Collision theory (surface area)

RATES OF REACTION HCCHEMISTRY

HCCHEMISTRY

calcium carbonate(marble)

hydrochloric acid

carbon dioxide

After completing this topic you should be able to :

FOLLOWING REACTIONS

RATES OF REACTION HCCHEMISTRY

HCCHEMISTRY

Importance of controlling reaction rate.

Learn how chemists control reaction rates by careful consideration of the influence of : surface area (particle size) concentration temperature pressure collision geometry

"If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in

the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon

being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a

little imagination and thinking are applied."Richard Feynman (Lectures in Physics)

Atomic theory

Collision theory is a theory proposed independently by Max Trautz in 1916 and William Lewis in 1918, that explains how chemical reactions occur and why reaction rates differ for different reactions

William Lewis British chemist (1885 - 1956) and Max Trautz German chemistry (1880 –1960)

Collision theory

Collision theory is sometimes summarized by the idiom: “You can’t react if you don’t collide.”

The collision must provide enoughenergy to break the bonds in the reactant molecules

Then new chemical bonds form to make product molecules.

Activation energy and collision geometry

When two billiard balls collide, they simply bounce off of each other.

This is also the most likely outcome if the reaction between A and B requires a significant disruption or rearrangement of the bonds between their atoms.

In order to effectively initiate a reaction, collisions must be sufficiently energetic (kinetic energy) to bring about this bond disruption.

This will be discussed in more detail later but for now all you need to know is:

Progress of a Reaction

time

Rate

C. Why does the graph curve?

A. Where is the reaction the quickest?

B. Why does the graph level off?

A

No more products formed.

The concentration of the reactants decrease with time.

Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products.

Factors that affect rate In your earlier studies you learned that 4 factors affect the rate of a reaction. List 4

ways to increase the rate of reactions:

• surface area of a solid reactant.• concentration or pressure of a reactant.• temperature.• presence/absence of a catalyst.

Increasing the pressure on a reaction involving reacting gases increases the rate of reaction.

Changing the pressure on a reaction which involves only solids or liquids has no effect on the rate.

Rates of reaction

The rate of reaction can be followed by measuring changes in

Concentration MassVolume of gas produced

Measuring reaction rates

Average rate of reaction =

change in mass of product or reactant time interval

Units g s-1

time (s)

Change in mass (g)

Products

Reactants

Measuring reaction rates

Average rate of reaction =

change in volume of product or reactantin time for the change to occur

Units cm3 s-1

time (s)

Change in volume (cm3)

Products

Reactants

Measuring reaction rates

Average rate of reaction =

change in concentration of product or reactant time interval

Units mol l-1 s-1

Time (s)

Change in concentration (mol l-1)

Products

Reactants

Effect of surface area

Particle size, the smaller the particles, the greater thesurface area, the greater the chance of successful collisions.

4X4= 16 cm2

16x6=96 cm2

2x2 = 4 cm2 24X8= 192 cm2

4X6= 24 cm2

Rate and Particle SizeOnly the particles on the surface of a solid can be involved in a collision

Crushing a solid increases the surface areamore particles are available for collision therefore increased rate of the reaction

Higher Chemistry Eric Alan and John Harris

Effect of surface area

Hydrochloric acid reacts with marble chips (calcium carbonate)

2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O (l)

How can we follow the reaction?

A gas is produced. What will happen to the gas if there is no lid on the container?

What will happen to the mass?

How can we follow the rate?

What to do

You are going to follow the rate of the reaction by

Measuring the volume of gases produced over time

Measuring the loss of mass over time

Measuring rate of reactionTwo common ways:

1) Measure how fast the products are formed

2) Measure how fast the reactants are used up

How can we follow the reaction?

• If we use a container fitted with a delivery tube we could measure the amount of gas produced. How?

What to do - Group 1

Measure 25 cm3 of 2 mol l-1 HCl into a conical flask

fitted with a stopper and a delivery tube

Set up an inverted measuring cylinder of water to collect the gas

Add 2g marble chips to the acid

Measure the volume of gas every 10 seconds

Repeat with 2g crushed marble chips

HOW TO CHANGE CONCENTRATION

To change the concentration of a solution (hydrochloric acid), the solution has to be mixed with different volumes of water.

HYDROCHLORIC ACID

2 mol l-1

The following shows how to dilute 2 mol l-1 hydrochloric acid to make 100 cm3 of lower concentrations.

HYDROCHLORICACID

1 mol l-1

HYDROCHLORICACID

0.5 mol l-1

HYDROCHLORICACID

1.5 mol l-1

50 cm3

2 mol l-1 acid+

50 cm3 water

25 cm3

2 mol l-1 acid+

75 cm3 water

75 cm3

2 mol l-1 acid+

25 cm3 water

wateracid

¾ concentration

½ concentration

¼ concentration

What to do - Group 1

Record your results in a table.

Plot a graph of volume vs time using the same axes for both sets of data

rate = change in volume ( the unit is cm3 s-1)

time interval

Calculate the rate for the 1st and 2nd 25 seconds for each set of results

What to do – Group 2

Weigh out 15 g marble chips

Measure 25 cm3 1 mol l-1 HCl into a conical flask

Place on balance and zero it

Add 2g marble chips to the boat. Now add it to the

acid and take mass readings every 10 seconds

Repeat using crushed chips

What to do – Group 2

Record your results in a the table.

Plot a graph of volume vs time using the same axes for both sets of data

rate = change in mass ( the unit is g s-1) time interval

Calculate the rate for the 1st and 2nd 25 seconds for each set of results

Swap results

Each group should have a sets of results which can be used to plot graphs.

time (s) 0 10 20 30 40 50 60 70 80

volume (cm3) C

volume(cm3) G

Method A resultsPlot the results on a graph with time on the x axis and volume on the y. Use the same set of axes for both sets of results.

0 14 26 36 44 50 50 50 500 22 40 48 50 50 50 50 50

Sample results

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

lumps

powder

Rate over 1st 25 seconds (cm3 s-1)

rate over 2nd 25seconds (cm3 s-1)

Whole chips (C)

32-025-0=1.3

50-3250-25=0.72

Ground chips (G)

45-025-0=1.8

50-4550-25=0.2

Work out the rate of reaction over the first 25 seconds and the second 25 seconds using the formularate = change in volume = _____________ cm3 s-1

time interval

Time (s)

Volume of gas cm3

Method B results• Plot the results on a graph with time on the x axis and

mass on the y. Use the same set of axes for both sets of results.

time (s) 0 20 40 60 80 100 120 140 160

Mass (g)C

mass(g)G

2.0 1.4 1.0 0.7 0.5 0.35 0.3 0.25 0.25

2.0 0.7 0.4 0.3 0.25 0.25 0.25 0.25 0.25

Sample results

Work out the rate of reaction over the first 25 seconds and the second 25 seconds using the formularate = change in mass The answer will have the units g s-1

time interval

Rate over 1st 25 seconds (g s-1)

rate over 2nd 25seconds (g s-1)

Whole chips (C)0.8-225-0=0.05

0.35 -0.850-25=0.018

Ground chips (G)

0.3-225-0=0.068

0.25-0.350-25=1x10-3

0 10 20 30 40 50 60 70 80 900

0.5

1

1.5

2

2.5

lumps

powder

Loss in mass (g)

Time (s)