Chapter 18 - Rates of Reaction

rate of reaction

change in concentration of a reactant/product per unit time

equation = change in conc/time

units = mol dm^-3 s^-1

order of a reactant

the power to which the concentration of a reactant is raised by in rate equation e.g. [A]^x

-can only be 0, 1 or 2

rate α [A]^x [B]^y

zero order with respect to reactant

-changing the concentration of reactant has no effect on rate of reaction e.g. ‘as [A] is doubled, the rate of reaction remains constant’

rate α [A]⁰

first order with respect to reactant

-the rate of reaction is directly proportional to concentration of reactant e.g. ‘as [A] is doubled, the rate of reaction is doubled’

rate α [A]¹

second order with respect to reactant

-the rate of reaction will equal to concentration of reactant squared e.g. ‘as [A] is doubled, the rate of reaction is quadrupled’

rate α [A]²

rate equation + constant

rate = k [A]^x [B]^y [C]^z

k = constant that links rate of reaction with order of reactants

overall order

-sum of individual orders in reaction equation

e.g. x + y + z

calculating k and units

-rearrange rate equation to work out k

-work out units the same as Kc, mol dm^-3 s^-1

measuring rate of reactions 

-measuring pH changes in titrations

-measure volume of gaseous product with gas syringe

-monitor colour changes using colorimeter

half life t_1/2

the time taken for the concentration of a reactant to decrease by half

concentration-time graph for ZERO ORDER (draw graph)

has no effect on rate so also no effect on half life

concentration-time graph for FIRST ORDER (draw graph)

half life remains constant over time

equation for k using HALF LIFE

k = ln2 / t_1/2

plotting RATE-CONCENTRATION graphs

x-axis = concentration, y-axis = rate

gradient = rate constant k

clock reaction

-method to measure initial rate of reaction

initial rate α 1/t

examples = visual colour change, formation of ppt increases over time, time taken for ppt depends on conc of reactants

rate determining step

the slowest step in a reaction mechanism

-determines the rate as it is the slowest

how to work out rate determining step 

-given overall chemical equation

-then given rate equation - reactant in equation is the only species present in rate determining step

-first equation uses reactant and forms products then used in second equation 

-then cancel out species present on both sides to gain chemical equation 

example of rate determining step

CH₃Br + OH^- → CH₃OH + Br^-

rate = k [CH3Br]

1) CH₃Br → CH₃⁺ + Br^-

2) CH₃⁺ + OH → CH₃OH

then cancel out CH₃⁺

Arrhenius equation - taking logs

k = Ae^-Ea/RT

k = rate constant, Ea = activation energy, T = temp in Klevin, A = pre-exponential factor, R= gas constant,

ln k = (-Ea/R x 1/T) + ln A

y = m x + c - y = ln k, m = -Ea/R, x = 1/T, c = ln A