Topic 6: Chemical Kinetics

the change in conc of reactants/products per unit time unit: mol dm-3 s-1

rate of rxn = Δ conc / time taken • conc can either be product or reactant

pH probe and meter

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)

• 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

• 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

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

• 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

• 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

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

• 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

• 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

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

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

↑ temp = ↑ KE = ↑ no. of successful collisions = ↑ reactants with energy ≥ Ea = ↑ rate of rxn many rxn speeds double for every 10°C/K increase

↑ 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

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

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

↑ pressure = ↑ conc = ↑ no of successful collisions = ↑ rate of rxn (only for reactions with gases)

sequence of steps in an observable reaction

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

same physical state as reactant e.g. breakdown of O3 catalysed by chlorine

catalyst is in different state to reactant e.g. Vanadium (V) oxide in contact process

• 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