18 Rates of Reactions

rate equation

rate = Δ[] / Δt

rate units

rate equation

overall order

finding units of rate constant k

continuous monitoring def

continuous measurements taken during the course of a reaction

• monitoring by gas collection

• monitoring by mass loss

• monitoring w a colorimeter

how does a colorimeter work

• the wavelength of the light passing through a coloured solution is controlled using a filter

• the amount of light absorbed by a solution is measured

what does the gradient of a []/t graph show

the rate of reaction

zero and first order on a []/t graph

second order on a []/t graph

half life def

time taken for half of a reactant to be used up

• first order reactions have a CONSTANT half life w the [] halving each time

exponential decay def (and which order is this)

• when the [] halves every half life

• first order

two ways of determining k from a first order rxn

• calculating rate constant from rate

• calculating rate constant from the half-life

how to calculate rate constant from rate

• draw tangents to curve at particular []

• gradient of tangent = rate

• rearrange rate eqn + sub in values of rate (gradient of tangent) and [] from where the tangent was drawn

how to calculate rate constant from half-life

orders of rxn from shape of rate/[] graph

initial rates method

• the initial rate is the instantaneous rate at the start of a rxn when t=0

• can be found by measuring grad of a tangent at t=0 on a []/t graph

• a clock rxn is a more convenient way of obtaining initial rate from a single measurement

• the time t from start of an experiment is measured for a visual change to be observed (a colour of ppt)

• initial rate proportional to 1/t

iodine clocks

• common clock rxn is formation of iodine

• starch usually added (blue-black colour)

rate determining step def

• in multi-step reactions, the steps take place at different rates

• the slowest step in the sequence is the RDS

effect of temp on k

as T increases, rate increases and value of k will also increase

• for many rxns, each 10°C rise doubles k and rate

how does temp increase affect rate

• increasing temp shifts Boltzmann distribution to right - proportion of particles that exceed Ea increases

• as temp increases, particles move faster and collide more frequently

• more particles colliding with sufficient Ea and correct orientation

Arrhenius equation

logarithmic form of arrhenius equation

how does the logarithmic form of arrhenius eqn relate to graphs

on a lnk and 1/T graph what does the gradient show

-Ea/R

on a lnk and 1/T graph what does the y-intercept show

lnA