Topic 6: Chemical Kinetics

Question 1

Rate of rxn

Answer 1

the change in conc of reactants/products per unit time

unit: mol dm-3 s-1

Question 2

rate of rxn equation

Answer 2

rate of rxn = Δ conc / time taken

• conc can either be product or reactant

Question 3

technique used to measure change in pH

Answer 3

using pH probe and a meter

Question 4

technique used to measure change in conductivity

Answer 4

conductivity probe and a meter

as no of ions increase, electrical conductivity increases… and vice versa

Question 5

technique used to measure change in mass

Answer 5

• reacting the mixture in a sealed beaker (e.g. cotton on the neck)
• having the beaker placed on a balance throughout the rxn

NOTE: will not work with H2 as its too light to give significant change in mass

Question 6

technique used to measure change in vol of gas

Answer 6

• downward displacement of gas: only works on gases with low solubility in water
• OR connecting a gas syringe to a closed system (e.g. sealed beaker) and allowing pressure to push the syringe handle

Question 7

technique to detect change in rxn

Answer 7

clock rxn

• measure for the rxn to reach a certain fixed point
• i.e. something observable that can be used as an arbitrary endpoint to stop the clock

Question 8

technique to measure change in colour

Answer 8

• for rxns involving transition metals or other coloured compounds
• equipment may be colorimeter, of spectrophotometer
• works by passing light of a selected wavelength through the soln being studied, to a sensitive photocell
• the photocell generates a current depending on the light intensity, which in turn depends on the conc of the coloured reactant/product
• measures absorbance against time

Question 9

collision theory

Answer 9

before a chem rxn can occur:

• the reactants must physically & directly collide
• reactants must have correct mutual orientation
• reacting particles must have sufficient KE to initiate the rxn (I.e. Ea, or activation energy)
• temp (in k) is proportional to avg KE of particles in a substance

Question 10

factors affecting rate of rxn: temp

Answer 10

higher temp = higher KE = higher successful collisions = more particles achieve Ea = higher rate of rxn

many rxn speeds increase by 2x for every 10 degree C/K increase

Question 11

Maxwell-Boltzmann energy distribution curve

Answer 11

a plot of the no of particles with a given KE, plotted against KE

Question 12

factors affecting rate of rxn: concentration

Answer 12

higher conc = higher no of particles per unit area = higher no of successful collisions = higher no of reactants achieving Ea = higher rate of rxn

doubling the conc of one of the reactants typically doubles rate of rxn

Question 13

factors affecting rate of rxn: particle size/SA

Answer 13

smaller particle size = bigger surface area = higher chance of contact = higher no of successful collisions = more reactants with Ea = higher rate of rxn

Question 14

factors affecting rate of rxn: catalyst

Answer 14

• provides an alternate pathway with less Ea requirement

- in reversible rxns, catalysts equally affect both forward and backward rxns so it doesn’t alter equilibrium positions

Question 15

homogeneous catalyst

Answer 15

same physical state as reactant

e.g. breakdown of O3 catalysed by chlorine

Question 16

heterogeneous catalyst

Answer 16

catalyst is in different state to reactant

e.g. Vanadium (V) oxide in contact process

Question 17

factors affecting rate of rxn: pressure

Answer 17

higher pressure = higher conc = higher no of successful collisions = higher no of reactants achieving Ea = higher rate of rxn

only for reactions of gases

Question 18

factors affecting rate of rxn

Answer 18

• temperature
• concentration
• particle size
• catalyst
• pressure

Question 19

properties of reactants that affect rate of rxn

Answer 19

• collision geometry/orientation
• frequency of collisions
• kinetic energy

Question 20

applications - interpreting volume-time graphs regarding rate of rxn

Answer 20

• rate = volume (of product)/time = slope of graph
• initially CO2 is produced quickly as the conc. of reactants are highest at the beginning
• as rxn progresses, rate decreases due to less frequent collisions as conc.s of reactants decrease
• curve becomes flat when one of the reactants is completely used up in the reaction

Question 21

rate-determining step

Answer 21

• slowest step in the rxn

- name is self-explanatory :)

Question 22

reaction intermediates

Answer 22

substances that are formed in 1 step and consumed in a later step (in the rxn)

Question 23

reaction mechanism

Answer 23

sequence of elementary reactions that must occur to go from reactants to products

Question 24

elementary reaction

Answer 24

• single-step reaction that has no intermediates or transition states
• all bond breaking/forming occurs in a single collision

Question 25

how to identify a correct rxn mechanism (for MCQs)

Answer 25

• add up rxns: if the overall rxn doesn’t match the actual rxn equation, the proposed rxn mechanism is incorrect
• if the overall rxn = actual rxn equation, check the reactants with the formula:
  rate = k [A]^a [B]^b

Question 26

rate constant unit

Answer 26

formula: (s-1 )/ (mol dm-3)m+n-1

zero order: M s^-1 dm^-3 (or any other unit of time)
first order: s^-1 (or any other unit of time)
second order: dm^3 M^-1 s^-1 (you get the drill…)
third order: dm^6 M^-2 s^-1

Question 27

factors affecting rate constant

Answer 27

temperature

Question 28

finding Ea from arrhenius plot

Answer 28

slope = - Ea / R