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Energy

Energy. The ability to do work. Not homework— real work. Types of energy. Radiant energy: E=h n (later) Thermal energy: q=ms D T q=nH v (or mH v ) OR q=nH f (or mH f ) Chemical energy: Enthalpy (H) and others of less interest to a chemist. Symbols and units.

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Energy

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  1. Energy The ability to do work. Not homework—real work

  2. Types of energy • Radiant energy: E=hn (later) • Thermal energy: q=msDT q=nHv (or mHv) OR q=nHf (or mHf) • Chemical energy: Enthalpy (H) • and others of less interest to a chemist

  3. Symbols and units • E :Energy, in J h :Planck’s Constant • n :frequency, /sq :heat, in J • m :mass, in g DT :change in temp.oC • n :# of moles H :enthalpy, J/mol • Hv :enthalpy of vaporization J/mol or J/g • Hf :enthalpy of fusion J/mol or J/g • s :specific heat capacity, in j/goC

  4. Law of conservation of energy • Energy is conserved.

  5. Law of conservation of energy • Energy is conserved. • Energy is neither created nor destroyed in any chemical or physical process, though it may be changed to another form.

  6. Heat is not temperature. • Heat cannot be measured directly, only by its affect on matter. • If you add heat to a sample, it may…

  7. Heat is not temperature. • Heat cannot be measured directly, only by its affect on matter. • If you add heat to a sample, it may… a) warm up. q=msDT b) melt q=nHf (or mHf) c) boil q=nHv (or mHv) d) expand (tough to calculate, don’t bother)

  8. Heat is not temperature. All endothermic physical processes! • Heat cannot be measured directly, only by its affect on matter. • If you add heat to a sample, it may… a) warm up. q=msDT b) melt q=nHf (or mHf) c) boil q=nHv (or mHv) d) expand (tough to calculate, don’t bother)

  9. Heat is not temperature. • Heat cannot be measured directly, only by its affect on matter. • If you add heat to a sample, it may… a) warm up. q=msDT b) melt q=nHf (or mHf) c) boil q=nHv (or mHv) d) expand (tough to calculate, don’t bother) Can you reverse these?

  10. Consider the Universe

  11. Consider the Universe • Dude.

  12. Consider the Universe System Surroundings

  13. Consider the Universe System Surroundings A system may be open, closed, or isolated

  14. Open system System Surroundings Matter Energy A system may be open, closed, or isolated

  15. Closed system System Surroundings Energy A system may be open, closed, or isolated

  16. Isolated system System Surroundings A system may be open, closed, or isolated

  17. Flow of Energy System Surroundings A process may be endothermic or exothermic

  18. Endothermic process System Surroundings Energy A process may be endothermic or exothermic

  19. Exothermic process System Surroundings Energy A process may be endothermic or exothermic

  20. For chemical and physical processes! System Surroundings A process may be endothermic or exothermic

  21. State functions • Some values assigned to a system are state functions —depending only on the state of the system • Other values are not state functions—they depend on how the system got there

  22. State functions • Some values assigned to a system are state functions —depending only on the state of the system Ex: Energy, P, V, T • Other values are not state functions—they depend on how the system got there Ex: Work

  23. Change • A change in a state function is defined as the difference between the pre- and post- states. • DE=Efinal-EinitialDH=Hfinal-Hinitial • DP=Pfinal-Pinitial DV=Vfinal-Vinitial • Detc.

  24. Heat • Heat energy is measured in Joules (like any energy) • Heat can be added to a system or lost by a system

  25. Chemical Energy • Chemical energy (enthalpy) is stored in bonds.

  26. Chemical Energy • Chemical energy (enthalpy) is stored in bonds. • Forming bonds releases energy • Breaking bonds requires energy

  27. Chemical Energy • Chemical energy (enthalpy) is stored in bonds. • Forming bonds is exothermic • Breaking bonds is endothermic

  28. Chemical Work. • When a gas expands against a pressure, it does work on its surroundings. • When a gas is compressed, its surroundings do work on it.

  29. Warning: • Dr. Chang reverses the sign on work. • Do not use his notation. • For everyone else in the world…

  30. Law of conservation of energy • Energy is conserved. • Therefore: • The change of a system’s energy is equal to the heat it gains, minus the work it does

  31. Law of conservation of energy • Energy is conserved. • Therefore: • The change of a system’s energy is equal to the heat it gains, minus the work it does DE=q-w

  32. Law of conservation of energy • Energy is conserved. • Therefore: • If no work is done, the change in energy is indicated only by the heat. In a chemical reaction, this is the enthalpy

  33. Law of conservation of energy • Energy is conserved. • Therefore: • If no work is done, the change in energy is indicated only by the heat. In a chemical reaction, this is the enthalpy DH=q

  34. Law of conservation of energy • Energy is conserved. • Therefore: • If no work is done, the change in energy is indicated only by the heat. In a chemical reaction, this is the enthalpy DH=q

  35. Work is done when… • A gas is produced • A gas is used up

  36. Work is done when… • A gas is produced The system does work on the surroundings • A gas is used up The surroundings do work on the system

  37. Work is done when… • A gas is produced The system does work on the surroundings The system loses energy • A gas is used up The surroundings do work on the system The system gains energy

  38. PV=nRT • D (PV) is work. • It could be P(DV) (at constant pressure) or (DP)V (at constant volume).

  39. PV=nRT • D (PV) is work. • It could be P(DV) (at constant pressure) or (DP)V (at constant volume). In either case: D(PV)=(Dn)RT The change in the number of moles of gas

  40. What is the change in energy? • If Dq=-115J and w=-10.J? • If a system gains 200.J and does 12 J of work? • If a system gains 124 J of heat as it expands from .028m3 to .043m3 at 88,000Pa? • If a system loses 58 J of heat as it is compressed from 12 L to 7 L at 1.5 atm? • If a reaction gives off 125 kJ as it produces 2.9 moles of gas at standard conditions?

  41. How about some stoichiometry? • The oxidation of copper releases 155.2 kJ/mol. • How much heat is produced from the oxidation of 15 g Cu?

  42. How about some stoichiometry? • The oxidation of copper releases 155.2 kJ/mol. • How much heat is produced from the oxidation of 15 g Cu? • How much work is done if this occurs at 300k?

  43. Calorimetry • --the measurement of heat.

  44. Calorimetry • --the measurement of heat. • If one thing gains heat…

  45. Calorimetry • --the measurement of heat. • If one thing gains heat… …something else lost it.

  46. If 75 g of a metal at 96oC is placed in 58 g of water at 21oC and the final temperature reaches 35oC, what is the specific heat capacity of the metal?

  47. Step 1 • How much heat did the water gain?

  48. Step 1 • How much heat did the water gain? q=msDT Mass of water, in grams Specific heat of water, 4.18 J/goC Change in the temperature of water, in oC

  49. Step 2 • How much heat did the metal lose?

  50. Step 2 • How much heat did the metal lose? • Heat lost = - heat gained • qlost=-qgained

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