# calculating & explaining energy changes

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• Calculating and explaining energy changes
• Measuring energy of chemical reactions
• measuring initial & final temp in a neutralisation reaction in an insulated container lets you calc the energy produced
• also used in solution reactions with solids
• Making and breaking bonds
• breaking chemical bonds = endothermic process
• forming bonds = exothermic
• If more energy is required to break old bonds than is released when new bonds are formed, the reaction is endothermic
• If more energy is released when the new bonds are formed than is needed to break old bonds, the reaction must be exothermic
• Measuring energy by calorimetry
• 4.2 joules to heat 1 gram of water by 1 degree c (calorie)
• 4.3 j = 1 calorie
• to measure temp change when fuel burns:
• 1Place 100g water in a glass/metal container
• find mass (g) of he fuel to be burned
• Burn fuel under water for a few minutes
• Record new temp and calc temp change
• Weigh fuel & calc mass of fuel used
• Energy released (Q) = mc delta T
• m = mass, c = 4.2 (a constant) and delta T = temp change
• Energy level diagrams
• exothermic: energy is given out (lost), so products have less energy than reactants
• endothermic: energy is taken in (gained), so products have more energy than reactants
• activation energy is needed to start reaction (to break old bonds)
• catalysts reduce activation energy needed (faster reaction)
• Hydrogen as a fuel
• can be used in combustion engines
• in fuel cells which produce electricity to power vehicles
• hydrogen + oxygen = water + heat

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