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Energy changes in reactions

Energy changes in reactions. Objectives. To understand that energy is always present in reactions To be able to describe energy changes in reactions To know how to work out how much energy is given in or taken out To understand the importance of this. Bonds. Energy is put in to break bonds

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Energy changes in reactions

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  1. Energy changes in reactions

  2. Objectives To understand that energy is always present in reactions To be able to describe energy changes in reactions To know how to work out how much energy is given in or taken out To understand the importance of this.

  3. Bonds Energy is put in to break bonds Energy is released when bonds are made Energy changes are different for each type of bond The amount of energy in the reactants and products is different If energy in product is more than in reactant we say the reaction is Endothermic If the energy in the product is less than the energy in the reactants we say the reaction is Exothermic

  4. Endothermic reactions More energy is required to break bonds of reactants than is produced by making bonds in products Good example is photosynthesis Energy therefore stored in the product Another example: Dissolving ammonium nitrate Decomposition of Carbon dioxide to carbon monoxide + oxygen (only occurs when a lot of heat energy is put in)

  5. Energy levels

  6. Exothermic reactions We meet lots of them They are very important Energy required to break bonds is less than energy given out when new bonds are made So energy given off usually as heat but not always Example combustion or respiration Others would include the reaction of metals and acids Bioluminescence

  7. Energy levels Exothermic

  8. Activation Energy This is the energy required to break bonds in reactants This can be provided by a spark or light This is lowered by catalysts so allowing rates of reaction to increase.

  9. Hydrogen + Chlorine ==> Hydrogen Chloride The symbol equation is: H2(g) + Cl2(g) ==> 2HCl(g) but think of it as: H-H + Cl-Cl ==> H-Cl + H-Cl (where - represents the chemical bonds to be broken or formed) The bond energies in kJ/mol are: H-H 436; Cl-Cl 242; H-Cl 431 Energy needed to break bonds = 436 + 242 = 678 kJ taken in Energy released on bond formation = 431 + 431 = 862 kJ given out The net difference between them = 678 - 862= -184 kJ given out (92 kJ per mole of HCl formed) More energy is given out than taken in, so the reaction is exothermic.

  10. Hydrogen Bromide ==> Hydrogen + Bromine The symbol equation is: 2HBr(g) ==> H2(g) + Br2(g) but think of it as: H-Br + H-Br ==> H-H + Br-Br (where - represents the chemical bonds to be broken or formed) The bond energies in kJ/mol are: H-Br 366; H-H 436; Br-Br 193 Energy needed to break bonds = 366 + 366 = 732 kJ taken in Energy released on bond formation = 436 + 193 = 629 kJ given out The net difference between them = 732-629 = 103 kJ taken in (51.5kJ per mole of HBr decomposed) More energy is taken in than given out, so the reaction is endothermic

  11. Plenary Try the printed examples. Work out the energy changes and say whether they are endothermic or exothermic.

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