kinetics

reaction kinetics

• measuring rate of reaction and sequence of steps by which it occurs

• allows us to

  • determine how fast reaction takes place

  • determine conditions for optimum rate

  • propose mechanism

rate of reaction

• decrease in concentration of a reactant per unit time OR increase in the concentration of product per unit time

• rate (mol dm^-3 s^-1) = – 1/n d[reactant]/dt = 1/n d[product]/dt

  • regardless of which reagent is used to calculate, rate will be the same: -1/a d[A]/dt = -1/b d[B]/dt = 1/c d[C]/dt

• determined from gradient of tangent of concentration-time graph

  • initial rate: gradient at t=0, infinitely small amount of reactant used up → most useful, can know initial concs of reactants. usually greatest rate since highest conc → highest freq of effective collisions

  • instantaneous rate: gradient at particular time t OR differentiate based on eqn of curve from excel (more accurate)

  • average rate: change in concentration over a period of time, gradient from start to end point

which reagent to measure concentration in order to determine rate?

• if whole reaction is aqueous, water is solvent so is in large excess → cannot measure + change in conc is negligible, can ignore in pseudo rate eqn

experimental procedures for measuring rate

• continuous measurement: track progress of reaction with time → “how much __ over time” (d/dt)

  • conc over time → using titration with quenching [the only chemical method]

  • vol of gaseous product over time → using gas syringe

  • mass loss over time

  • colour intensity → using colorimeter

  • electrical conductivity → using conductivity meter

• discontinuous measurement

  • stopclock method

continuous method: titration with quenching

1. reactants of known conc and vol, start stop-watch at point of mixing

2. pipette oout samples of reaction mixture at regular time intervals

3. quench each sample: slow down/stop reaction

   • add large vol of cold solvent: dilutes reactants + reduces temp → slows down reaction

   • add negative catalyst/inhibitor → slows down reaction

   • add quenching reagent: immediately reacts with a reactant → stops reaction

4. titrate quenched sample with suitable reagent to determine conc of unreacted reactant/product formed at that time

• to find rate, plot vol against time

continuous method: gaseous product volume

• use gas syringe, measure volume at regular intervals

• to find rate wrt gas, plot vol of gas collected against time

• to find rate wrt acid, plot (V_{gas at end} – V_{gas after time t}) against time

(to add photo of setup)

continuous method: colour intensity

(to add photo of setup)

• using colorimeter

• beer-lambert law: colour intensity is proportional to concentration → higher intensity, higher absorbance

• can only use when there is one coloured substance

• to find rate, plot conc against time

continuous method: electrical conductivity

(to add photo of setup)

• using conductivity meter (conductance bridge) → measure resistance of solution

• changes because total no. of ions (mobile charge carriers) changes over time

• OR type of ion present changes over time even if total no. same. eg: heavier CH₃CHOO^- with lower conducting mobility replaces OH^- ions, conductivity of solution decreases

discontinuous method: stop clock method

• measure time taken for appearance/disappearance of a colour

• average rate ∝ 1/t₁+t₂+t₃

  • diff experiments, vary conc of reactants

• relative rate ∝ 1/t₁

1. beaker standing on piece of white paper with cross, look vertically down

2. at amount of ppt increases, cross fades → until completely obscured (take as time)

disadvantages

• only gives average rate and relative rate: cannot plot conc against time → cannot measure initial/instantaneous rate

• precision is subjective, based on perception

• larger % unc if time too short but not accurate if time too long

why is naked flame banned in coal mine?

• provides energy to start initial combustion of coal dust in air

• combustion will release energy to trigger subsequent reactions

• fine suspended particles with large surface area increases rate of reaction, subsequent combustions escalates into explosion

half life

time taken for concentration of reactant to decrease to half of its original value

• zero order: [A₀]/2k

• first order: ln2/k

• second order: 1/k[A₀]

integrated rate laws

• zero order: [A] = -kt + [A₀]

• first order: ln[A] = -kt + ln[A₀]

• second order: 1/[A] = kt+1/[A₀]

linearises eqn, able to find gradient and y intercept

finding order of reaction

1. inspection method: compare between data sets

2. substitution method: r₁/r₂ = (c₁/c₂)ⁿ where n is order of reaction

molecularity

no. of reactant particles involved in each elementary step
unimolecular, bimolecular, termolecular