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 decreasing 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)

describe two factors that contribute to the increased rate and rate constant when temperature increases

1) Boltzmann distribution shifts to the right, increasing the proportion of particles that exceed the activation energy (main factor)

2) particles move faster and collide more frequently