18.1 Orders, rate equations, and rate constants

define rate of reaction

change in concentration of a reactant or product per unit of time

formula for calculating rate of reaction at a given instant

rate = change in concentration / time

units of rate

mol dm^-3s^-1

define zero order

when the concentration of a reactant has no effect on the rate

define first order

the rate depends on its concentration raised to the power if 1

define second order

the rate depends on its concentration raised to the power of 2

standard rate equation

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

symbol for rate constant

k

formula for overall order

sum of orders in respect to each reactant

shape of concentration-time graph of a zero order reaction

straight line with a negative gradient

shape of concentration-time graph of a first order reaction

downward curve with a decrreasing gradient over time

constant half-life

shape of concentration-time graph of a second order reaction

downward curve that is steeper at the start but tails off more slowly

define half life t_1/2

the time taken for half of a reactant to be used up

how to calculate the rate constant from the rate

draw a tangent on the concentration-time graph at a particular concentration

calculate the gradient of the tangent

rearrange the rate equation so k =

how to calculate rate constant from the half-life

k = ln2 / t_1/2

shape of rate-concentration graph of a zero order reactant

horizontal straight-line with zero gradient

incerpet on y-axis = k

shape of rate-concentration graph of a first order reactant

straight-line graph through (0,0)

rate is directly proportional to concentration

k = gradient

shape of rate-concentration graph of a second order reactant

upward curve with increasing gradient

by plotting a second graph of the rate against the concnetration², a graph with a straight line through the origin is produced

gradient of straight line graph = k

define initial rate

the instantaneous rate at the start of a reaction when time = 0

describe iodine clock reaction

starch is added

colourless to blue once iodine is formed

H₂O₂ (aq) + 2I^- (aq) + 2H⁺ → I₂ (aq) + 2H₂O (l)

2S₂O₃^2- (aq) + I₂ (aq) → S₄O₆^2- (aq) + 2I^- (aq)