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
pH probe and meter
technique used to measure change in conductivity
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)
technique used to measure change in mass
- 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
technique used to measure change in vol of gas
- 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
technique to detect change in rxn
stopwatch
- i.e. using something observable as an arbitrary endpoint for when to stop the watch
technique to measure change in colour
- 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
technique to measure change in concentration
- 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
non-continuous method to measure rate of reaction
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
collision theory
- 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
Maxwell-Boltzmann energy distribution curve
- 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
factors affecting collision success
- energy of reactants (KE)
- orientation/geometry of collision
- frequency of collisions
factors affecting rate of rxn
- temp
- pressure
- catalyst
- particle size
- conc.
factors affecting rate of rxn: temp
↑ temp = ↑ KE = ↑ no. of successful collisions = ↑ reactants with energy ≥ Ea = ↑ rate of rxn
many rxn speeds double for every 10°C/K increase
factors affecting rate of rxn: conc
↑ 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
factors affecting rate of rxn: particle size
↓ particle size = ↑ SA = ↑ chance of contact = ↑ no of successful collisions = ↑ rate of rxn
factors affecting rate of rxn: catalyst
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
factors affecting rate of rxn: pressure
↑ pressure = ↑ conc = ↑ no of successful collisions = ↑ rate of rxn
(only for reactions with gases)
reaction mechanism
sequence of steps in an observable reaction
how is rate expression derived for a 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
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
applications - interpreting volume-time graphs for 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 constant unit
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
special feature of first order reactions
- constant half-life
- time taken for reactant conc to decrease to half the original value is not dependent on its starting conc
finding Ea from arrhenius plot
slope = - Ea / R
Writing rate expressions with consideration to reaction mechanism
- trick question: when writing rate expressions, DO NOT CONSIDER REACTION MECHANISM
- only look at the original reaction equation
