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

using pH probe and a meter

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

• 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

• 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

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

• 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

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

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

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

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

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

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

higher pressure = higher conc = higher no of successful collisions = higher no of reactants achieving Ea = higher rate of rxn only for reactions of gases

• temperature • concentration • particle size • catalyst • pressure

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

• 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

• slowest step in the rxn - name is self-explanatory :)

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

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

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