The rate expression is an equation Determined experimentally Shows dependence of rate on concentrations

of reactants Rate is found whilst changing concentration

of a reactant

Rate Expression

Rate Expression

Order of Reaction

Experimental Data

Experimental Data

If doubling concentration has no effect then zero order (w.r.t. that reactant)

If doubling concentration doubles rate then first order

If doubling concentration quadruples rate then it is second order

What are orders for A, B, and C on page 171? Therfore what is rate expression?

Order of Reaction

Please read pages 170 to 173 Make any notes you need. Then try questions 1 to 7 from page 173.

Reading and Questions

Most reactions occur in a number of steps At each stage only two particles collide

successfully Intermediates therfore exist Slowest step is the rate determing step

Reaction Mechanism

Rate determing Step

Reaction Mechanism

Unimolecular step

Bimolecular

See page 176, figure 618

Relating mechanism to Rate Expression

Activation Energy

Maxwell-Boltzmann distribution

The Activation energy can be determined using an equation

The Arrhenius Equation This shows the effect of temperature on the

rate constant for a reaction

Arrhenius Equation

The Arrhenius Equation

The rate constant, k, depends exponentially on the temperature

This is why temperature has a large effect on reaction rate

Using logs and rearranging the equation…..

Arrhenius

Arrhenius equation converted to…

What is intercept ? Gradient ? Y and x axis ?

Compare to y=mx + c

Ea can be determined graphically….

So the activation energy can be found by measuring the rate at different temperatures, with all the other conditions unchanged and then plotting In(rate) against 1/T (in Kelvin)

Finding Ea