1 Chapter 26Burning of Fuels and Fire Fighting 26.1Heat changes in chemical reactions 26.2Burning of...

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Chapter 26 Burning of Fuels and Fire Fighting

26.1 Heat changes in chemical reactions

26.2 Burning of fuels

26.3 Fire fighting

26.4 Safe use of fuels at home

26.5 Potential dangers associated with storage of fuels

CONTENTS OF CHAPTER 26

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26.1 HEAT CHANGES IN CHEMICAL

REACTIONS

EXOTHERMIC AND ENDOTHERMIC REACTIONS

An EXOTHERMIC REACTION is one that gives out heat.

An ENDOTHERMIC REACTION is one that takes in heat.

26.1 HEAT CHANGES IN CHEMICAL REACTIONS

Exothermic reactions

Most chemical reactions are exothermic. Here are some

examples:

(1) All combustion reactions. For example,

S(s) + O2(g) SO2(g) + heat

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Figure 26.1

A combustion reaction.

oxygen

burning sulphur


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(2) All precipitation reactions. For example,

Cu2+(aq) + 2OH–(aq) Cu(OH)2(s) + heat


Figure 26.2

A precipitation reaction.

copper(II) hydroxide precipitate

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(3) All displacement reactions. For example,

Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s) + heat

copper wire

silver nitrate solution

silver crystals

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(4) All acid-alkali neutralizations.

(5) Addition of a little water to an anhydrous salt. For example,

CuSO4(s) + 5H2O(l) CuSO4 • 5H2O(s) + heat

anhydrous copper(II) sulphate add a few

drops of water

Figure 26.4 Addition of a little water to an anhydrous salt (CuSO4).

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Endothermic reactions

Only a few reactions are endothermic. Two examples are given

here:


(6) Reaction between calcium oxide and water.

CaO(s) + H2O(l) Ca(OH)2(s) + heat

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(1) Cracking of oil fractions.

(2) Thermal decomposition of calcium carbonate.

CaCO3(s) + heat CaO(s) + CO2(g)

H NOTATION FOR HEAT CHANGE

The heat change during a reaction is the difference between the

total heat content of products (Hp) and that of the reactants (Hr).

That is:

H = Hp Hr

Heat change is measured in kilojoules (kJ).


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ENERGY LEVEL DIAGRAMS

Energy level diagram for an exothermic reaction

H has a negative value.

During an exothermic reaction, the reaction mixture becomes

hotter.


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reactants

energy given out, H is negative

products

exothermic reaction taking place

heat

heat

heat

heat

Ene

rgy

(hea

t co

nten

t)

Hr

Hp

(a) (b)

Figure 26.6

An exothermic reaction:

(a) Energy level diagram. (b) Heat is lost to the surroundings.


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Energy level diagram for an endothermic reaction

H has a positive value.

During an endothermic reaction, the reaction mixture becomes

colder.


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Figure 26.7

An endothermic reaction:

(a) Energy level diagram. (b) Heat is gained from the surroundings.

reactants

energy taken in, H is positive

products

heat

heat

heat

heat

Ene

rgy

(hea

t co

nten

t)

Hp

Hr

(a) (b)

endothermic reaction taking place

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THERMOCHEMICAL EQUATION AND HEAT OF

REACTION

2Mg(s) + O2(g) 2MgO(s) H = 1204 kJ mol1

A THERMOCHEMICAL EQUATION is an equation written

alongside with the H value.

HEAT OF REACTION (H) is the heat change, when the number

of moles of reactants represented by the equation of a reaction

react completely.


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A26.1

(a) Endothermic

(b) Exothermic

(c) Endothermic

(d) Exothermic

(e) Exothermic


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26.2 BURNING OF FUELS

FUELS TO PROVIDE HEAT ENERGY

Combustion of fossil fuels and hydrocarbons

Burning is also called combustion.


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Figure 26.10

Combustion of fuel is what

makes motor cars move. Petrol

is the fuel used — a mixture of it

and air is burnt in the car

engine.


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The fuels most commonly used today are fossil fuels. Most of

them consist of hydrocarbons.

A hydrocarbon (CxHy), when burnt completely in plenty of air,

forms carbon dioxide and water as the only products.

CxHy + (x + ) O2(g) xCO2(g) + H2O(l) H = z kJ mol1

On the other hand, when oxygen supply is poor, the

combustion of hydrocarbons would not be complete. Carbon

monoxide and carbon are formed at the same time.

y

4

y

2


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A26.2

(a) No. Electricity is an important source of energy, but not a

fuel. Electricity is a flow of electrons, not a substance that can

be burnt to give out heat.

