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Understanding Energy in Chemical Reactions and Thermodynamics

Explore the role of energy in chemical reactions, including the First Law of Thermodynamics, energy forms, fuels, temperature scales, and energy conversions. Learn about the different terms related to energy exchange processes, energy profile curves, bond energies, and the Second Law of Thermodynamics. Delve into chemical kinetics, focusing on reaction rates and mechanisms influenced by temperature, concentration, and catalysts.

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Understanding Energy in Chemical Reactions and Thermodynamics

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  1. Chapter 5 The Role of Energy in Chemical Reactions

  2. First Law of Thermodynamics • Energy is conserved. It is neither created nor destroyed. • Implication: It is transferred from place to place. Energy is echanged in chemical processes. • Implication: It can take several forms; heat, light, nuclear, etc. • These forms are either kinetic (motion) or potential (stored) energy.

  3. Fuels contain carbon, hydrogen, and, often, oxygen Burning wood (cellulose): C6000H12002O5001 + 6000O2→ 6000CO2 + 5001H2O Burning candles (paraffin): C21H44 + 32O2→ 21CO2 + 22H2O

  4. Temperature Scales • The Kelvin scale is directly related to the average kinetic energy of a system. • The Celsius scale is in common use throughout the world. 1oC = 1 K • The Fahrenheit scale is commonly used in the U.S. 1.8oF=1oC=1 K

  5. The ElectromagneticSpectrum

  6. Terms We Use with Energy Exchange Processes Exergonic: gives of energy Endergonic: absorbs energy Exothermic: gives off heat Endothermic: absorbs heat

  7. Energy Profile Curves

  8. Units of Energy 1 calorie = 4.184 joules (J) 1 Calorie = 1000 calories = 4.184 kJ 1 Btu = 1054.5 joules

  9. Energy Conversions How many Calories (“food calories”) are available in a hamburger that can provide 2.15 x 106 J of energy?

  10. Solution: Energy Conversions How many Calories (“food calories”) are available in a hamburger that can provide 2.15 x 106 J of energy? = 514 Calories

  11. Bond Energies Bond breaking requires energy. Bond formation releases energy. Whether heat will be required or released in the overall process depends upon the which energy change is larger.

  12. Exercise 5.3 H2(g) + Cl2(g) → 2HCl(g) Breaking reactant bonds 1 mol H-H bonds x 432 kJ/mol = 432 kJ 1 mol Cl-Cl bonds x 240 kJ/mol = 240 kJ Total = +672 kJ (endothermic) Forming product bonds 2 mol H-Cl bonds x 428 kJ/mol = 856 kJ Total = –856 kJ (exothermic)

  13. Exercise 5.3 H2(g) + Cl2(g) → 2HCl(g) Bond-breaking: +672 kJ Bond-making: –856 kJ _____________________________________ Net energy change: –184 kJ/2 moles of HCl

  14. Second Law of Thermodynamics The unavoidable tendency of the entropy of the universe to increase.

  15. Chemical Kinetics Concerned with the rates and mechanisms of chemical reactions. • Things that affect reaction rates: • temperature • concentration • catalysts

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