Energy changes

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Hougang secondary school science chemistry

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Energy changes

  1. 1. Chapter 15 Energy Changes
  2. 2. Conservation of energy <ul><li>Recall: Principle of Conservation of Energy </li></ul><ul><li>- Energy can neither be created nor destroyed. It can only be changed from one form to another. </li></ul><ul><li>In chemical reactions, energy is transferred in the form of heat energy or light energy. </li></ul>
  3. 3. Expt I : Addition of NaOH to water Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Sodium hydroxide pellets are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding sodium hydroxide.
  4. 4. Expt II : Addition of NH 4 C l to water Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Ammonium chloride crystals are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding ammonium chloride.
  5. 5. Results of Both Experiments Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Experiment Solid Initial temperature (°C) Final temperature (°C) I NaOH 28 34 II NH 4 C l 28 22
  6. 6. What conclusions about energy changes can be made from these 2 experiments? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. In experiment I, heat energy was given out when the solid dissolved in water. Thus, the temperature of the solution rose . We say this change is exothermic .
  7. 7. What conclusions about energy changes can be made from these 2 experiments? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. In experiment II, heat energy was taken in when the solid dissolved in water. Thus, the temperature of the solution dropped . We say this change is endothermic .
  8. 8. 19.1 Exothermic & Endothermic Changes <ul><li>Exo thermic reaction </li></ul><ul><li>- Gives out heat (or light) energy to the surroundings </li></ul><ul><li>- Temperature rises as heat is given out to surroundings. </li></ul><ul><li>- E.g. : Condensation, freezing, respiration, combustion of fuels, neutralisation (acid + alkali), rusting of iron </li></ul>
  9. 9. Initially, the temperature of the reaction mixture rises until the highest temperature is reached. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. How does temperature change in an exothermic reaction? When the reaction is completed, the temperature of the reaction mixture falls until it reaches room temperature.
  10. 10. Energy Level Diagram – Exothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. This means that the total energy of the products is less than that of the reactants. Hence, the energy level of the products is lower. Consider an exothermic reaction. Heat is given out to the surroundings. reactants heat of reaction (Δ H = negative) products Energy level
  11. 11. 19.1 Exothermic & Endothermic Changes (cont’d) <ul><li>Endo thermic reaction </li></ul><ul><li>- Takes in heat (or light) energy from the surroundings </li></ul><ul><li>- Temperature falls as heat is taken in from surroundings </li></ul><ul><li>- E.g. : Melting, boiling, photosynthesis, action of light on photographic film, thermal decomposition </li></ul>
  12. 12. How does temperature change in an endothermic reaction? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Initially, the temperature of the reaction mixture falls until the lowest temperature is reached. When the reaction is completed, the temperature of the reaction mixture rises until it reaches room temperature.
  13. 13. Energy Level Diagram – Endothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. This means that the total energy of the products is more than that of the reactants. Consider an endothermic reaction. Heat is taken in from the surroundings. reactants heat of reaction (Δ H = positive) products Energy level
  14. 14. Exothermic & Endothermic Changes (cont’d) <ul><li>Enthalpy change (heat of reaction),  H </li></ul><ul><li>- The amount of heat energy given out or taken in during a chemical reaction </li></ul><ul><li>- Positive ( + ) for endothermic reactions </li></ul><ul><li>- Negative ( - ) for exothermic reactions </li></ul><ul><li>- Unit: kilojoule (kJ) </li></ul>
  15. 15. All reactions need energy in order to get started. For example, a mixture of hydrogen and oxygen will not explode unless it is ignited. Many reactions that do not occur at room temperature will proceed rapidly when the reactants are heated. Heat provides the energy to change less energetic particles into more energetic particles and thus to start a reaction. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Energy changes in actual reactions
  16. 16. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. The minimum energy that reacting particles must possess in order for a chemical reaction to occur is called the activation energy, E a . What is activation energy?
  17. 17. Energy Profile Diagram for an Exothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. reactants activation energy ( E a ) enthalpy change (Δ H is negative) products Energy Progress of reaction
  18. 18. Energy Profile Diagram for an Endothermic Reaction Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. reactants activation energy ( E a ) enthalpy change (Δ H is positive) products Progress of reaction Energy
  19. 19. What causes energy changes in chemical reactions? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. The breaking of bonds or the making of new bonds during a reaction leads to energy changes.
  20. 20. When bonds between atoms are broken , heat energy is absorbed . Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. When bonds are formed between atoms, heat energy is given out . • Bond breaking is an endothermic process. • Bond formation is an exothermic process. Most reactions involve both bond breaking and bond making. What causes energy changes in chemical reactions?
  21. 21. How can you tell if a reaction is exothermic or endothermic? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Exothermic reaction: Δ H bond breaking < Δ H bond making Endothermic reaction: Δ H bond breaking > Δ H bond making
  22. 22. How do we determine whether a reaction is exothermic or endothermic? Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Is the reaction 2H 2 (g) + O 2 (g)  2H 2 O(g) exothermic or endothermic? To find out, we calculate the energy involved in bond breaking and bond forming in the reaction.
  23. 23. Calculating the Δ H bond breaking Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Energy required to break 1 mol of H–H bonds = +436 kJ (given) 2H 2 (g) + O 2 (g)  2H 2 O(g) Energy required to break 2 mol of H–H bonds = 2 × (+436) kJ = +872 kJ Δ H bond breaking = (+872) + (+496) = +1368 kJ Energy required to break 1 mol of O=O bonds = +496 kJ (given) “ +” because bond breaking has positive enthalpy.
  24. 24. Calculating the Δ H bond making Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Each molecule of H 2 O is made up of 2 O–H bonds 2H 2 (g) + O 2 (g)  2H 2 O(g) Energy required to make 4 mol of O–H bond = 4 × ( −926) kJ = −3704 kJ Δ H bond making = −3704 kJ Energy liberated to make 1 mol of O–H bonds = − 926 kJ (given) *** Overall, Δ H reaction = (+1368) + (−3704) kJ = −2336 kJ Hence, the reaction is exothermic . “ – ” because bond breaking has negative enthalpy.

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