ib chem 15/16 common questions

would confirmation of the rate expression prove its correct

• no because diff mechanisms can have the same rate expression

why does increase in temp increase rate of reaction (3)

• more frequent collisions per unit time

• increase kinetic energy

• greater proportion of molecules have energy greater than or equal to activation

would confirmation of the rate expression prove its correct

• no because diff mechanisms can have the same rate expression

why does increase in temp increase rate of reaction (3)

• more frequent collisions per unit time

• increase kinetic energy

• greater proportion of molecules have energy greater than or equal to activation

why does increasing particle size of solid reactant decrease rate

less surface area, $ of collisions decrease

2 situations where rate = constant

• zero order

• every conc is 1 mol

describe collision theory (2)

particles must collide at ccorrect orientation

particles must have sufficient energy to be greater or equal to Ea

how and why does rate change with time (3)

conc of rxts per unit volume decrease, making collision less frequent = decrease reaction rate

in some cases, increasing conc of reactant doesnt increase rate. how may this occur

reactant is not involved in the rate-determining step

how can the Ea be determined (3) graph

• determine the rxn rate at a range of temperatures

• calulate k for each temperature

• graph ln k with 1/t

in another experiment something was added while all conc remained unchanged. why did color change occur faster and how to confirm hypothesis (2)

• more rapid = catalyst is the thing

• see if the thing comes out at the end of the reaction

define activation energy Ea

• minimum energy requried for reactants to successfully collide/react/transitionstate

on the graph sketch line you would expect if catalyst is added (arrhenius graph)

any line is ok as long as line is less steep

how to determine the order of experiment

• do many experiments at same temperature but diff concs of reactant, plot conc/time graph and calculate initial slope of eachcurve at the start and then deduce relationship between rate and conc

intermediate vs transition state graph

guessing RDS steps

1. determine possible mechanism and see which one fits the rate equation

eg. a + b + c = abc, rate = k [a][b]

try: a + b = ab, then ab + c = abc

or try a + c = ac then ac + b = abc etc…

the first one is more likely with the first reaction as the RDS