Topic 6: Chemical Kinetics Flashcards
Rate of rxn
the change in conc of reactants/products per unit time
unit: mol dm-3 s-1
rate of rxn equation
rate of rxn = Δ conc / time taken
- conc can either be product or reactant
technique used to measure change in pH
using pH probe and a meter
technique used to measure change in conductivity
conductivity probe and a meter
as no of ions increase, electrical conductivity increases… and vice versa
technique used to measure change in mass
- 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
technique used to measure change in vol of gas
- 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
technique to detect change in rxn
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
technique to measure change in colour
- 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
collision theory
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
factors affecting rate of rxn: temp
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
Maxwell-Boltzmann energy distribution curve
a plot of the no of particles with a given KE, plotted against KE
factors affecting rate of rxn: concentration
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
factors affecting rate of rxn: particle size/SA
smaller particle size = bigger surface area = higher chance of contact = higher no of successful collisions = more reactants with Ea = higher rate of rxn
factors affecting rate of rxn: catalyst
- 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
homogeneous catalyst
same physical state as reactant
e.g. breakdown of O3 catalysed by chlorine
heterogeneous catalyst
catalyst is in different state to reactant
e.g. Vanadium (V) oxide in contact process
factors affecting rate of rxn: pressure
higher pressure = higher conc = higher no of successful collisions = higher no of reactants achieving Ea = higher rate of rxn
only for reactions of gases
factors affecting rate of rxn
- temperature
- concentration
- particle size
- catalyst
- pressure
properties of reactants that affect rate of rxn
- collision geometry/orientation
- frequency of collisions
- kinetic energy
applications - interpreting volume-time graphs regarding rate of rxn
- 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
rate-determining step
- slowest step in the rxn
- name is self-explanatory :)
reaction intermediates
substances that are formed in 1 step and consumed in a later step (in the rxn)
reaction mechanism
sequence of elementary reactions that must occur to go from reactants to products
elementary reaction
- single-step reaction that has no intermediates or transition states
- all bond breaking/forming occurs in a single collision
how to identify a correct rxn mechanism (for MCQs)
- 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
rate constant unit
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
factors affecting rate constant
temperature
finding Ea from arrhenius plot
slope = - Ea / R
