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Chapter 5Chapter 5
The Role of Energy in Chemical The Role of Energy in Chemical ReactionsReactions
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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:Exergonic: gives of energy
Endergonic:Endergonic: absorbs energy
Exothermic:Exothermic: gives off heat
Endothermic:Endothermic: absorbs heat
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Energy Profile Curves
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Units of Energy
1 calorie = 4.184 joules (J)
1 CCalorie = 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: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= 514 Calories
kJ 4.184
Calorie 1 x
J 1000
kJ 1 x kJ 2150
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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 kJ1 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 kJTotal = –856 kJ (exothermic)
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Exercise 5.3
H2(g) + Cl2(g) → 2HCl(g)
Bond-breaking: +672 kJ
Bond-making: –856 kJ
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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