5A- Rate equation

what is the order of reaction

the power to which the concentration of a reactant is raised in the rate equation

what is the overall order

the sum of the individual orders of reactants in the rate equation

what is the rate equation

for mA + nB → products, rate= k [A]^m [B]ⁿ

what is the rate constant

k, the constant that links rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation

when order is 0…

concentration of reactant changing doesn’t effect rate of reaction

when order is 1 (1st order)…

rate is directly proportional to concentration, doubling concentration doubles rates

when order is 2 (2nd order)

if concentration triples then rate of reaction increases by factor 3² 

what is the time concentration graph of a zero order reactant

what is the time concentration graph of a first order reactant

what is the time concentration graph of a second order reactant

what is the concentration rate graph of a zero order reactant

what is the concentration rate graph of a first order reactant

what is the concentration rate graph of a second order reactant

what is the graph of a time concentration of product graph

what order is this graph

second order (reactant concentration is squared)

define initial rate

the instantaneous rate at the start of the reaction

when writing exam answers on finding order from initial rates how should it be structured

• say which experiments you are comparing

• how concentration of each reactant changes

• how rate changes

• hence determine order of reaction wrt to the reactant

why when order of reaction for a reactant is investigated using the initial rates method do the concentrations of the other reactants not need to be in large excess

as the rate is only being measured over a very short period of time the change in concentration of the other reactants will not have a significant impact on rate

why are initial rates used to measure rate

concentration of all reactants are known

how can you determine order when you have the concentration of a reactant in time intervals

• plot graph of conc. vs time

• find rate at different concentrations by finding gradients of tangents

• plot graph of rate vs conc.

• observe the shape

define half life

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

what happens to the half life of a zero order reaction as reactant concentration falls

it decreases

what happens to the half life of a first order reaction as reactant concentration falls

it remains constant

what happens to the half life of a second order reaction as reactant concentration falls

it increases

what is the equation for 1st order reactions with half life

half life= ln2/k

what happens to rate constant when temperature increases

rate constant increases

what equation describes how value of rate constant changes with temperature

arrhenius equation

how can k be found using half lives

k= ln2/half life

how can rate constant be found from a zero order rate vs conc. graph

y-intercept

how can rate constant be found from a first order rate vs conc. graph

gradient of line

how can rate constant be found from a second order rate vs conc. graph

it can’t be directly found, you could rearrange rate equation or plot rate against conc.²

how can you find k in a second order reaction with a conc. vs time graph

find rate at particular concentration using a tangent, substitute values into rate equation

what are ways of monitoring changes in concentration

colorimeter, gas syringe, balance, conductivity probe (continuous monitoring methods)

how do you monitor changes in concentration where there are ions in a re action being used up/produced

use a conductivity probe

what equation is initial rate proportional to

1/time

what is the clock method of monitoring rates

when a significant change is suddenly detected after a little delay, the only variable being measured is time

what are example of clock reactions

sulfur disappearing cross, iodine clock

what is the iodine clock reaction

add a fixed amount of an extra reactant, thiosulfate, to allow the reaction to be monitored as a clock. once reaction begins the additional thiosulfate reacts away any iodine produced leaving a colourless solution, when thiosulfate runs out the iodine build up and the starch indicator turns blue-black,

what are the issues with clock reactions

1/time is not the same as initial rate, initial rate is instantaneous rate at t=0s, in a clock reaction 1/time is just the average rate over the time needed to form a specific amount of product

quantities need to be chosen carefully

what are the pros of clock methods

quick and simple to carry out, easy to process results

what are the pros of the continuous monitoring methods

more accurate

what are the cons of the continuous monitoring methods

more work to monitor several reactions over time, processing of results takes longer

when is using a balance not suitable for monitoring rates

when the gas has a low Mr e.g. H2

define rate determining step

the slowest step in the reaction mechanism of a multi-step reaction

how does the rate determining step link to the rate equation

the rate equation tells us which species react together in the rate determining step (or reactants that make intermediates used in the rate determining step) and in what numbers

how can you tell if a reaction has multiple steps

in a single step it is highly unlikely that more than 2 particles can collide together to react