Revision: Year 12

5 Factors affecting the rate of reaction?

Collision theory*The Maxwell-Boltzmann Distribution

Thermodynamic and kinetic stability

Catalysts

HETEROGENEOUS CATALYST THEORY:

Homogeneous catalysis theory

1

16 How fast? - KINETICS II (AKA rates)

The rate of reaction is usually expressed as the

“rate of decrease of the concentration of one of the reactants”

(NB. The rate of increase of the conc. of the products will be the same or a simple multiple of this)

Consider the reaction:

A + B C + D

Rate = - small change in [A] = -d[A] small change in time dt

Units?

2

How do we measure this concentration? 1. Sampling of reacting solutions 2. Colorimetry (UV spectrophotometry) 3. Gas pressure changes or volume of gas evolved. 4. Mass of gas evolved

We keep T constant, and we plot the (converted ) concentrations against time.

The gradient gives the rate of reaction at a given point.

3

GCSE: What is rate dependent upon? Concentration of reactants (pressure) Temperature Catalyst Surface area of reactants

1. Sampling 100cm3 of methyl ethanoate (0.05M) +

100cm3 NaOH (0.05M)t = 0 min

t = 5 min20cm3 sample taken Quenched in 10cm3 0.05M HCl at 0ºC

Titrated with 0.05M NaOH Titre = v1 cm3

20cm3 sample taken Quenched in 10cm3 HCl

at 0ºC

20cm3 sample taken Quenched in 10cm3 HCl

at 0ºC

t = 10 min

t = 15 min

Titrated with 0.05M NaOH Titre = v2 cm3

Titrated with 0.05M NaOH Titre = v3 cm3

..etc

Write the equations for the reactions…

4

Show that the concentration of the NaOH or the ester at time t is given by:

[ester] = 0.0025 (10-v) mol dm-1

Sketch graph of [ester] against t

The rate at any point on the graph is given by the gradient (negative)

5

2. Measuring the volume of gas produced

Demonstration: Decomposition of H2O2

Constant temperature

Measured amount of MnO2

Measured amount of H2O2 , known concentration

Draw a diagram of the apparatus

Plot a graph of oxygen produced against time

Calculate the initial rate of reaction wrt. H2O2

How could we:

• Show the effect of temperature on the initial rate

• Show the effect of concentration?

• Show the effect of the catalyst on initial rate?6

3. Measuring the mass of gas produced

Design an experiment to show how the initial rate of reaction is affected by concentration:

Calcium Carbonate and Hydrochloric acid

Plan? Apparatus?

7

4. Colorimetry

Any investigation of the rate of a reaction requires a method to measure the concentration for a given time t.

Here, if a reactant or product is coloured, the intensity of the colour is proportional to it’s concentration. A colorimeter measures this.

Method:

1. The colorimeter must first be calibrated, i.e. known concentrations of the coloured substance in the reaction are used to convert the data into concentrations.

2. The reaction is carried out and a graph of the converted readings against time is produced. A datalogger can help in this respect.

Examples and questions

8

Text Book pp142-144

Other ways to measure rate…Describe very briefly how each of the following could be used to measure rate (p142 and Google!)

Electrical conductivity

Dilatometry

Chirality

Refractive index

The general rate equation, which we find EXPERIMENTALLY is…

Rate = k [A]m [B]n

• m is the ORDER of the reaction wrt. A

• n is the order wrt. B, etc

• Overall order = m + n + …

• m and n may be the same as the stoichiometric ratios but not necessarily!!

• k is called the rate constant (it is not actually a true constant as it varies with temperature and is affected by the use of a catalyst. Its units vary according to the expression, just as with Kc (see later)).

10

Consider the reaction…

A + B C + D

The general rate equation or expressionThese are definitions - no questions yet!)

Determining the order of reaction

Using initial rate

We repeat the reaction varying the concentration of the reactants.

Eg. A + B C + D

What effect does doubling [A] have?

Reaction rate doubles: Order = 1 wrt. A

Reaction rate cuadruples: Order = 2 wrt. A

Reaction rate unchanged: Order = 0 wrt. A

11

Reactions involving more than one stage

One of the stages of the reaction (the slowest) is the rate determining step of the reaction.

This explains why the order of a reaction cannot be determined by the equation for this reaction (and many others). The order depends on the number and nature of the molecules involved in the RDS.

Eg. A + B + C D + E

Step 1: A + B X Slow

(X is an intermediate, not TS)

Step 2: X + C D + E Fast

Here, rate = k [A] [B]

Order wrt C is zero. We have used kinetics to support one particular hypothesis about the reaction mechanism, i.e. it could be multi-stage. 12

Text Book pp145-147

Graphs and the order of reaction

Orders of reaction are found through experiment and reaction rate can be determined in 2 main ways: Continuous monitoring and Initial rate measurement

13

Continuous monitoring

We can use sampling to find out the the concentration of one of the reactants (or products) at time, t, and draw a concentration-time graph. The shape of the graph tells us about the kinetics involved.

