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8. Kinetics

8.1a Collision Theory

Learning Objectives

1.Understand that reactions can only occur when collisions take place between particles having sufficient energy

2.Define the term “activation energy”

3.Understand that most reactions do not lead to a reaction

What does rate of reaction mean?

The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

What is the rate of these reactions?

The speed of a reaction is called the rate of the reaction.

rusting baking explosion

slow fast very fast

Rates of reaction

Why are some reactions faster than others?

Reactions, particles and collisions

Reactions take place when particles collide with a certain amount of energy.

The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction.

The rate of a reaction depends on two things:

the frequency of collisions between particles

the energy with which particles collide.

If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

Not all collisions result in a reaction

1. Particles may not collide with enough energy (activation energy)

2. Particles may not collide with the correct orientation.

• Collision Theory

Not all collisions result in a reaction

Review

• The rate of reaction depends on what two factors?

• Describe collision theory.

• Describe the term “activation energy”.

• Why is it that most collisions do not result in a reaction?

What is a chemical reaction?

A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products).

reactants products

For example:

carbon carbon dioxideoxygen

+

+

Chemical Energy

• All molecules store chemical energy in the chemical bonds.

• Molecules held together by bonds takes energy to break

carbon carbon dioxideoxygen

+

+

What is a chemical reaction?

Usually two parts to a chemical reaction:1.Breaking of chemical bonds.2.Formation of new bonds.

carbon carbon dioxideoxygen

+

+

Energy

Usually two parts to a chemical reaction:1.Breaking of chemical bonds Takes Energy2.Formation of new bonds Give Out Energy

carbon carbon dioxideoxygen

+

+

Endothermic vs. Exothermic

• Endothermic = requires energy, process absorbs energy

• Exothermic = gives out energy, process releases energy

(think exo like exit)

Making and breaking chemical bonds

Most chemicals will break up (decompose) if they are heated strongly enough. This means that energy is needed to break chemical bonds – an endothermic process.

energy

absorbed

Because bond-breaking is endothermic, bond-making must therefore be exothermic. This means that energy is released when chemical bonds are made.

energy

released

Activation Energy

• Activation energy = energy needed to start the reaction (break bonds)

• High activation energy = need to put in a LOT of energy (strong bonds)

• Low activation energy = little energy needed, weak bonds

Enthalpy Profile Diagram

Review

• What are the two steps in a chemical reaction?

• Which absorbs energy? Which releases energy?

• What do the terms “Exothermic” and “Endothermic” mean?

Overall Reaction

Endothermic Reactions Energy absorbed (activation) > Energy released

Overall energy absorbed (feels cold)

Exothermic Reactions

Energy absorbed (activation) < Energy released Overall energy released (feels hot)

1.An exothermic reaction is one which releases heat energy to the surroundings

2.The temperature of the surroundings increases

3.The energy released from forming new bonds is greater than the energy needed to break old bonds

Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions

Exothermic reactions

Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions

Exothermic reactions

Exothermic reactionsExothermic reactions release thermal energy (heat) into their surroundings. Exothermic reactions can occur spontaneously and some are explosive.

What are some examples?

combustion

respiration

neutralization of acids with alkalis

reactions of metals with acids

the Thermit Process.

1.An endothermic reaction is on which takes in heat energy from the surroundings

2.The temperature of the surroundings decreases

3.The energy needed to break old bonds is greater than the energy released from forming new bonds

Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions

Endothermic reactions

Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions

Endothermic reactions

Endothermic reactions absorb thermal energy, and so cause a decrease in temperature.

thermal decomposition, e.g. calcium carbonate in a blast furnace

photosynthesis

some types of electrolysis

sherbet!

Endothermic reactions

What are some examples?

Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions

Exothermic reactions

Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions

Endothermic reactions

8.1b Rates of Reaction

Learning Objectives

1.Understand the effect of temperature on rate of reaction.

2.Understand the effect of concentration on rate of reaction.

Changing the rate of reactions

increased temperature

increased concentration of dissolved reactants, and increased pressure of gaseous reactants

increased surface area of solid reactants

use of a catalyst.

Anything that increases the number of successful collisions between reactant particles will speed up a reaction.

