7 reaction kinetics

rate of reaction

that of the change of concentration of reactant/product per unit time

rate eqn

experimental relationship b/w the reaction rate and conc of reactants

order of reaction

power of reactant’s concentration terms in the experimentally determined rate eqn

rate constant

proportionality constant in the experimentally determined rate eqnfir

reaction orders

zero order

• ror is independent of [A]

first order

• half life: time taken for conc of reactant to be halved

  • ln2/k

  • fraction of reactant remaining = (1/2)ⁿ

second order

• doubling [A] will quadruple the rate

• half life gets longer

pseudo order

• when [A] is in large excess, it is regarded as a const

reaction mechanism

slowest reaction is the rate-determining eqn

• rmb to substitute the intermediates as they cannot appear in rate qn

collision theory

reactant particles must collide effectively before reacting

• must collide with activation energy

• appropriate collision geometry (minimise steric repulsion)

transition state theory

• particles must collide with Ea and appropriate geometry to form TS, which decomposes back into reactant

transition states & Ea

• bonds are broken & formed in TS

• to get to TS, particles must possess Ea to:

  • weaken bonds in reactant particles

  • overcome steric repulsion between particles as they approach eo

  • oritentate themselves in the appropriate orientation

• high Ea → slower rxn

thermodynamic & kinetic stability

• thermo

  • endo: thermodynamically unstable and not feasible

  • exo: thermodynamically stable + feasible

• kinetic:

  • high Ea: slow rxn, kinetically stable

  • low Ea: fast rxn, kinetically unstable

intermediates

• energy minimum with energy level that is higher than both reactant & product

how conc of reactant particles affect ror

conc inc, rate inc

• no. of reactant particles per unit vol inc, freq of collision inc, freq of eff collision inc

how surface area of reactant particles affect ror

sa inc, rate in

• when reactant particles are in finly divided state, there is larger SA of contact b/w reactant particles, freq of collision inc, freq of eff collision inc

how temp affect ror

temp inc, rate inc

• avg KE of particles inc, freq of collision inc

• more particles possess energy =/> Ea, freq of eff collision inc

how catalysts affect ror

catalyst: substance that increases rate of chemical rxn but remain chemically unchanged at end of reaction

characteristics:

• provides alternative rxn pathway with lower Ea

• specific (only affects one reaction)

• doesn’t affect yield

• regenerated at end of rxn

homogenous catalyst

• reactant and catalyst are in same phase

  • same physical state

• forms intermediate w reactant, which forms product and regenerates catalyst

  • catalyst takes place in rate-determining step

heterogenous catalyst

• not in same phase

• adsorption theory:

  • adsorption: reactant particles form weak ond to active site on catalyst surface (this ensures weak bonds are formed within reactant particles, making it more reactive)

  • reaction: reactant particles are held onto catalyst surface in cloxe proximity & in correct orientation so they can readily react

  • desorption: weak bonds b/w catalyst surface and reactant particles are broken (products diffuse away from catalyst surface and active site is available agn)

autocatalysis

• product catalyses rxn

  • initial rate is low

  • as rxn progress, [product] inc → since pd catalyses rxn, rate inc

  • rate dec when [reactant] dec as there isn’t enough reactant alr

enzymes

• enzyme specificity → active site must match shape of substrate molecule (complemetary)

• pH sensitivity → each enzyme has its own optimum pH

  • changes in acidity/alkalinity will affect 3D structure of active site, affects its ability to bind w substrate molecule

• temp sensitivity → too high/low temp will denature/deactivate enzyme

  • too high: disrupt idid/pdpd/hydrogen bonding in structure of enzyme, affects 3D structure of enzyme

  • too low: lower enzymatic activity, lower freq of collision

determining ror (continuous/discontinuos methods)

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