6. Kinectics

Question 1

Word Definition of Rate of reaction

Answer 1

change in concentration of the product or reactant per unit time

Question 2

Word Equation of Rate of Reaction

Answer 2

Rate of Reaction = Final Concentration - Initial Concentration/Time taken for reaction to complete

Question 3

Mathematical Equation of Rate of Reaction

Answer 3

Rate of reaction = d [reactant/product]/dt

Units: mol dm-3 s-1 or mol dm-3 min-1 or mold dm-3 h-1

Question 4

Initial Rate

Answer 4

• start of the reaction
• gradient of the tangent at t = 0
• usually the greatest rate
• because of highest initial conc. of reactants results in highest freq. of effective collisions

Question 5

Instantaneous rate

Answer 5

• rate at a particular instant in time

- time = t

Question 6

Average rate

Answer 6

total change in concentration of reactant or product over a particular period of time

Question 7

Name two ways to determine reaction rate experiemntally

Answer 7

• Direct Chemical Method

- Physical Method

Question 8

Definition of quenching

Answer 8

to stop the reaction, in this case so that the concentration of the chemical species to be analysed will not change further

Question 9

ways of quenching

Answer 9

• Cooling sample rapidly via ice bath
• Add large amounts of (preferably cold) solvent to dilute the sample, slowing it significantly but not stopping completely
• Add a known excess of a quenching agent

Question 10

Examples of a quenching agent

Answer 10

For acidic mediums, NAOH or NAHCO3, provided that it does not catalyse the reaction

Question 11

Examples of the physical method volume measurement

Answer 11

• dilute acid on a carbonate (such as CaCO3)
• dilute acid on reactive metals
• decomposition of H2O2

Question 12

Different physical methods

Answer 12

• Volume Measurement
• Mass Measurement
• Pressure Measurement (Manometric Method)
• Colour Intensity Measurement (Colorimetric Method)
• Conductivity Measurement (conductometric method)

Question 13

Definition of rate equation

Answer 13

an experimentally determined mathematical expression/relationship between the reaction rate and the concentration of reactants, not stoichiometrically deduced

Question 14

Rate of a reaction usually dependent on

Answer 14

initial concentration of reactants

Question 15

Definition of rate constant (K)

Answer 15

a proportionality constant for a given reaction at a certain temp.

Question 16

Definition of order of reaction

Answer 16

• In the experimentally determined rate equation, with respect to a reactant, it is the power of that reactant’s concentration
• can be integers or fractions
• not related to stochiometric coefficients
• related to mechanism of the reaction

Question 17

Definition of overall order of reaction

Answer 17

the sum of the orders of reactions

Question 18

Zero order reaction

Answer 18

• independent of reactant [A]
• changing the concentration of reactant A will not affect the rate
• units of K = mol dm-3 time -1

Question 19

Rate constant is affected by

Answer 19

• temp
• Ea (presence of a catalyst which lowers the Ea)
  from the Arrhenius equation

Question 20

First order reaction

Answer 20

• directly proportional to reactant [A]
• when [A] doubles, the rate of reaction doubles, etc.
• units of K = time -1

Question 21

Second order reaction

Answer 21

• rate of reaction is directly proportional to reactant [A]2
• when [A] doubles, the rate of reaction quadruples (x 4)
• units of k = mol-1 dm3 time-1

Question 22

Half Life (t1/2)

Answer 22

time taken for conc of a reactant to be halved

Question 23

half life for first order reaction is

Answer 23

t1/2 = ln 2/ k = 0.693/k

Question 24

Reaction Mechanism Definition

Answer 24

shows the sequence or series of steps in a reaction which occurs in multi-steps called elementary steps

Question 25

The molecularity of each elementary step is the

Answer 25

number of reactant particles (molecules, atoms or ions) taking part in that particular step. ex: unimolecular step involves one reactant species in that step, bimolecular step involves two reactants species in that step

Question 26

Rate determining step (r.d.s.)

Answer 26

- the slowest step in the multi- step process | - determines overall rate of reaction

Question 27

in a reaction mechanism, the sum of the elementary steps is equal to

Answer 27

the overall stoichiometirc equation

Question 28

in a reaction mechanism, the slowest step

Answer 28

determines the rate of the reaction

Question 29

Collision Theory

Answer 29

- chemical reaction takes place as a result of effective collisions - reactant particles must collide 1. in the correct orientation 2. possess Ea

Question 30

Ea

Answer 30

minimum amount of kinetic energy

Question 31

Ea required to

Answer 31

- overcome repulsive forces between reacting particles when approaching in close proximity - break bonds in reacting particles

Question 32

Transition State

Answer 32

After colliding, molecules form an activated complex or transition state, it then decomposes or rearranges to form the products - some existing bonds are broken - new bonds formed 0 maximum energy and highly unstable, cannot be isolated

Question 33

Energy Profile Diagram

Answer 33

Tracks the energy changes of the reaction as it progresses from reactants to products

Question 34

Factors that affect Rate of reaction

Answer 34

- conc of reactant particles (pressure for gaseous reactants) - surface area of solid reactant - temp. - catalyst

Question 35

How conc of reactant affect rate of reactions

Answer 35

- as it increase, it increase no. of collisions and therefore more effective collisions and rate increase

Question 36

how surface area of reactants affect rate

Answer 36

- reaction between solid and liquid/gas, occurs at the surface of the solid - the smaller the size, the great er the SA exposed for reaction - faster rate

Question 37

how temp affect rate

Answer 37

- average kinetic energy of particles increase, collisions and effective collisions increase, rate increase

Question 38

catalyst

Answer 38

- increase reaction rate by providing an alternative reaction pathway that has lower Ea - does not undergo any permanent chemical change i.e. regnerated

Question 39

Types of Catalyst

Answer 39

- hetergenous catalyst - homogenous catalyst - auto-catalyst - biological catalyst or enzyme

Question 40

Heterogenous catalyst

Answer 40

- in different phases as the reactants | - generally catalyst is solid, reactants are either aq or g

Question 41

Homogenous Catalysis

Answer 41

- same phase as reactants

Question 42

Auto catalysis

Answer 42

one of the product formed in the reaction catalyses the reaction