The Role of Energy in Chemical Reactions

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Chapter 5

• The Role of Energy in Chemical Reactions

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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.

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Fuels contain carbon, hydrogen, and, often, oxygen

• Burning wood (cellulose)
• C6000H12002O5001 6000O2 ? 6000CO2
  5001H2O
• Burning candles (paraffin)
• C21H44 32O2 ? 21CO2 22H2O

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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

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The Electromagnetic Spectrum
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Terms We Use with Energy Exchange Processes

• Exergonic gives of energy
• Endergonic absorbs energy
• Exothermic gives off heat
• Endothermic absorbs heat

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Energy Profile Curves
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Units of Energy

• 1 calorie 4.184 joules (J)
• 1 Calorie 1000 calories 4.184 kJ
• 1 Btu 1054.5 joules

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Energy Conversions

• How many Calories (food calories) are
  available in a hamburger that can provide
• 2.15 x 106 J of energy?

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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

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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.

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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)

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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

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Second Law of Thermodynamics

• The unavoidable tendency of the entropy of the
  universe to increase.

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Chemical Kinetics

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