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5.1 Energy changes

5.1 Energy changes. Exothermic reaction , e.g. respiration Energy ‘exits’ reaction – heats surroundings Thermometer readings rises. Endothermic reaction , e.g. photosynthesis Energy ‘enters’ reaction – cools surroundings Thermometer readings fall. 5.2 Energy and reversible reactions.

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5.1 Energy changes

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  1. 5.1 Energy changes Exothermic reaction, e.g. respiration • Energy ‘exits’ reaction – heats surroundings • Thermometer readings rises Endothermic reaction, e.g. photosynthesis • Energy ‘enters’ reaction – cools surroundings • Thermometer readings fall

  2. 5.2 Energy and reversible reactions Hydrated Anhydrous copper sulphate copper sulphate + water

  3. 5.3 Haber process (again!) N2 + 3H2 2NH3

  4. 5.3 Haber process (again!) N2 + 3H2 2NH3

  5. 5.3 Haber process (again!) Temperature: - Forward reaction is exothermic, so low temperature is preferred - But this makes reaction slow - Compromise by using 450OC N2 + 3H2 2NH3 Catalyst: - Iron - Speeds up both sides of reaction Pressure: - The higher the better - High pressure is dangerous! - Compromise by using 200-350 atm

  6. C3

  7. 4.2J raises temp of 1 g of water by 1 degree Acids & Alkalis Energy from fuels Calorimeter A + B  C If 0.1 mole of reactants. Total mass of A and B is 100g. Temp start is 19.6, temp max is 26.1 Work out diff….6.5 (Don’t need to learn this, you would get this) So for 0.1 moles = 2730J For 1 mole 2730 x 10 = 27300J (27.3kJ) …..exothermic reaction ( as temp rise) = -27.3kJ/mol Think HSW! Energy change = mass x 4.2 x temp change Bomb calorimeter Energy Food high in carbs and fats have lots of energy!!  more than body needs  obesity

  8. Acids & Alkalis Energy changes Reaction = bond breaking ( endo) and bond making ( exo) EXOTHERMIC Energy required to break bonds in less than energy released when new bonds are formed ENDOTHERMIC Energy required to break bonds in greater than energy released when new bonds are formed CATALYST…. Lowers activation energy ∆H = + ve ∆H = - ve Energy

  9. Acids & Alkalis Bond energies Identify the bonds…..stick diagrams! CH4(g) + 2O2(g)  2H2O(l) + CO2(g) Add up on the bonds in the reactants. This is bond energy needed to break the bonds Add up on the bonds in the products. This is bond energy needed to make new bonds. Energy REMEMBER… making new bonds is an exothermic reaction…so it is always a –ve number ∆H = bond breaking - bond making

  10. Example 1 Hydrogen + Nitrogen Ammonia a) Complete the balanced symbol equation. b) Draw the structural formula. c) Calculate the energy in for the reactant bonds being broken. d) Calculate the energy out for the product bonds being made. e) Work out the overall change in energy BOND ENERGY (kJ/mol) H-H 436 N-H 391 N≡N 945

  11. Example 2 Hydrogen + Oxygen Water a) Complete the balanced symbol equation. b) Draw the structural formula. c) Calculate the energy in for the reactant bonds being broken. d) Calculate the energy out for the product bonds being made. e) Work out the overall change in energy BOND ENERGY (kJ/mol) O=O 498 H-H 436 O-H 464

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