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Lecture 11

Lecture 11. Energy, Chemistry and Society April 27, 2005. What is energy?. Energy: the capacity of a physical system to do work Work (W): equal to a force (F) multiplied by a distance (d); force exerted to move an object a certain distance

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Lecture 11

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  1. Lecture 11 Energy, Chemistry and Society April 27, 2005

  2. What is energy? Energy: the capacity of a physical system to do work Work (W): equal to a force (F) multiplied by a distance (d); force exerted to move an object a certain distance Force (F): the change of velocity for a mass; mass (m) x acceleration (a)

  3. Energy: Two Kinds • Kinetic • Anything that is moving • Heat • Electricity • Potential • Stored energy • Chemical – remember bond = spring model • Hydroelectric • Roller coaster

  4. Energy Units • Joules (J) 1 J = amount of energy needed to lift a 1 kg book, 10 cm against Earth’s gravity = amount of energy in 1 heartbeat • calories (cal) = 4.184 J 1 cal = amount of heat needed to raise the temperature of 1 g of water 1 °C • Calories (food calories) 1 Cal = 1 kilocalorie = 1 kcal • Other energy units are British Thermal Units (btu), ergs, & foot-pounds. • Energy of fuels are usually expressed as kJ/mole or kcal/mole.

  5. Law: Conservation of Energy • Energy is always conserved. • It can change forms, but is never created or destroyed. • Can be mysterious… always consider the dissipation of energy to surrounding environment in the form of sound, heat, etc. • Ultimately, most energy on earth comes from the sun. • Biology: photosynthesis yields fossil fuels • Weather: wind, hydroelectric

  6. First Law of Thermodynamics • Also called, the Law of Conservation of Energy and Mass. • Thermodynamics: Physics that deals with the relationships and conversions between heat and other forms of energy. • Good Jeopardy! question… We’ll call these Ken Jennings points

  7. Converting Heat to Work to Electricity

  8. Efficiency Ex. from book discusses converting electricity into heating a home – this final electric heater has an efficiency of 98% Overall efficiency = 0.60 x 0.90 x 0.75 x 0.95 x 0.90 x 0.98 = 0.34 = 34% Why? Because heat cannot be completely converted into work

  9. Second Law of Thermodynamics • States: it is impossible to completely convert heat into work without further changes to the universe. • Entropy in a closed system cannot decrease.

  10. Definitions Heat or Thermal Energy: • Random motion of molecules Entropy: • a measure of the amount of energy in a physical system that cannot be used to do work • Disorder, randomness • High entropic states are more probable than lower ones

  11. Combustion • What makes a usable fuel? • Most common energy-generating reaction is combustion. • Recall that combustion is the combination of fuel with oxygen. CH4(g) + 2O2(g) CO2(g) + 2H20 (g) + energy Energy on reactant side = Endothermic Energy on this side (products) means the reaction is Exothermic

  12. Exothermic vs. Endothermic

  13. Experimentally determining heat energy

  14. Calculating energy • We’ll mainly consider heat • Historically, heat has been abbreviated as q • Transfers of heat cause changes in temperature – Heat transferred in: temperature increases – Heat transferred out: temperature decreases • q ∝ ΔT

  15. Calculating energy • Amount of heat needed to change temperature directly proportional to mass – More heat needed to raise temperature of 50 g of something than 10 g of something • q ∝ mΔT

  16. Calculating energy • Some things are harder to heat than others – Very easy to heat metals: small amount of heat needed to change temperature by 1 °C – Very hard to heat wood, water: large amount of heat needed to change temperature by 1 °C • Specific heat capacity: amount of heat needed to change temperature of 1 g of a material by 1 °C • q = msΔT

  17. Table of Specific Heat Capacities (s) for various compounds

  18. Example • How much heat is required to heat 100 g of water from 15 to 25 °C? q = msΔT q = 100g x 4.184 J/g•°C x 10 °C q = 4184 J = 4.184 kJ

  19. Example • To what final temperature can 75 g of copper, initially at 80 °C, be raised by the addition of 750 J of heat? q = msΔT final temp = 80 °C + 26 °C = 106 °C q ms 750 J 75 g x 0.387 J/g •°C ΔT = = = 26 °C

  20. Example • How much heat is released when 32 g of aluminum are cooled from 15 to 5 °C? q = msΔT q = 32 g x 0.900 J/g •°C x 10 °C q = 288 J

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