1. CHAPTER 6 Energy General, Organic, & Biological Chemistry Janice Gorzynski Smith

2. CHAPTER 6: Energy 2 Smith. General Organic & Biolocial Chemistry 2nd Ed. Learning Objectives:  Definition of Energy, Kinetic Energy, Potential Energy  Heat transfer in reactions  Enthalpy  Exothermic and endothermic  Energy unit conversions and calculations  Bond Strength  Energy diagrams  How to change the rate of a reaction  Catalysts  Equilibrium: definition and calculations  Re-establishing equilibrium and Le Chatlier’s principle

3. 3 Energy Definition of Energy Smith. General Organic & Biolocial Chemistry 2nd Ed. Total Energy Potential Energy Kinetic Energy = + Energy is the capacity to do work. Potential energy is stored energy. The law of conservation of energy states that the total energy in a system does not change. Energy cannot be created or destroyed. Kinetic energy is the energy of motion.

4. 4 Energy Kinetic Energy Smith. General Organic & Biolocial Chemistry 2nd Ed. http://www.petervaldivia.com/technology/energy/ http://scienceisntscary.wordpress.com/tag/kinetic-energy/ Kinetic Energy (KE) Energy of motion KE = ½mv 2

5. 5 Energy Potential Energy Smith. General Organic & Biolocial Chemistry 2nd Ed. Potential Energy = Stored energy Exists in natural attractions and repulsions Chemical Energy PE possessed by chemicals Stored in chemical bonds Breaking bonds requires energy Forming bonds releases energy PE ReactantsProducts Lower PE Favorable Stable Higher PE Unfavorable Unstable R P P

6. 6 Energy Units of Energy Smith. General Organic & Biolocial Chemistry 2nd Ed. A calorie (cal) is the amount of energy needed to raise the temperature of 1 g of water by 1 o C. A joule (J) is another unit of energy. 1 cal = 4.184 J Both joules and calories can be reported in the larger units kilojoules (kJ) and kilocalories (kcal). 1,000 J = 1 kJ 1,000 cal = 1 kcal 1 kcal = 4.184 kJ

7. 7 Energy Example: Energy in a Gummy Bear Smith. General Organic & Biolocial Chemistry 2nd Ed. A gummy bear is 9.000 Calories (nutritional calories). How much energy is stored in a gummy bear in units of Joules? 9.000 Cal = 9.000 kcal x 1000 cal = 9000. cal 9000. cal x 4.184 J = 37660. J = 37.66 kJ 1 kcal 1 cal Gummy Bear Video: https://www.youtube.com/watch?v=6YWGnfnEmgM&src_vid=Jzoi7dJAiSc&feature=iv&annotation_id=annotation_713078

8. 8 Energy Breaking and Forming Bonds Smith. General Organic & Biological Chemistry 2nd Ed. Cl To cleave this bond, 58 kcal/mol must be added.  H = +58 kcal/mol Endothermic To form this bond, 58 kcal/mol is released.  H = −58 kcal/mol Exothermic Breaking bonds requires energy Forming bonds releases energy  H is the energy absorbed or released in a reaction; it is called the heat of reaction or the enthalpy change.

9. 9 Energy Enthalpy Change & Bond Dissociation Energy Smith. General Organic & Biological Chemistry 2nd Ed. The bond dissociation energy is the  H for breaking a covalent bond by equally dividing the e − between the two atoms. Bond dissociation energies are positive values, because bond breaking is endothermic (  H > 0). Bond formation always has negative values, because bond formation is exothermic (  H < 0). HH H + H HH  H = +104 kcal/mol  H = −104 kcal/mol

10. 10 Energy Bond Strength Smith. General Organic & Biological Chemistry 2nd Ed. The stronger the bond, the higher its bond dissociation E.  H indicates the relative strength of the bonds broken and formed in a reaction:  H negative: Exothermic reaction: more energy required to break products then reactant bonds: products have stronger bonds.  H positive: Endothermic reaction: less energy required to break products then reactant bonds: products have weaker bonds.

11. 11 Energy Endothermic & Exothermic Smith. General Organic & Biological Chemistry 2nd Ed.

12. 12 Reactions Energy Diagrams Smith. General Organic & Biological Chemistry 2nd Ed. The orientation of the two molecules must be correct as well. For a reaction to occur, two molecules must collide with enough kinetic energy to break bonds.

