Energy Transfers Using a calorimeter Worked example: A fuel heated 40g of water which started at temperature of 20 degrees Celsius and finished at a temperature of 39 degrees. The fuel before burning had a mass of 23.6g but now has a mass of 23.5g. The fuel has a formula mass of 28. You can work out four things: Energy released by fuel in Joules Energy released by fuel in kilojoules Energy released by fuel in Kilojoules/gram Energy released by fuel in Kilojoules/mol

Your go: A fuel heated 50g of water which started at temperature of 16 degrees Celsius and finished at a temperature of 41 degrees. The fuel before burning had a mass of 25.8g but now has a mass of 25.6g. The fuel has a formula mass of 46. Work out the energy released in joules, kilojoules, kilojoules/gram and kilojoules/mol.

Energy Level Diagrams

An ______________________ can be used to show how the energy stored in chemicals changes during a chemical reaction. In ______________ reactions the products have more energy than the ___________. In exothermic reactions the ____________ have less energy than the __________. A catalyst __________ the __________ energy. It does this by providing __________________________ ________. A catalyst is not ________________________. energy level diagram endothermic reactants products reactants lowers Copy and complete: activation an alternate route for the reaction. used up during the reaction.

3.4 Bond Energies Objective: to explain how the balance of energy changes when bonds are broken or made, and how this affects the overall energy change of a reaction Outcomes: All: I can state what happens to energy when bonds are made and broken (D/E) Most: I can assign bond energies to an energy level diagram (C) Some: I can calculate the energy transferred in reactions using supplied bond energies (A/B) Keywords: energy level diagram, exothermic, endothermic, bond energy Homework: Revise for test on 3.1, 3.2 and 3.3 Due: Thursday 21 March

bond breaking = energy IN bond making = energy OUT ENDOTHERMIC EXOTHERMIC I can state what happens to energy when bonds are made and broken (D/E)

Each bond has a different bond energy. The amount of energy it takes to make OR break it. A H-H bond has a bond energy of 436kJ/mol Data on bond energies can be used to calculate the TOTAL energy of a reaction. This is the difference between the energy put in when bonds are made and the energy given out when bonds are broken.

CH 4 (g) + 2O 2 (g)CO 2 (g) +2H 2 O(l) Work out the bond energies for: a) the reactant molecules b) the product molecules Hence c) workout the overall energy change 804

I can assign bond energies to an energy level diagram (C) ____________ ________ _____ Add labels and energy values to the energy level diagram for the reaction. CH 4 (g) + 2O 2 (g)CO 2 (g) +2H 2 O(l)

Fuel Issues

Briefly Summarise: Why Hydrogen fuel cells are less harmful to the environment than another example fuel (3).

3.4 Bond Energies Objective: to explain how the balance of energy changes when bonds are broken or made, and how this affects the overall energy change of a reaction Outcomes: I can state what happens to energy when bonds are made and broken (D/E) I can assign bond energies to an energy level diagram (C) I can calculate the energy transferred in reactions using supplied bond energies (A/B) Keywords: energy level diagram, exothermic, endothermic, bond energy Homework: Revise for test on 3.1, 3.2 and 3.3 Due: Thursday 21 March

Example 1 BOND ENERGY (kJ/mol) C-H 413 O=O 498 C-C 347 C=O 732 O-H 464 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 Ethanol + Oxygen Water + Carbon Dioxide

Example 2 BOND ENERGY (kJ/mol) C-H 413 O=O 498 C-C 347 C=O 732 O-H 464 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 Propane + Oxygen Water + Carbon Dioxide

Example 3 BOND ENERGY (kJ/mol) H-H 436 N-H 391 N ≡ N 945 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 Hydrogen + Nitrogen Ammonia

Example 4 BOND ENERGY (kJ/mol) O=O 498 H-H 436 O-H 464 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 Hydrogen + Oxygen Water

CHM2 Jan 2009 Example 5 – A/S level question