# C3.3 Calculating + explaining energy changes

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• C3.3 Calculating + explaining energy changes
• Energy is transferred in chemical reactions because old bonds are broken and new are formed.
• Endothermic processes- Energy must be supplied to break existing bonds.
• In endothermic reactions the energy required is greater than the energy released when new bonds are formed. The extra energy needed is absorbed from the surroundings and the temperature falls.
• Exothermic processes-Energy is released when new bonds are formed.
• In exothermic reactions the energy released in bond formation is greater than the energy required in breaking old bonds. The left over energy is released into the surroundings and the temperature rises.
• Bond Energy - the energy required to break a bond or amount of energy released when a bond is made.
• Energy per Gram
• Mass of fuel,g = initial mass of fuel - final mass of fuel
• Energy per gram, J/g = Energy charge, J / Mass of fuel, g
• Energy per Mole
• Moles of each reactant, mol = Mass, g / Molar Mass
• Energy per mol, J/mol = energy change, J / Moles, mol
• Calculating energy of different fuels by burning
• The amount of energy transferred from the burning fuel to the water in the calorimeter can be calculated if you know the mass of water heated and rise in temperature.
• Fuel Cells
• Fuels cells can produce electricity by: hydrogen + oxygen --> water
• Water is the only product and is clean
• non-polluting
• Hydrogen is hard to store safely as it is explosive
• Shorter driving range than traditional vehicles
• Special, expensive engine needed
• Requires expensive catalysts - e.g. platinum
• Not many Hydrogen refueling stations
• Current fuels
• Ethanol
• CO2
• Land use
• Crops not used for food
• Fermentation for ethanol production uses a lot of energy
• Petrol and diesel
• CO2
• Increases greenhouse effect
• SO2 + NO
• Acid rain