Unit 25
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Transcript of Unit 25
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Unit 25
Burning of fuels and fire-fighting
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Heat content
Heat content – the total amount of energy stored in a substance.
Symbol for the heat content: H
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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
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Exothermic Reaction
heat content
reactants (CH4(g) + 3O2(g)
products (CO2(g) + 2H2O(l))
H is negative.
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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)
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Endothermic reaction
heat content
reactants
products
H is positive
CaCO3(s)
CaO(s) + CO2(g)
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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.)
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Fire Triangle
Necessary conditions for burning Fuel – combustible materials Oxidant – usually oxygen from air. Heat – a high enough temperature
to start the ignition (burning).
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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.
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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.
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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.
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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
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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
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Water type fire extinguisher
sulphuric acid container
sodium hydrogencarbonate solution
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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
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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.
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Foam fire extinguisher
Foam keeps the air out. Suitable for putting fire due to
flammable liquids. Not suitable for electrical fire.
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Carbon dioxide fire extinguisher
A denser gas than air, covers the fire and keeps the air out.
Used for metal fire and electrical fire.
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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.
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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
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Ionic equation for the soda acid fire extinguishers
CO32-
(aq) + 2H+(aq) → CO2(g) + H2O(l)
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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).
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Using LPG
In limited supply of air’ Carbon soot, carbon monoxide and
steam are formed. Keep the room well ventilated
(with windows open)
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In case of gas leakage, what should we do?
Read Chapter 25 P. 58
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Fire-fighting equipments in the laboratory
Carbon dioxide fire extinguisher Dry powder fire extinguisher A bucket of sand Fire blanket