(b) Yes. It can be burnt to give out heat.

2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(l)


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A26.3

(a) Charcoal, wood or gaseous fuel.

(b) Petrol or diesel oil.

(c) Kerosene, ethanol or methylated spirit.

FIRE AND FIRE TRIANGLE

A fire needs three conditions:

Fuel — anything that can burn (combustibles)

Oxidant — usually oxygen from the air


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Heat — to reach a temperature high enough to start the fire

(ignition temperature) and keep it going.

These three things make up the Fire Triangle.


The fire triangle.

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26.3 FIRE FIGHTING

FIRE FIGHTING AND FIRE TRIANGLE

To put out a fire, we have to remove one or more of the three

conditions in the Fire Triangle:

Cutting off the fuel (See Example A)

Cutting off the air supply (See Example B)

Removing the heat (See Example C)

26.3 FIRE FIGHTING

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Figure 26.14

Fire ruins lives and

properties!

26.3 FIRE FIGHTING

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WAYS OF PUTTING OUT FIRES

(A) Sometimes, it is best to remove the fuel, e.g. in case of a fire

due to gas leakage.

26.3 FIRE FIGHTING

Putting out fires by breaking the fire triangle – by removing fuel.

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Figure 26.16

A gas fire can be put out by

cutting off the fuel gas supply.

26.3 FIRE FIGHTING

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(B) Most small fires are best put out by cutting off the air supply

— for example, covering burning oil in a pan with its lid.

26.3 FIRE FIGHTING

Figure 26.17

Fire fighting using foam to

cut off the air supply.

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Figure 26.18

Fire fighting using water.

26.3 FIRE FIGHTING

(C) A big fire needs a lot of cooling before it can be put out. So

water from a fire hose is usually used to remove heat from a

house on fire.

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A26.4

(a) Heat

(b) Fuel

(c) Oxidant (air supply)

(d) Heat (also fuel – wax vapour)

FIRE EXTINGUISHERS

Common types of fire extinguisher are:

(1) Water-type fire extinguisher

26.3 FIRE FIGHTING

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Water is used to put out fires since it has a cooling effect.

Water-type fire extinguishers are used for fires caused by

solid combustibles (e.g. wood, paper, cloth). They cannot be used

on fires involving flammable liquids (e.g. petrol, oil) or electricity.

sulphuric acid container

sodium hydrogencarbonate solution

Figure 26.23

A soda-acid fire extinguisher.

26.3 FIRE FIGHTING

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(2) Foam fire extinguisher

The foam cuts off the air supply and cools the fire. It stays in place

long enough for natural cooling.

Foam fire extinguishers are often used for fires caused by

flammable liquids (e.g. kerosene). They cannot be used on

electrical fires because the foam contains water.

26.3 FIRE FIGHTING

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Figure 26.24

A foam fire extinguisher.

26.3 FIRE FIGHTING

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(3) Carbon dioxide fire extinguisher

Carbon dioxide is denser than air. It forms an invisible layer over

the fire to keep out air.

Carbon dioxide fire extinguishers can be used for most fires.

In particular, they are used for fires caused by electrical faults and

by flammable liquids.

26.3 FIRE FIGHTING

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Figure 26.26

A carbon dioxide fire

extinguisher.

26.3 FIRE FIGHTING

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(4) Halon fire extinguisher

In contact with burning substances, the halon liquid absorbs heat

and vaporizes. Thus halon has a cooling effect. Besides, the thick

vapour layer surrounds the fire to cut off the air supply.

Halon fire extinguishers can be used for most fires. They are

particularly effective for electrical fires and fires involving

flammable liquids.

26.3 FIRE FIGHTING

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Figure 26.27

A halon fire extinguisher.

26.3 FIRE FIGHTING

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Figure 26.29

BTM is used for fire fighting in MTR

stations.

26.3 FIRE FIGHTING

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(5) Dry powder fire extinguisher

The powder forms a surface layer on the burning material,

preventing air from reaching it. Powder-type fire extinguishers can

be used for all types of fires.

26.3 FIRE FIGHTING

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Figure 26.30 Figure 26.31

A powder-type fire A dry powder fire extinguisher in a MTR coach.

extinguisher.

26.3 FIRE FIGHTING

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26.3 FIRE FIGHTING

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Table 26.1 Characteristics of different types of fire extinguishers.

Activity 2

(a) Fire extinguishers (water, dry powder and/or carbon dioxide),

sand bucket and fire blanket.

(b) Dry powder fire extinguisher.

26.3 FIRE FIGHTING

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A26.5

(a) Carbon dioxide or dry powder fire extinguisher.