What factors affect the rate of reactions?

Temperature and collisions

How does temperature affect the rate of particle collision?

Effect of temperature on rate

Higher temperature = higher rate of reaction

Why does increased temperature increase the rate of reaction?

•higher temperature = more energy•more energy = faster particles•faster particles = more frequent collisions more successful collisions = more reactions.Also...more total energy = more collision with enough Ea (more forceful collisions.

Temperature and particle collisions

Temperature and batteries

Why are batteries more likely to rundown more quickly in cold weather?

At low temperatures the reaction that generates the electric current proceeds more slowly than at higher temperatures.

This means batteries are less likely to deliver enough current to meet demand.

How does temperature affect rate?

The reaction between sodium thiosulfate and hydrochloric acid produces sulfur.

Sulfur is solid and so it turns the solution cloudy.

How can this fact be used to measure the effect of temperature on rate of reaction?

hydrochloricacid

sodiumchloride

sulfursodium

thiosulfate + + watersulfur

dioxide ++

Na2S2O3

(aq)2HCl(aq)

2NaCl(aq)

S(s)++

SO2

(g)H2O(l)

+ +

The effect of temperature on rate

Effect of concentration on rate of reaction

higher the concentration = faster rate of a reaction

Why does increased concentration increase the rate of reaction?

• higher concentration = more particles • more likely to collide more likely to react.

higher concentrationlower concentration

Concentration and particle collisions

The effect of concentration on rate

Effect of pressure on rate of reaction

The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

As the pressure increases, the space in which the gas particles are moving becomes smaller.

lower pressure higher pressure

Effect of surface area on rate of reaction

Any reaction involving a solid can only take place at the surface of the solid.

If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?

smaller the pieces = larger the surface area = more collisions greater chance of reaction.

larger SA = larger area for the reactant particles to collide with = faster rate of reaction

low surface area high surface area

Surface area and particle collisions

Reaction between a carbonate and acid

Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide.

The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.

hydrochloricacid

calciumchloride

calciumcarbonate + + water + carbon

dioxide

CaCO3 (aq)

2HCl(aq)

CaCl2

(aq)++

H2O(aq)

+ CO2

(g)

The effect of surface area on rate

8.2 Maxwell-Boltzman distribution

Learning Objectives

1.Have a qualitative understanding of the Maxwell-Boltzman distribution of molecular energies in gases.

2.Be able to draw and interpret distribution curves for different temperatures.

Maxwell-Boltzman distribution curve

Effect of temperature

8.3 Catalysts

Learning Objectives

1.Know the meaning of the term catalyst.

2.Understand how catalysts work to increase the rate of reaction.

reaction (time)

ener

gy

(kJ)

What are catalysts?

Catalysts are substances that change the rate of a reaction without being used up in the reaction.

Catalysts never produce more product – they just produce the same amount more quickly.

Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea).

Ea withcatalyst

Ea withoutcatalyst

Effect of catalyst

How does a catalyst speed of a reaction?

• Catalysts provide an alternate reaction pathway.

• They lower the activation energy required to start the reaction.

• Catalysts are reformed at the end of the reaction, they are not used up so are not considered reactants.

Everyday catalysts

Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).

Many catalysts are transition metals or their compounds. For example:

Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.

Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).

Catalysts in industry

Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes.

Why are catalysts so important for industry?

Products can be made more quickly, saving time and money.

Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.

Collisions and reactions: summary

Glossary

activation energy – The amount of energy needed to start a reaction.

catalyst – A substance that increases the rate of a chemical reaction without being used up.

concentration – The number of molecules of a substance in a given volume.

enzyme – A biological catalyst.

rate of reaction – The change in the concentration over a certain period of time.

Anagrams

Rates of reaction: summary

Multiple-choice quiz

Kinetics Quiz1 Define the term activation energy.

2 Draw, with labelled axes the Maxwell-Boatman distribution. Label this T1. Draw on the same axes a curve to represent same sample at lower temperature.

3 Add a label for activation energy.

4 Draw the effect of adding a catalyst.

5 Explain why most collisions do not result in a reaction.

6 Describe three ways to increase the rate of reaction and explain why using ideas about collision theory.