5.1.1 Reaction rates

Rates of reaction (quantitative)

Zero order with respect to reactant

Changing conc. of reactant has no effect
on rate

First order with respect to reactant

Rate is directly proportional to conc. of reactant

Second order with respect to reactant

change in rate= change in conc. squared

How is overall order of reaction calculated?

Sum of individual orders

Units of k for a first order reaction

S-1

Units of k for a second order reaction

Mol-1 dm3 s-1

Units of k for a zero order reaction

Mol dm-3 s-1

How could you measure rate of reaction experimentally?

If there is a colour change: Use a colourimeter at suitable intervals
If gas is evolved: use a gas syringe to collect volume of gas
Measure change in mass of reaction mixture

Half life

The time taken for concentration of a reactant to decrease by half (t1/2)

Relationship between first order reactions and half life

First order reactants have constant half lives
(Draw on the graph)

Equation to determine rate constant in a first order reaction (using half life)

k= ln2/ half life

Rate determining step

The slowest step in a reaction with multiple steps

What affects the value of the rate constant?

Temperature only

Arrhenius equation

ln k= -Ea/RT + ln A

k= rate constant for reaction
A= pre-exponential factor (number of collisions between reactant molecules)
R= gas constant (8.314)
T= temperature in Kelvin
Ea= activation energy for reaction (in Joules)

The Arrhenius equation to plot a graph

Graph of lnk against 1/T is a straight line
Gradient= -Ea/R
Y intercept= lnA

Concentration time graph for a zero order reactant

Concentration time graph for a first order reactant

Rate concentration graph for a zero order reactant

Rate concentration graph for a first order reactant

Rate concentration graph for a second order reactant