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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 : 

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Unit 25

Unit 25

Burning of fuels and fire-fighting


Heat content
Heat content

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

  • Symbol for the heat content: H


Heat of reaction
Heat of reaction

  • Symbol : 

  • The heat of reaction is the difference in the heat content of the products and the heat content of the reactants.

  •  = Hproducts - Hreactants



Exothermic reaction1
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 reaction1
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
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
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
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
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
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
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


Water type fire extinguisher1
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
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 fire extinguisher

  • Foam keeps the air out.

  • Suitable for putting fire due to flammable liquids.

  • Not suitable for electrical fire.


Carbon dioxide fire extinguisher
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
Dry powder fire extinguisher

  • The powder is solid sodium hydrogencarbonate.

  • Covers the fire and keeps the air out.

  • Decomposes to solid sodium carbonate and carbon dioxide which help to exclude the air.

  • For most fires, including electrical fire.


Haloform fire extinguisher
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
Ionic equation for the soda acid fire extinguishers

  • CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)


Safety precautions in using gaseous fuels
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
Using LPG

  • In limited supply of air’

  • Carbon soot, carbon monoxide and steam are formed.

  • Keep the room well ventilated (with windows open)



Fire fighting equipments in the laboratory
Fire-fighting equipments in the laboratory

  • Carbon dioxide fire extinguisher

  • Dry powder fire extinguisher

  • A bucket of sand

  • Fire blanket


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