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Ks4 energy transfer


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  • 1. KS4: Energy Transfer in Reactions © Boardworks Ltd 2003
  • 2. Exothermic Reactions Exothermic reactions increase in temperature.• Examples include: – Burning reactions including the combustion of fuels. – Detonation of explosives. – Reaction of acids with metals. Magnesium reacting with acid Thermit reaction © Boardworks Ltd 2003
  • 3. Activity Say whether these processes are exothermic. 1. Charcoal burning yes 2. A candle burning. yes 3. A kettle boiling no 4. Ice melting no 5. A firework exploding yes You have to put heat in for boiling and melting. You get heat out from all the other processes © Boardworks Ltd 2003
  • 4. Exothermic Reactions• Magnesium + Hydrochloric acid 25o C 45o C magnesium Heat energy given Gets hot out Hydrochloric acid © Boardworks Ltd 2003
  • 5. Exothermic Reactions• If heat is given out this energy must have come from chemical energy in the starting materials (reactants). 45o C 25o Reactants convert chemical energy to heat energy. The temperature rises. © Boardworks Ltd 2003
  • 6. Exothermic Reactions• Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. 25 45o C Chemical energy becomes heat energy. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. It follows that reaction products have less chemical energy than the reactants had to start with. © Boardworks Ltd 2003
  • 7. Energy Level Diagram for an Exothermic Reaction Reactants have more reactants chemical energy. Energy / kJ) Some of this is lost as heat which spreads out into the room. Products now have products less chemical energy than reactants. Progress of reaction (time) © Boardworks Ltd 2003
  • 8. Energy Level Diagram for anExothermic Reaction 2. Exothermic Reaction ∆H is how much energy reactants is given out Energy / kJ ∆H is negative ∆H=negative because the products have less energy than products the reactants. Progress of reaction © Boardworks Ltd 2003
  • 9. Exothermic Reaction - DefinitionExothermic reactionsgive out energy. Thereis a temperature riseand ∆H is negative. reactants Energy / kJ) ∆H is negative products Progress of reaction © Boardworks Ltd 2003
  • 10. Activity © Boardworks Ltd 2003
  • 11. Endothermic Reactions Endothermic reactions cause a decrease in temperature.• Endothermic chemical reactions are relatively rare.• A few reactions that give off gases are highly endothermic - get very cold.• Dissolving salts in water is another process that is often endothermic. © Boardworks Ltd 2003
  • 12. Endothermic ReactionsEndothermic reactions cause a decrease in temperature. Heat Ammonium energy nitrate taken in as the Cools mixture returns back to Water room temp.Starts 25°C Cools to 5°C Returns to 25°C © Boardworks Ltd 2003
  • 13. Endothermic Reactions• Extra energy is needed in order for endothermic reactions to occur. 5o CC 25o• This comes from the thermal energy of the reaction mixture which consequently gets colder. Reactants convert heat energy into chemical energy as they change into products. The temperature drops. © Boardworks Ltd 2003
  • 14. Endothermic Reactions• The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. 5o o C 25oC The reactants gain energy. This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy. © Boardworks Ltd 2003
  • 15. Energy Level Diagram for an Endothermic Process This is how much energy is taken in products Energy / kJ) This is positive ∆H=+ because the products have reactants more energy than the reactants. Progress of reaction © Boardworks Ltd 2003
  • 16. Endothermic Reaction DefinitionEndothermic reactionstake in energy. There isa temperature drop and∆H is positive. products Energy / kJ ∆H=+ reactants Progress of reaction © Boardworks Ltd 2003
  • 17. © Boardworks Ltd 2003
  • 18. Activity Are these endothermic or exothermic? 1. A red glow spread throughout the mixture exo and the temperature rose. 2. The mixture bubbled vigorously but the endo temperature dropped 150C. 3. Hydrazine and hydrogen peroxide react so explosively and powerfully that they exo are used to power rockets into space. 4. The decaying grass in the compost maker was considerably above the outside temperature. exo © Boardworks Ltd 2003
  • 19. Activity Sketch the two energy diagrams and label exothermic and endothermic as appropriate. reactants products Energy / kJ) ∆H=- Energy / kJ ∆H=+ products reactants Progress of reaction Progress of reaction © Boardworks Ltd 2003
  • 20. Breaking chemical bonds• Most chemicals will decompose (break up) if we heat them strongly enough.• This indicates that breaking chemical bonds requires energy – is an endothermic process. Heat taken in Energy in chemicals Energy needed to overcome the bonding between Energy needed the atoms © Boardworks Ltd 2003
  • 21. Making chemical bonds• It is reasonable to assume that bond making will be the opposite of bond breaking• Energy will be given out in an exothermic process when bonds are formed. Heat given out Energy in chemicals Energy given out as bonds form between atoms Energy given out © Boardworks Ltd 2003
  • 22. Changes to chemical bonds Endothermic Reactions• In most chemical reactions some existing bonds are broken (endothermic)• But new bonds are made (exothermic) Energy given Energy in chemicals out as new Energy taken Overall bonds form in as old endothermic bonds break in this case products ∆H reactants © Boardworks Ltd 2003
  • 23. Changes to chemical bonds Exothermic Reactions• Again some existing bonds are broken (endothermic)• And new bonds are formed (exothermic) Energy in chemicals Energy taken in Overall as old bonds Energy given out break exothermic – as new bonds in this case form reactants ∆H products © Boardworks Ltd 2003
