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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