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Unit 25

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

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  1. Unit 25 Burning of fuels and fire-fighting

  2. Heat content • Heat content – the total amount of energy stored in a substance. • Symbol for the heat content: H

  3. 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

  4. Exothermic Reaction

  5. 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)

  6. Endothermic reaction

  7. 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.)

  8. Fire Triangle • Necessary conditions for burning • Fuel – combustible materials • Oxidant – usually oxygen from air. • Heat – a high enough temperature to start the ignition (burning).

  9. 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.

  10. 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.

  11. 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.

  12. 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

  13. 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

  14. sulphuric acid container sodium hydrogencarbonate solution Water type fire extinguisher

  15. 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

  16. 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.

  17. Foam fire extinguisher • Foam keeps the air out. • Suitable for putting fire due to flammable liquids. • Not suitable for electrical fire.

  18. Carbon dioxide fire extinguisher • A denser gas than air, covers the fire and keeps the air out. • Used for metal fire and electrical fire.

  19. 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.

  20. 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

  21. Ionic equation for the soda acid fire extinguishers • CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)

  22. 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).

  23. Using LPG • In limited supply of air’ • Carbon soot, carbon monoxide and steam are formed. • Keep the room well ventilated (with windows open)

  24. In case of gas leakage, what should we do? • Read Chapter 25 P. 58

  25. Fire-fighting equipments in the laboratory • Carbon dioxide fire extinguisher • Dry powder fire extinguisher • A bucket of sand • Fire blanket

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