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Energy

Energy. Section 3-3 (somewhat). Energy. Physical and chemical changes are always accompanied by energy changes. Released (Exothermic) Absorbed (Endothermic). Energy Transfer. The most common form is HEAT The energy transferred as a result of temperature difference

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Energy

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  1. Energy Section 3-3 (somewhat)

  2. Energy • Physical and chemical changes are always accompanied by energy changes. • Released (Exothermic) • Absorbed (Endothermic)

  3. Energy Transfer • The most common form is HEAT • The energy transferred as a result of temperature difference • Represented by q or ΔH • If a system is undisturbed, energy will continue to transfer until all temperatures are equal.

  4. Energy Units • Energy is measured in joules (J) • Derived SI Unit James Prescott Joule

  5. Energy Units • Sometimes energy is measured in calories (c) • One calorie = 4.184 joules • The “calories” in food are actually kilocalories (upper case “C”) A whole bunch of Calories

  6. Energy Units • Calories can easily be converted to joules: • 1cal = 4.184 joules • Convert 10 calories to joules: • (10 cal) x (4.184 J/cal) = 41.84 J

  7. Practice: • Convert 4 calories to joules • Convert 9,000 calories to joules. • If 4 Thin Mint Girlscout Cookies contain 160 Calories, how many joules of energy do they contain? Big C!!!

  8. Energy and Chemical Change • Chemical changes are always accompanied by a change in energy. • Endothermic Reaction – energy is absorbed by the reaction • Exothermic Reaction – energy is released by the reaction • Rule of Thumb: Nature tends to run downhill… Exothermic reactions tend to take place spontaneously (without outside help).

  9. Activation Energy • All reactions need energy to start reacting – called activation energy • The energy that pushes a reaction to happen

  10. Measuring Energy Changes Section 3-3 Pages 65-69

  11. Calorimeter • Used to measure the energy given off or absorbed during a chemical or physical change

  12. Specific Heat • To change the temperature of a substance, heat must be added or removed. • Some things need more heat to change than others • Water requires 4.184 joules to change 1 degree celsius • Aluminum requires 0.902 joules • BIG difference!!!

  13. Specific Heat • Specific Heat (Cp) – The heat needed to raisethe temperature of one gram of a substance by one Celsius degree • (Cp of water is 4.184 J/gC°) • Every substance has its own specific heat • Use Appendix Tables A-3 and A-5 in back of book

  14. Law of Conservation of Energy • Energy cannot be created nor destroyed • Must be transferred from one substance to another substance • Heat lost by one thing is gained by another • Two things of different temperatures will continue to exchange energy until they are both the same temperature

  15. Law of CoE continued • Here is the relationship: (heat gained or lost) = (mass in grams)(change in temperature) (specific heat) q = (m) (ΔT) (Cp) • To calculate change in temperature: ΔT = Tf– Ti

  16. In a calorimeter • We add water and measure the change in the temperature of the water to determine the amount of heat released. • (m)(ΔT)(4.184 J/gC°) = (m)(ΔT)(Cp) • Must assume that the calorimeter does not absorb or release energy. Other Substance Water

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