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

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

2. CHAPTER 6: Energy • 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 Smith. General Organic & Biolocial Chemistry 2nd Ed.

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

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

5. Energy Potential Energy Reactants Products 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 P Higher PE Unfavorable Unstable R PE P Lower PE Favorable Stable Smith. General Organic & Biolocial Chemistry 2nd Ed.

6. Energy Units of Energy • A calorie (cal) is the amount of energy needed to • raise the temperature of 1 g of water by 1 oC. • 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 Smith. General Organic & Biolocial Chemistry 2nd Ed.

7. Energy Example: Energy in a Gummy Bear 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 Smith. General Organic & Biolocial Chemistry 2nd Ed.

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

9. Energy Enthalpy Change & Bond Dissociation Energy 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). H H H + H H = +104 kcal/mol Bond formation always has negativevalues, because bond formation is exothermic (H < 0). H + H H H H = −104 kcal/mol Smith. General Organic & Biological Chemistry 2nd Ed.

10. Energy Bond Strength 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 weakerbonds. Smith. General Organic & Biological Chemistry 2nd Ed.

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

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

13. Reactions Energy Diagrams Ea, 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. Smith. General Organic & Biological Chemistry 2nd Ed.

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

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

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

17. Reactions Rates of Reactions 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 catalystis a substance that speeds up the rate of a reaction and can be recovered unchanged. • Catalysts lower Activation Energy, Ea. Smith. General Organic & Biological Chemistry 2nd Ed.

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

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

20. Reactions Equilibrium A reversible reaction can occur in either direction. The forward reaction proceeds to the right. CO(g) + H2O(g) CO2(g) + H2(g) The reverse reaction proceeds to the left. • The system is at equilibriumwhen 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. Smith. General Organic & Biological Chemistry 2nd Ed.

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

22. Reactions Equilibrium N2(g) + O2(g) 2 NO(g) [NO]2 [N2] [O2] equilibrium constant = K = *The coefficient becomes the exponent. Smith. General Organic & Biological Chemistry 2nd Ed.

23. Reactions Equilibrium HOW TO Calculate the Equilibrium Constant for a Reaction Write the expression for the equilibrium constant from the balanced equation. Step [1] [AB]2 A2 + B2 2 AB K = [A2][B2] Substitute the given concentrations in the equilibrium expression and calculate K. Step [2] [AB]2 [0.50]2 0.25 K = = = = 4.0 [A2][B2] [0.25][0.25] 0.0625 Smith. General Organic & Biological Chemistry 2nd Ed.

24. Reactions Le Châtelier’s Principle If a chemical system at equilibrium is disturbedor 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 Smith. General Organic & Biological Chemistry 2nd Ed.

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