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Energy Changes in Reactions

Energy Changes in Reactions. Learning objectives. Perform simple energy calculations using different energy units Apply specific heat concept to heat calculations Distinguish between endothermic and exothermic reactions Calculate specific heat from calorimetry

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Energy Changes in Reactions

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  1. Energy Changes in Reactions

  2. Learning objectives • Perform simple energy calculations using different energy units • Apply specific heat concept to heat calculations • Distinguish between endothermic and exothermic reactions • Calculate specific heat from calorimetry • Calculate heat changes in calorimetry

  3. Energy is capacity to do work • Energy comes in different forms • Kinetic energy is energy due to motion • Potential energy is energy due to position or state

  4. Energy comes in many forms • Kinetic energy is energy due to motion • Heat • Rotation • Vibration • Translation • Potential energy is energy due to position or state • Height • Chemical • Electrical • Coiled spring

  5. Energy is interchangeable • Processes convert energy from one form to another • Chemical reaction • (potential → heat) • Turbine • (heat → mechanical) • Generator • (mechanical → electrical) • Light bulb • (electrical → light) • Photosynthesis • (light → chemical) • Some energy is always wasted during conversion

  6. Measuring energy: calories, Calories and joules • calorie is energy required to raise temperature of 1 g of water 1 degree C • Calorie is the food version = 1,000 cal • Raises temperature of 1 pint of water 3.8ºF • Joule is SI unit derived from mechanical work: work done when force of 1 Newton is applied for 1 meter 1 cal = 4.18 J

  7. Specific heat • Specific heat is energy required to raise temperature of 1 g of substance by 1ºC • Units are J/gºC or cal/gºC

  8. Champagne from a styrofoam cup: Measuring heat change • Calorimetry is process used to measure heat change of reaction • Heat change in calorimeter is: Q = mH2O x ΔTH2O x SHH2O Weigh Measure Know

  9. Principle of Calorimetry • Conservation of energy: • Exothermic: • Temperature in calorimeter increases • Heat lost by process (system) = heat gained by H2O (surroundings) • Endothermic • Temperature in calorimeter decreases • Heat gained by process = heat lost by H2O

  10. Exo-thermic and endo-thermic • H2 + O2 gives out energy – exothermic • The system (chemical bonds) lose potential energy) • N2 + O2 absorbs energy – endothermic • The system gains potential energy

  11. Calorimetry example: Calculating specific heat • What is specific heat of lead if 57.0 J are required to raise temperature of 35.6 g Pb by 12.5ºC? Q = ms x ΔTs x SHs

  12. Specific heat metal example • 35.2 g metal at 100ºC are placed in calorimeter containing 42.5 g H2O at 19.2ºC. Final temperature is 29.5ºC. What is SH of metal? SH H2O = 4.18 J/g ºC • Heat lost by metal = heat gained by H2O • Qmetal = massmetal x ΔTmetal x SHmetal • Qwater = masswater x ΔTwater x SHwater

  13. Heat of reaction • Heat of reaction is the enthalpy change when reactants are converted into products • Note sign: • Exothermic (energy out) is – • Endothermic (energy in) is +

  14. Nutrients and energy • Macronutrients supply energy • Fats (9 Cal/g) (Main form of energy storage) • Carbohydrates (4 Cal/g) • Proteins (4 Cal/g)

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