Unit 25

Burning of fuels and fire-fighting

Heat content

Heat content – the total amount of energy stored in a substance.

Symbol for the heat content: H

Heat of reaction

Symbol : The heat of reaction is the difference i

n the heat content of the products and the heat content of the reactants.

= Hproducts - Hreactants

Exothermic Reaction

heat content

reactants (CH4(g) + 3O2(g)

products (CO2(g) + 2H2O(l))

H is negative.

Exothermic reaction

Heat is given out. The total heat content of products

is less than the total heat content of reactants.

Feeling warm (temperature increases)

Endothermic reaction

heat content

reactants

products

H is positive

CaCO3(s)

CaO(s) + CO2(g)

Endothermic reaction

Heat is absorbed. The total heat content of products

is more than the total heat content of reactants.

Feeling cold (heat is absorbed and temperature drops.)

Fire Triangle

Necessary conditions for burning Fuel – combustible materials Oxidant – usually oxygen from air. Heat – a high enough temperature

to start the ignition (burning).

Fire-fighting

Remove one or more conditions for putting put fire.

e.g., cooling down the fire by spraying cold water.

Excluding air (oxidant) by sand / fire-blanket.

Cut off the fuel supply.

Hill fire

Cut down a line of trees in front of the fire.

Dig a trench to separate the fire from other trees.

Removing fuels.

Fire in a cooking pan

Cover the fire with a lid. For excluding air. Don’t spray water onto the pan.

Why ? Oil does not mix water. Spraying

water helps to spread the fire.

Types of fire

Class A – burning ordinary materials such as paper, and wood.

Class B – Burning of flammable liquids. Class C – Burning of fiammable gases Class D – Burning metals

Fire extinguishers

4 types Water-type fire extinguisher

Water fire extinguisher / soda acid fire extinguisher

Foam type fire extinguisher Carbon dioxide fire extinguisher Dry powder fire extinguisher Haloform fire extinguisher

Water type fire extinguisher

sulphuric acid container

sodium hydrogencarbonate solution

Water type fire extinguisher

Producing a spray of cold water to lower the temperature of fire (to cool the fire)

Not for electrical fire For putting fire involving the

burning of papers and woods

2 types of water type fire extingusihers

Water fire extinguisher – forcing a jet of water out when a samll can of compressed carbon dioxide is broken.

Soda acid fire extinguisher – carbon dioxide is formed from the reaction of carbonate witrh dilute acid. The building up of pressure forces a jet of water out.

Foam fire extinguisher

Foam keeps the air out. Suitable for putting fire due to

flammable liquids. Not suitable for electrical fire.

Carbon dioxide fire extinguisher

A denser gas than air, covers the fire and keeps the air out.

Used for metal fire and electrical fire.

Dry powder fire extinguisher

The powder is solid sodium hydrogencarbonate.

Covers the fire and keeps the air out. Decomposes to solid sodium carbona

te and carbon dioxide which help to exclude the air.

For most fires, including electrical fire.

Haloform fire extinguisher

BCF ? BTM ? BCF : bromochlorodifluoromethane BTM: Bromotrifluoromethane Volatile and denser than air Forms a blanket over the fire. Keeps the air out and put out the fire. For electrical fire Poisonous vapours

Ionic equation for the soda acid fire extinguishers

CO32-

(aq) + 2H+(aq) → CO2(g) + H2O(l)

Safety precautions in using gaseous fuels

Why town gas is poisonous? Carbon monoxide, one of the component in

town gas, combines with the haemoglobin in red blood cell to form a stable complex – carboxyhaemoglobin which can no longer carries oxygen. Man will be then killed because of lack of oxygen.

To keep the room well ventilated (with windows open).

Using LPG

In limited supply of air’ Carbon soot, carbon monoxide and

steam are formed. Keep the room well ventilated

(with windows open)

In case of gas leakage, what should we do?

Read Chapter 25 P. 58

Fire-fighting equipments in the laboratory

Carbon dioxide fire extinguisher Dry powder fire extinguisher A bucket of sand Fire blanket