Title: Lesson 4 Determining Reaction Order and Rate Equation

Learning Objectives:– Build on our understanding of reaction order to construct

rate equations.

– Analyse experimental data in order to determine a rate equation.

Main Menu

Lesson 6: Rate Equations

Objectives:

Build on our understanding of reaction order to construct rate equations.

Analyse experimental data in order to determine a rate equation.

Main Menu

What is a rate equation? A rate equation allows us to calculate the rate we would

expect for any concentration of reactants.

For example in the reaction:

A + B + C D + E

The rate equation is:

Rate = k[A]x[B]y[C]z

Where: [A], [B] an [C] are the concentrations of each reactant x, y, and z are the order of reaction with respect to each reactant k is the ‘rate constant’

Main Menu

The rate constant, k The rate constant k is essentially a measure of

how readily a reaction will take place: Higher k faster reaction Lower k slower reaction

k is dependent on temperature As temperature increases, so does k As temperature decreases, so does k

k is independent of concentration: Provided the temperature is fixed, k is always the

same

Main Menu

Units of k vary depending on the overall order of reaction

Main Menu

For example (from last lesson):

The reaction is 0th order w.r.t reactant A, and 1st order w.r.t reactant B

Therefore, the rate equation is: Rate = k[B]

The value of k is given by (using values for Experiment 1):k = Rate / [B]k = 1.25x10-2 / 1.00 = 1.25x10-2 s-1

Experiment Initial [A] ([A]0)

Initial [B] ([B]0)

Initial Rate (v0)

1 1.00 M 1.00 M 1.25 x 10-2 M/s

2 1.00 M 2.00 M 2.5 x 10-2 M/s

3 2.00 M 2.00 M 2.5 x 10-2 M/s

Main Menu

Another example (from last lesson):Experiment Initial [NO] /

mol dm–3

Initial [H2] /mol dm–3

Initial rate /mol (N2) dm–3

s–1

1 0.100 0.100 2.53×10–6

2 0.100 0.200 5.05×10–6

3 0.200 0.100 1.01×10–5

4 0.300 0.100 2.28×10–5

The reaction is 1st order w.r.t [H2], and second order w.r.t [NO]

Therefore, the rate equation is: Rate = k[H2][NO]2

The value of k is given by (using values for Experiment 2):k = Rate / [H2][NO]2

k = 5.05x10-6 / (0.200x0.1002) = 2.53x10-3 mol-2 dm6 s-1

Main Menu

Main Menu

Solutions

Main Menu

Main Menu

Rate Equations in Practice In this activity you will need to determine the rate

equation for the following reaction

You will use real-life data collected from the lab, which will be messy and lumpy in way that the data in exams won’t be.

Follow the instructions here

You will need to download the spreadsheet ‘Iodination of Propanone’ from the blog

Main Menu

Review

Rate = k[A]x[B]y[C]z

To determine k, divide the rate by the concentration of reactants.

To determine the units of k, sub the rate and concentration units into the above, and cancel units to express in their lowest terms.