How fast? rate of reaction

rate of reaction

speed at which a chemical reaction takes place, can be expressed as change in conc of a particular reactant/product per unit time

calculating rate of reaction (mol dm^-3 s^-1 )

change in conc of reactants or products / time

3 techniques of measuring rate

• mass loss

• gas production

• colorimetry

4 factors of collision theory

• for a reaction between 2 particles to occur, the particles must collide with:

  • the appropriate collision geometry

  • sufficient kinetic energy

• the minimum amount of energy required = the activation energy

factors affecting rate of reaction

• concentration

• pressure

• temperature

• SA

• catalysts

concentration affecting rate

• an increase in conc causes an increase in number of particles per unit volume

• this causes increase in collision frequency, hence increased frequency of successful collisions

pressure affecting rate

• increase in pressure causes less space for particles to move in

• number of successful collisions increases due to increased collision frequency

temperature affecting rate

• increase in temp cause particles to move faster as they gain KE, so collide more frequently

• at higher temp, higher proportion of particles have the activation energy, meaning higher proportion of collisions are successful

surface area affecting rate

• increase in surface area means more particles are on the surface and able to collide with particles of reactant

  • means more collisions in total meaning more successful collisions

catalyst affecting rate

• catalyst provides reactants with alternative reaction pathway which is lower in activation energy

• more collisions are successful

• catalyst remains unchanged

activation energy

• minimum energy colliding particles need to collide successfully leading to a reaction

homogenous & heterogenous catalyst

• homo: catalyst is in the same phase as reactant

• hetero: catalyst is in a different phase to the reactant

energy profiles of catalyst reactions

effect of temp on maxwell boltzmann

• when temp increases, curve flattens and the peak shifts to right

• because higher proportion of successful collisions

rate equation

• Rate of reaction = k [A]^m [B]ⁿ

• A and B are concs of reatants

• catalysts and products may feature

order of reaction

• order shows how conc affects the rate

  • the power to which the conc of the reactant is raised in the rate eqution

• zero order

  • changing conc has no effect

• first order

  • conc of reactant is directly proportional to rate

• second order

  • rate is directly proportional to the square of conc of reactant

determining rate equation from data

• find 2 experiments where conc of 1 reactant changes but concs of others are constant

• calculate what happens to concentration and rate

• deduce order

reaction orders from conc-time graphs

• zero order

  • as time goes on, conc decreases, graph is straight line

  • rate = k

• first order

  • conc decreases with time, graph is a curve which plateaus

• second order

  • conc decreases more steeply with time, graph is steeper curve which plateaus

reaction orders from rate-conc graphs

• zero order

  • rate remains constant, graph is horizontal line

  • rate = k

• first order

  • rate increases as conc increases,

  • graph is straight line

  • rate = k(A)

• second order

  • rate increases more as conc increases, graph is curved line

  • rate = k(A)²

calculating the rate constant and its units

• zero order: mol dm^-3 s^-1

effect of temp on rate constant

• increasing temp increases value of rate constant as concentration stays same

• this is because as rate of reaction increases, rate constant increases

• relationship is not linear though

reaction mechanisms

• each step in reaction is called elementary step, involves small number of particles

• some products of an elementary step exist as intermediates and react in subsequent steps

• sum of elementary steps must equal overall reaction equation

  • intermediates cancel out if appear on both sides

rate determining step

• the slowest step in the reaction

• if reactant appears in rate determining step, conc of the reactant appears in rate equation

• the order with respect to a reactant is the number of particles of that reactant that take part in rate-determining step

energy profiles in single step reaction

• when reacting molecules collide, with bond breaking/formation occurring, they will be in an unstable, high-energy state temporarily

  • transition state: higher energy state than reactants/products, corresponds to AE

energy profiles in multi step reactions

• the rate determining step is the step with greatest activation energy

molecularity

• number of reacting particles taking part in an elementary step

  • unimolecular: one reactant particle involved

  • bimolecular: two reactant particles involved

  • termolecular: three reactant particle involved

• usually discussed in relation to the rate determining step

arrhenius equation

• k = rate constant

• A = arrhenius factor (nature of reactants)

• Ea= activation energy J mol-1

• R= gas constant

• T= temperature K

  • k and T are the only variables

  • used to describe reactions involving gases, occurring in solution, or reactions on surface of catalyst

using the arrhenius equation

determining Ea and the arrhenius factor

• once k at different temperatures for a reaction has been determined, can be used to determine Ea and A

• graph of ln k against 1/T can be plotted

  • gradient: -Ea / R

  • y intercept: ln A

• Ea = -gradient x R

• A = e^y-intercept