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): C 6000 H O O 2 → 6000CO H 2 O Burning candles (paraffin): C 21 H O 2 → 21CO H 2 O

4

5 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. 1 o C = 1 K The Fahrenheit scale is commonly used in the U.S. 1.8 o F = 1 o C = 1 K

6

7 The Electromagnetic Spectrum

8 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

9 Energy Profile Curves

10 Units of Energy 1 calorie = joules (J) C 1 C alorie = 1000 calories = kJ 1 Btu = joules

11 Energy Conversions How many Calories (“food calories”) are available in a hamburger that can provide 2.15 x 10 6 J of energy?

12 Solution: Solution: Energy Conversions How many Calories (“food calories”) are available in a hamburger that can provide 2.15 x 10 6 J of energy? = 514 Calories

13 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.

14

15 Exercise 5.3 H 2 (g) + Cl 2 (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)

16 Exercise 5.3 H 2 (g) + Cl 2 (g) → 2HCl(g) Bond-breaking:+672 kJ Bond-making:–856 kJ _____________________________________ Net energy change: –184 kJ/2 moles of HCl

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

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