We are trying to determine the order of reaction with respect to each of the reactants so that we can write the rate expression.

“Half life” and concentration

First order reactions: The concentration of the reactant takes the same time to fall to half of its value. This characteristic can be used to identify a first order reaction.

NB. Half life is independent of concentration – the shorter the half life, the faster the reaction!

14

Consider the following graph showing the rate of decomposition of Hydrogen Peroxide. The red lines show the tangents to the curves that could be used to calculate the initial rate and the rate at time t=10h

These are both examples of instantaneous reaction rates

Estimate the rate at t=10h and give the units.

The same procedure can be followed with volume-time graph…

We can use the initial rate of several distinct reaction mixtures of the same reaction to find the order of reaction and determine the rate expression and constant…

Finding the order from a Rate-Concentration Graph

We can measure the time, t, taken for a fixed amount of product to be made or reactant to be used up and this gives us a measure of the initial rate…

Rate 1 t

This is really a measure of the mean rate over time, t, and it works because we are usually woking with integer orders of reaction. If t gets too big, the method loses validity.

How could we obtain a straight line for a second order reaction? Text Book pp148-152

Rate equations and mechanisms: Propanone and Iodine

How could we follow the reaction rate of the following reaction?

20

You are going to use the sampling method in Core Practical 13a to follow the rate in this reaction.

We typically use a large concentration of both propanone and hydrogen ions in order to investigate the effect of the concentration of iodine on the reaction rate, and the same can be done for the other 2 reagents.

The rate equation turns out to be…

What does this tell us about the molecules involved in the RDS?

One possibility for mechanism is as follows…

Which fits nicely with the rate equation, but this is not considered to be the solution. Look at p155-6 of the text book and explain why.

Rate equations and mechanisms: Halogenoalkanes There are 2 possible routes for the hydrolysis of halogenoalkanes to take place:

22

SN1 versus SN2

Consider the following reactions…

They priced by 2 different reaction mechanisms which can be confirmed by experiment

Simultaneously, the nucleophile begins to form a covalent bond with the carbon atom attached to the halide, and the bond with the latter begins to break.

Rate = k [OH-] [R-X]

(first order wrt. OH- and first order wrt. R-X)

This RDS can be considered “bimolar” as there are 2 molecules involved

The transition state is when these two bonds are of equal strength (the peak of the energy diagram).

The reaction has 2 stages: 1. The C-X bond is broken forming an intermediate carbocation 2. The nucleophile combines with the carbocation

The first step is the slowest and is the RATE DETERMINING STEP The reaction is said to follow first order kinetics (overall order 1 and the RDS is unimolar)

i.e. Rate = k [halogenoalkane]

SN1 or SN2?

This depends on the stability of the carbocation formed:

Tertiary halogenoalkane are more likely to be SN1

Primary halogenoalkanes, SN2.

Secondary halogenoalkanes: both routes may be possible and the order of the reaction can be fractional.

The reaction mechanism is confirmed by experiment.

25

Text Book pp153-156

Catalysts

What is activation energy?

What is the transition state?

26

What types of catalysis exist?

Name the catalyst in the following reactions/processes and categorise them: - Contact process - Iodine and propanone - Depletion of ozone - Haber process - Decomposition of hydrogen peroxide - Oxidation of tartrate ions with hydrogen peroxide (Demo)

Where and how does reaction take place?

A

R

D

Heterogeneous Catalysis

What are “active sites”?

What happens if all of the active sites are occupied?

How is a catalyst poisoned?

Text Book pp157-159

28

How does temperature affect the rate constant?Firstly, why does temperature increase the rate of reaction? Read the first paragraph on p160 of the book and spot the mistake!

Notes:

Temperature, T in kelvin.

The gas constant, R (from the equation, pV=nRT)

Activation energy, Ea (the minimum energy required in a collision, expressed in joules per mole)

e = 2.71828 . . . a magic mathematical number (look on your calculator).

The frequency factor, A (sometimes called the pre-exponential factor or the steric factor - to do with frequency of collisions and the orientation required during a collision for reaction to take place. It is almost a constant). Don’t worry about this - it will always be given.

29

Can be transformed into…

These expressions can be used to calculate Ea for a give reaction by plotting a graph. How?

A historically useful generalization supported by Arrhenius' equation is that, for many common chemical reactions at room temperature, the reaction rate doubles for every 10 degree Celsius increase in temperature. Wikipedia

30Text Book pp160-163