13. 13 Reactions Energy Diagrams Smith. General Organic & Biological Chemistry 2nd Ed. E a, the energy of activation, is the difference in energy between the reactants and the transition state. It can be thought of as the energy barrier that must be overcome for the reaction to occur.

14. 14 Reactions Energy Diagrams Smith. General Organic & Biological Chemistry 2nd Ed.  H is positive, the reaction is endothermic:  H is negative, the reaction is exothermic: When the E a is high, few molecules have enough energy to cross the energy barrier, and the reaction is slow. When the E a is low, many molecules have enough energy to cross the energy barrier, and the reaction is fast.

15. E Reactants Products EXOTHERMIC ENDOTHERMIC Heat released Heat absorbed ENDOTHERMIC Heat + Reactants  Products Products have weaker bonds and a higher energy then Reactants. Heat is absorbed by the system. ΔE +ΔH + EXOTHERMIC Reactants  Products + heat Products have stronger bonds and a lower energy then Reactants. Heat is released by the system. ΔE - ΔH - PE increases as bonds break PE decreases as bonds form Summary Energy & Reactions

16. 16 Reactions Ex: Splitting Water Smith. General Organic & Biological Chemistry 2nd Ed. Requirments: Very Endothermic o Need a minimum of 1.23 V to split water o Kinetically infrared light could do this, but the reaction is very slow o The potential really needs to be at least 3.0 V to utilize the full spectrum of light

17. 17 Reactions Rates of Reactions Smith. General Organic & Biological Chemistry 2nd Ed. Increasing the concentration of the reactants: Increases the number of collisions Increases the reaction rate Increasing the temperature of the reaction: Increases the kinetic energy of the molecules Increases the reaction rate A catalyst is a substance that speeds up the rate of a reaction and can be recovered unchanged. Catalysts lower Activation Energy, E a.

18. 18 Reactions Catalysts Smith. General Organic & Biological Chemistry 2nd Ed. The uncatalyzed reaction (higher E a ) is slower. The catalyzed reaction (lower E a ) is faster.   H is the same for both reactions.

19. 19 Reactions Catalysts: Photosystem II Smith. General Organic & Biological Chemistry 2nd Ed. PQ + H2O --> PQH2 + O2 (g) The overall reaction of Photosystem II is the oxidation of water and the reduction of plastoquinone.

20. 20 Reactions Equilibrium Smith. General Organic & Biological Chemistry 2nd Ed. A reversible reaction can occur in either direction. CO(g) + H 2 O(g)CO 2 (g) + H 2 (g) The forward reaction proceeds to the right. The forward reaction proceeds to the right. The reverse reaction proceeds to the left. The reverse reaction proceeds to the left. The system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. The net concentrations of reactants and products do not change at equilibrium.

21. 21 Reactions Equilibrium Smith. General Organic & Biological Chemistry 2nd Ed. a A + b Bc C + d D equilibrium constant =K = [C] c [D] d [A] a [B] b = [products] [reactants] The relationship between the concentration of the products and the concentration of the reactants is the equilibrium constant, K. *Brackets, [ ], are used to symbolize concentration in moles per liter (mol/L).

22. 22 Reactions Equilibrium Smith. General Organic & Biological Chemistry 2nd Ed. N 2 (g) + O 2 (g) 2 NO(g) equilibrium constant =K= [N 2 ] [O 2 ] [NO] 2 *The coefficient becomes the exponent.

23. 23 Reactions Equilibrium Smith. General Organic & Biological Chemistry 2nd Ed. HOW TO Calculate the Equilibrium Constant for a Reaction A 2 + B 2 2 AB Step [1] Write the expression for the equilibrium constant from the balanced equation. [AB] 2 [A 2 ][B 2 ] K = Step [2] Substitute the given concentrations in the equilibrium expression and calculate K. [AB] 2 [A 2 ][B 2 ] K == [0.50] 2 [0.25][0.25] = 0.25 0.0625 = 4.0

24. 24 Reactions Le Châtelier’s Principle Smith. General Organic & Biological Chemistry 2nd Ed. If a chemical system at equilibrium is disturbed or stressed, the system will react in a direction that counteracts the disturbance or relieves the stress. Some of the possible disturbances: 1) Concentration changes 2) Temperature changes 3) Pressure changes

25. 25 Reactions Le Châtelier’s Principle Smith. General Organic & Biological Chemistry 2nd Ed.