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

pH probe and meter

Question 4

technique used to measure change in conductivity

Answer 4

conductivity probe and meter

scenarios this can be used in: rxns in which products increase no of ions in the system, thus increasing electrical conductivity (and vice versa)

Question 5

technique used to measure change in mass

Answer 5

• reacting the mixture in an open beaker
• 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 if the gas collected has 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

stopwatch

- i.e. using something observable as an arbitrary endpoint for when to stop the watch

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
• light of a selected wavelength is passed 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

technique to measure change in concentration

Answer 9

• taking samples from the solution at regular intervals
• the ‘quenching’ technique must be used (stopping the reaction in the solution)
• then titrating each sample against a known ‘standard’
• to determine the concentration of one of the reactants/products in each sample

Question 10

non-continuous method to measure rate of reaction

Answer 10

AKA clock reactions

• stopping the stopwatch when a certain condition is met
  e. g. time taken for an Mg strip submerged in acid to no longer be visible

Question 11

collision theory

Answer 11

• rate of rxn depends on number of successful collisions
• temp (in k) is proportional to avg KE of particles in a substance

conditions necessary for a chem rxn to occur:

• reactants must physically and directly collide
• reactants must have correct mutual orientation (collision geometry)
• reactant KE ≥ Ea

Question 12

Maxwell-Boltzmann energy distribution curve

Answer 12

• a plot of the no of particles, plotted against KE
• shows the distribution of KE
• also shows no of particles with energy ≥ Ea
• ↑ energy: curve moves broader (↔) and lower (↓)
• catalyst present: Ea cutoff moves to right

Question 13

factors affecting collision success

Answer 13

• energy of reactants (KE)
• orientation/geometry of collision
• frequency of collisions

Question 14

factors affecting rate of rxn

Answer 14

• temp
• pressure
• catalyst
• particle size
• conc.

Question 15

factors affecting rate of rxn: temp

Answer 15

↑ temp = ↑ KE = ↑ no. of successful collisions = ↑ reactants with energy ≥ Ea = ↑ rate of rxn

many rxn speeds double for every 10°C/K increase

Question 16

factors affecting rate of rxn: conc

Answer 16

↑ conc = ↑ no. of particles per unit area = ↑ no of successful collisions = ↑ reactants with energy ≥ Ea = ↑ rate of rxn

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

Question 17

factors affecting rate of rxn: particle size

Answer 17

↓ particle size = ↑ SA = ↑ chance of contact = ↑ no of successful collisions = ↑ rate of rxn

Question 18

factors affecting rate of rxn: catalyst

Answer 18

catalyst present = ↓ Ea required = ↑ no of successful collisions = ↑ rate of rxn

• 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 19

factors affecting rate of rxn: pressure

Answer 19

↑ pressure = ↑ conc = ↑ no of successful collisions = ↑ rate of rxn

(only for reactions with gases)

Question 20

reaction mechanism

Answer 20

sequence of steps in an observable reaction

Question 21

how is rate expression derived for a reaction?

Answer 21

rate = k [A]^m [B]^n
k: rate constant (reaction and temp dependent)
A and B: reactants
m and n: individual orders of reaction

Question 22

homogeneous catalyst

Answer 22

same physical state as reactant

e.g. breakdown of O3 catalysed by chlorine

Question 23

heterogeneous catalyst

Answer 23

catalyst is in different state to reactant

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

Question 24

applications - interpreting volume-time graphs for rate of rxn

Answer 24

• 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 25

rate constant unit

Answer 25

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

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

Question 26

special feature of first order reactions

Answer 26

• constant half-life

- time taken for reactant conc to decrease to half the original value is not dependent on its starting conc

Question 27

finding Ea from arrhenius plot

Answer 27

slope = - Ea / R

Question 28

Writing rate expressions with consideration to reaction mechanism

Answer 28

• trick question: when writing rate expressions, DO NOT CONSIDER REACTION MECHANISM
• only look at the original reaction equation