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Chapter 26 Burning of Fuels and Fire Fighting

Chapter 26 Burning of Fuels and Fire Fighting. 26.1 Heat changes in chemical reactions. 26.2 Burning of fuels. 26.3 F ire 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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Chapter 26 Burning of Fuels and Fire Fighting

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  1. Chapter 26 Burning of Fuels and Fire Fighting 26.1Heat changes in chemical reactions 26.2Burning of fuels 26.3Fire fighting 26.4Safe use of fuels at home 26.5Potential dangers associated with storage of fuels CONTENTS OF CHAPTER 26

  2. 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. Exothermic reactions Most chemical reactions are exothermic. Here are some examples: (1) All combustion reactions. For example, S(s) + O2(g)  SO2(g) + heat 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  3. oxygen burning sulphur Figure 26.1 A combustion reaction. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  4. copper(II) hydroxide precipitate (2)All precipitation reactions. For example, Cu2+(aq) + 2OH–(aq)  Cu(OH)2(s) + heat Figure 26.2 A precipitation reaction. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  5. copper wire silver crystals silver nitrate solution (3) All displacement reactions. For example, Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s) + heat 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  6. (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). 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  7. (6) Reaction between calcium oxide and water. CaO(s) + H2O(l)  Ca(OH)2(s) + heat Endothermic reactions Only a few reactions are endothermic. Two examples are given here: 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  8. (1) Cracking of oil fractions. (2) Thermal decomposition of calcium carbonate. CaCO3(s) + heat CaO(s) + CO2(g) DH 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: D H = Hp- Hr Heat change is measured in kilojoules (kJ). 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  9. ENERGY LEVEL DIAGRAMS Energy level diagram for an exothermic reaction D H has a negative value. During an exothermic reaction, the reaction mixture becomes hotter. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  10. heat reactants Hr Energy (heat content) energy given out, DH is negative heat heat exothermic reaction taking place Hp products heat (a) (b) Figure 26.6 An exothermic reaction: (a) Energy level diagram. (b) Heat is lost to the surroundings. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  11. Energy level diagram for an endothermic reaction D H has a positive value. During an endothermic reaction, the reaction mixture becomes colder. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  12. heat products Hp Energy (heat content) energy taken in, DH is positive heat heat Hr reactants heat (a) (b) Figure 26.7 An endothermic reaction: (a) Energy level diagram. (b) Heat is gained from the surroundings. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS endothermic reaction taking place

  13. THERMOCHEMICAL EQUATION AND HEAT OF REACTION 2Mg(s) + O2(g)  2MgO(s) D H = -1204 kJ mol-1 A THERMOCHEMICAL EQUATION is an equation written alongside with the D H value. HEAT OF REACTION (D H) is the heat change, when the number of moles of reactants represented by the equation of a reaction react completely. 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  14. A26.1 (a) Endothermic (b) Exothermic (c) Endothermic (d) Exothermic (e) Exothermic 26.1 HEAT CHANGES IN CHEMICAL REACTIONS

  15. 26.2 BURNING OF FUELS FUELS TO PROVIDE HEAT ENERGY Combustion of fossil fuels and hydrocarbons Burning is also called combustion. 26.2 BURNING OF FUELS

  16. 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. 26.2 BURNING OF FUELS

  17. y 4 y 2 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) D H = -z kJ mol-1 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. 26.2 BURNING OF FUELS

  18. 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) 26.2 BURNING OF FUELS

  19. 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 26.2 BURNING OF FUELS

  20. 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. 26.2 BURNING OF FUELS

  21. 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

  22. Figure 26.14 Fire ruins lives and properties! 26.3 FIRE FIGHTING

  23. 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. Putting out fires by breaking the fire triangle – by removing fuel. 26.3 FIRE FIGHTING

  24. Figure 26.16 A gas fire can be put out by cutting off the fuel gas supply. 26.3 FIRE FIGHTING

  25. (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. Figure 26.17 Fire fighting using foam to cut off the air supply. 26.3 FIRE FIGHTING

  26. (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. Figure 26.18 Fire fighting using water. 26.3 FIRE FIGHTING

  27. 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

  28. sulphuric acid container sodium hydrogencarbonate solution 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. Figure 26.23 A soda-acid fire extinguisher. 26.3 FIRE FIGHTING

  29. (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

  30. Figure 26.24 A foam fire extinguisher. 26.3 FIRE FIGHTING

  31. (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

  32. Figure 26.26 A carbon dioxide fire extinguisher. 26.3 FIRE FIGHTING

  33. (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

  34. Figure 26.27 A halon fire extinguisher. 26.3 FIRE FIGHTING

  35. Figure 26.29 BTM is used for fire fighting in MTR stations. 26.3 FIRE FIGHTING

  36. (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

  37. Figure 26.30 Figure 26.31 A powder-type fire A dry powder fire extinguisher in a MTR coach. extinguisher. 26.3 FIRE FIGHTING

  38. 26.3 FIRE FIGHTING

  39. 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

  40. 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’sozone layer.) 26.3 FIRE FIGHTING

  41. 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 26.4 SAFE USE OF FUELS AT HOME

  42. Figure 26.33 Delivery of LPG needs special safety precautions. 26.4 SAFE USE OF FUELS AT HOME

  43. 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. 26.4 SAFE USE OF FUELS AT HOME

  44. Figure 26.34 A newspaper cutting on a gas blaze. 26.4 SAFE USE OF FUELS AT HOME

  45. 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. 26.4 SAFE USE OF FUELS AT HOME

  46. 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. 26.4 SAFE USE OF FUELS AT HOME

  47. 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). 26.4 SAFE USE OF FUELS AT HOME

  48. (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. 26.4 SAFE USE OF FUELS AT HOME

  49. Figure 26.38 A warning label from the Hong Kong and China Gas Company. 26.4 SAFE USE OF FUELS AT HOME

  50. 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. 26.5 POTENTIAL DANGERS ASSOCIATED WITH STORAGE OF FUELS

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