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Endothermic and exothermic reactions

Endothermic and exothermic reactions. Objectives: To explain the difference between an exothermic and an endothermic reaction. Outcomes: All students will classify reactions observed in previous lesson as either exothermic or endothermic

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Endothermic and exothermic reactions

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  1. Endothermic and exothermic reactions

  2. Objectives: To explain the difference between an exothermic and an endothermic reaction Outcomes: All students will classify reactions observed in previous lesson as either exothermic or endothermic Most students will explain the terms exothermic and endothermic reaction in terms of bond making and bond breaking Some students will perform calculations to deduce whether a reaction is endo or exo thermic Endothermic and exothermic reactions

  3. Temperature rise Temperature drop Endothermic Exothermic

  4. Temperature rise Temperature drop Reactants Energy Heat given out Products Endothermic Exothermic Products Energy Heat taken in Reactants

  5. What happens in a chemical reaction? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H H • Bonds between the reactants have to be broken: • 4 x C-H bonds • 2 x O=O bonds • This requires energy to be put into the system ie this stage is endothermic • Bonds between products have to be made: • 2 x C=O • 4 x H-O • Energy is given out when bonds are made ie this stage is exothermic

  6. Reactants Products Energy Energy Heat given out Heat taken in Products Reactants Endothermic-breaking bonds Exothermic- making bonds

  7. So: why is a reaction exothermic or endothermic overall? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H H • Bonds between the reactants have to be broken: • 4 x C-H bonds • 2 x O=O bonds • This requires energy to be put into the system ie this stage is endothermic • Bonds between products have to be made: • 2 x C=O • 4 x H-O • Energy is given out when bonds are made ie this stage is exothermic It depends on the individual bond energies

  8. Energy Energy given out Energy taken in Reactants CH4 + 2O2 Products CO2 + 2H2O

  9. Exothermic or endothermic overall? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H H • Bonds between the reactants have to be broken (energy in): 4 x C-H bonds 2 x O=O bonds = (4 x 412 kJ/mole) = 1648 kJ/mole = (2 x 496 kJ/mole) = 992 kJ/mole Total energy put in = 1648 + 992 = 2640 kJ/mole • Bonds between products have to be made (energy out): 2 x C=O 4 x H-O = (2 x 743 kJ/mole) = 1486 kJ/mole =(4 x 463 kJ/mole) = 1852kJ/mole Total given out = 1486 + 1852 = 3338kJ/mole • Overall energy change: + 2640 kJ/mole– 3338 kJ/mole = -698 kJ/mole

  10. Energy given out = -3338 kJ/mole Energy taken in = + 2640 kJ/mole Energy Energy change = - 698 kJ/mole Reactants CH4 + 2O2 Products CO2 + 2H2O

  11. Energy in (bonds broken) Energy out (bonds made) Calculating energy changes Energy difference = energy in – energy out

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