  • 24. Summary – Bond Changes• Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic.• Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic. Exo Endo Bonds break Energy in chemicals Energy in chemicals Bond forming Bonds form Bonds break products reactants ∆H ∆H products reactants © Boardworks Ltd 2003
  • 25. Activity • The formation of nitrogen (IV) oxide (formula NO2) from reaction of nitrogen with oxygen in car engines has a ∆H value of +33.2kJ per mol of nitrogen oxide. 1. Write a word equation for the reaction. 2. Write a chemical equation for the reaction. 3. Is ∆H positive or negative? 4. Is the reaction exothermic or endothermic? 5. Draw an simple energy diagram for the reaction (not showing bond breaking and forming.) 6. Which involves the biggest energy change: bond breaking or bond forming? © Boardworks Ltd 2003
  • 26. Answer 1. Nitrogen + oxygen  nitrogen(IV)oxide 2. N2 + 2O2  2NO2. 3. ∆H positive (+33.2kJ/mol). 4. The reaction is endothermic. 5. Energy diagram 6. Bond breaking involves products the biggest energy change. Energy / kJ reactants Progress of reaction © Boardworks Ltd 2003
  • 27. Burning Methane • This is an exothermic reaction C HH HH Bond O O O O Breaking BondEnergy in chemicals Forming H O O C O O O H H H ∆H H H O O C O H H Progress of reaction © Boardworks Ltd 2003
  • 28. Activity • Hydrogen peroxide decomposes as shown: H Bond Energy (kJ) O O H H-O 464 H O-O 146 O O H O=O 498 1. Calculate energy for bond breaking. O O 2. Calculate the energy from bond making O 3. What is the value of ∆H for the H H O reaction shown H H © Boardworks Ltd 2003
  • 29. Answer Bond Energy (kJ) H H-O 464 O O O-O 146 H H O=O 498 O O H Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ O O Bond forming: (exothermic) 4(O-H) + 1(O=O) O =1856+498 = -2354kJ H H ∆H = +2148 – 2354 = -206kJ O H H (Exothermic) © Boardworks Ltd 2003
  • 30. Activation Energy.• Most chemical reactions, including exothermic reactions, seem to need an input of energy to get the reaction started.• This fits completely with what we have already explained: – Before new bonds can be formed we need to break at least some existing chemical bonds. – This requires an energy input –known as the activation energy (Ea or Eact) – Once an exothermic reaction is underway it can provide its own activation energy (from the bond forming stage) and so sustains the reaction. © Boardworks Ltd 2003
  • 31. Activation Energy and Exothermic Reactions Ea= + reactants Activation energy Energy / kJ) ∆H= - products Progress of reaction © Boardworks Ltd 2003
  • 32. Activation Energy and Endothermic Reactions Ea= + Activation products energy Energy / kJ) ∆H=+ reactants Progress of reaction © Boardworks Ltd 2003
  • 33. Activity Copy the energy diagram and use it ActivationE to help you explain nergy why garages can Energy / kJ) store petrol safely Petrol + but always have oxygen notices about not smoking near the Carbon dioxide + petrol pumps. water Progress of reaction © Boardworks Ltd 2003
  • 34. Answer The reaction is exothermic but ActivationE requires the Activation nergy energy to be provided Energy / kJ) before the reaction can get underway. Petrol + This is necessary to oxygen break some of the bonds in the oxygen or petrol before new Carbon dioxide + bonds can start water forming. Progress of reaction © Boardworks Ltd 2003
  • 35. Activity Copy the summary using the words from the box to fill in the gaps: endothermic lose positive exothermic common 1. Exothermic reactions are _____. common 2. Reactions that get cold are called _____. endothermic 3. Bond forming is an _______ process. exothermic 4. Endothermic reactions have a _____ ∆H. positive 5. In exothermic reactions the chemicals ___ lose chemical energy. continued © Boardworks Ltd 2003
  • 36. Activity Copy the summary using the words from the box to fill in the gaps: more endothermic activation 1. The energy needed to start off a reaction is called the activation energy ______ 2. In endothermic reactions bond breaking requires ___ energy than is produced by more bond forming. endothermic 3. Bond breaking is an _________ process. © Boardworks Ltd 2003
  • 37. Summary• Exothermic reactions: – Are common, – Give out heat. – Have a negative ∆H. – Bond forming gives out more energy than bond breaking consumes. – Have reactants that contain more chemical energy than the products.• Endothermic reactions – are the opposite!• Bond breaking is endothermic.• Bond forming is exothermic.• Reactions require an activation energy to help start the bond breaking process. © Boardworks Ltd 2003
  • 38. Which of the following is an endothermic process?A. Burning woodB. Reacting an acid and alkaliC. Reacting magnesium with acidD. Dissolving ammonium nitrate in water © Boardworks Ltd 2003
  • 39. Which of the following is true for an exothermic process?A. The reactants lose energy to the surroundings productsB. The reaction gets cold Energy / kJC. The energy diagram will be as shownD. The reaction will have a reactants ∆H that is positive (+). Progress of reaction © Boardworks Ltd 2003
  • 40. Which of the following is true when chemicalbonds are broken?A. The process is exothermicB. Energy is given outC. A physical change is occurringD. The reaction will have a ∆H that is positive(+). © Boardworks Ltd 2003
  • 41. Which of the following is true for an exothermic reaction?A. Bond breaking involves a bigger energy change than bond makingB. Bond making involves a bigger energy change than bond breakingC. Bond making involves the same energy change as bond breaking © Boardworks Ltd 2003
  • 42. Which of the following is the activation energy:A. XB. YC. Z © Boardworks Ltd 2003