensity of reactant

Question 23

What are the two physical clock methods?

Answer 23

Sulfur clock and Iodine clock

Question 24

Define elementary step, reaction intermediate, rate determining step.

Answer 24

An elementary step is simplest step which cannot be broken down into simpler steps the sequence of these elementary steps is referred to as reaction mechanism

Reaction intermediates are produced over course of reaction and used up subsequent steps.
rate determining step has a higher Ea than other elementary steps

Question 25

Two types of catalysts

Answer 25

Homogenous and heterogeneous

Question 26

What is Homogenous catalyst?

Answer 26

catalysts acts in same phase as reactants and is uniformly mixed with them, providing an alternative pathway with lower activation energy by first forming an intermediate, gets chemically changed but will be regenerated eg. transition metals as catalytic depend on variable oxidation states

Question 27

What’s a heterogenous catalyst?

Answer 27

acts in different phase from reactant, provides an alternate pathway with a lower activation energy by means of adsorption, which is the adhesion of a substance surface of another substance, these adsorption weakens the chemical bonds in reactant particles,lowering the activation energy of the reaction. Adsorption also increases the concentration of the reacting particles on the catalyse surface, increasing the rate of reaction. These catalysts have available vacant of partially filled 3-D orbitals, which allows the reactant particles to be adsorbed onto the catalyst surface

Question 28

Stages involved in heterogenous catalysis

Answer 28

Adsorption, reactant molecules diffuse towards the sonic, catalyst surface and are adsorbed onto the active sites at the surface, they then become chemisorbed, this process increases the concentration of the reactant molecules on the catalyst surface Reaction at the surface: intra molecular bonds within the reactant molecules are weakened. Thus lowering the activation energy. Reactor molecules are brought into close contact and are correctly orientated for reaction. desorption: product, molecules, disorbes, and diffuses away from the catalyst surface, freeing up active sites on the catalyst.

Question 29

How does catalytic oxidation of SO2 work?

Answer 29

NO2 (car exhaust) acts as catalyst to oxidise atmosphere SO2 (burning fossil fuels) to SO3 NO2(g)+SO2(g) > SO3(g)+NO(g) NO is rapidly re-oxidesed to NO2 by oxygen SO3 dissolves in water vapour in atmosphere to form sulfuric acid causing acid rain. SO3(g)+H2O(g)>H2SO4(g)

Question 30

How do catalytic converters work?

Answer 30

PT, Pd and Rh are coated on ceramic honeycomb structure. NOx are reduced to N2 by excess CO with Rh as catalyst 2NO(g) + 2CO(g) > 2CO2(g) + N2 (g) Unburnt hydrocarbons and CO oxidised to CO2 and H2O with Pt and Pd catalyst. 2CO (g) + O2(g) > 2CO2(g) combustion of hydrocarbon formula

Question 31

Define enzymes

Answer 31

enzymes are biological catalyst that speed up or alter rate of reaction in living system while remaining chemically unchanged at end of reaction

Question 32

How to use initial rate to find out order of reaction?

Answer 32

to find order of reaction wrt curve reactant. for graph with [B] = x mol dm-3, initial rate = y/t1 for graph with [B] = 2x mol dm-3, initial rate = y/t2 when [A] is kept constant and [B] doubled: if rate doubles, order of reaction wrt B is 1 if rate quadruples, order of reaction wrt B is 2

Question 33

How to use half life to find order of reaction?

Answer 33

[A] decreases with time while [B] relatively constant if straight line observed order of reaction wrt A is 0 if t1/2 constant order of reaction wrt A is 1 from graph of [B] = 2x mol dm-3 t1/2 is constant at W seconds, order of reaction wrt A is 1 (curly bracket, label t1/2, dotted line)