(b) Carbon dioxide or dry powder fire extinguisher.

(c) Water-type, foam, carbon dioxide or dry powder fire

extinguisher. (Water-type would be the best.)

(d) Water-type, foam, carbon dioxide or dry powder fire

extinguisher. (Water-type would be the best.)

(e) Foam, carbon dioxide or dry powder fire extinguisher.

(Avoid using BCF fire extinguishers whenever possible, since

BCF would destroy the Earth’s ozone layer.)

26.3 FIRE FIGHTING

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26.4 SAFE USE OF FUELS AT HOME


DANGERS ASSOCIATED WITH USE OF HOUSEHOLD

FUELS

In Hong Kong, the commonest household fuels are town gas and

LPG. Two potential dangers are associated with these gaseous

fuels:

Risk of carbon monoxide poisoning

Risk of fire and explosion

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Figure 26.33 Delivery of LPG needs special safety precautions.


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Carbon monoxide poisoning

Town gas contains carbon monoxide (about 3%). If there is gas

leakage, carbon monoxide will diffuse into air.

Besides, carbon monoxide is also produced whenever town

gas or LPG is burnt incompletely.

Carbon monoxide is a highly dangerous gas. It is toxic, yet

colourless and odourless.

Fire and explosion

If a gaseous fuel leaks, it forms a mixture with air. The mixture

can be dangerously explosive. A small flame or spark may ignite

the mixture, causing a fire or even an explosion.


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Figure 26.34

A newspaper cutting on a

gas blaze.


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PRECAUTIONS IN USING HOUSEHOLD FUELS

General precautions

Ensure that gas cookers are installed and regularly checked

by a qualified technician.

Check the rubber hose regularly.

Place gas cookers away from windows, so that wind will not

blow out the flame to cause gas leakage.

Do not go out of the house while a gas cooker is being used.

Open windows wide to ensure good ventilation.


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If you smell any gas or suspect of a leak, you must

turn off the gas meter control valve.

extinguish any naked flames nearby.

open windows and doors wide.

If the smell persists, there are things you must not do:

Figure 26.35 Things you must not do in case there is a gas leak at home.


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Gas water heaters

Gas water heaters installed in bathrooms are usually of the

following types:

(1) Balanced-flue water heater (Figure 26.36a).

(2) Fan-assisted water heater (Figure 26.36b).

(3) Open-flue water heater (Figure 26.36c).

(4) Flueless water heater (Figure 26.36d).


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(a) (b) (c) (d)

(safest) (safe) (safe) (unsafe)

Figure 26.36

Gas water heaters:

(a) Balanced-flue type (b) Fan-assisted type (c) Open-flue type (d) Flueless type.


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Figure 26.38

A warning label from the

Hong Kong and China Gas

Company.


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26.5 POTENTIAL DANGERS ASSOCIATED WITH STORAGE OF FUELS

26.5 POTENTIAL DANGERS ASSOCIATED

WITH STORAGE OF FUELS

There are potential dangers of storing fuels since they are

flammable.

Figure 26.39

Oil storage tanks on fire.

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STORAGE OF DIESEL OIL AND PETROL

The oil companies take very strict measures for fire prevention.

Moreover, newly-built oil storage tanks are located as far away

from residential areas as possible.

STORAGE OF LPG TANKS AT HOME

People should not store more than the legally permitted number of

LPG tanks at home. They must also take all necessary safety

precautions, e.g. storing the LPG tanks away from sunlight and

heat.


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Figure 26.40

Fuel storage tanks on Tsing Yi Island.

(a) Before (oil tanks adjacent to (b) Now (oil tanks and residential areas

residential areas) separated by a hill)


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SUMMARY

1. There are energy changes (usually heat changes) associated

with a chemical reaction. H is used to indicate the heat

change.

2. An exothermic reaction gives out heat; H is negative.

3. An endothermic reaction takes in heat; H is positive.

SUMMARY

4. The equation for the complete combustion of a hydrocarbon

CxHy in air is:

CxHy + (x + ) O2(g) xCO2(g) + H2O(l)

H = z kJ mol1

y

4

y

2

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SUMMARY

5. The three conditions for combustion are:

fuel

oxidant

heat

These make up the Fire Triangle.

6. Putting out a fire involves the removal of one or more

conditions for combustion.

7. Different types of fire extinguishers can be used for different

types of fires.

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SUMMARY

8. There are two potential dangers associated with the use of

household fuels:

Risk of carbon monoxide poisoning

Risk of fire and explosion

We should therefore use fuels safely.

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