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

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

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. 2

3 Energy Definition of Energy Total Energy = Potential + Energy Kinetic Energy Energy is the capacity to do work. 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. 3

4 Energy Kinetic Energy Kinetic Energy (KE) Energy of motion KE = ½mv 2 Smith. General Organic & Biolocial Chemistry 2nd Ed

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

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 o C. A joule (J) is another unit of energy. 1 cal = 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 = kj Smith. General Organic & Biolocial Chemistry 2nd Ed. 6

7 Energy Example: Energy in a Gummy Bear A gummy bear is Calories (nutritional calories). How much energy is stored in a gummy bear in units of Joules? Cal = kcal x 1000 cal = cal 1 kcal cal x J = J = kj 1 cal Gummy Bear Video: Smith. General Organic & Biolocial Chemistry 2nd Ed. 7

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 Cl Cl Endothermic To form this bond, 58 kcal/mol is released. H = 58 kcal/mol Exothermic H is the energy absorbed or released in a reaction; it is called the heat of reaction or the enthalpy change. 8

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 negative values, because bond formation is exothermic ( H < 0). H + H H H H = 104 kcal/mol 9

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 weaker bonds. 10

11 Energy Endothermic & Exothermic 11

12 Energy Diagrams For a reaction to occur, two molecules must collide with enough kinetic energy to break bonds. The orientation of the two molecules must be correct as well. 12

13 Energy Diagrams 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. 13

14 Energy Diagrams 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. H is negative, the reaction is exothermic: H is positive, the reaction is endothermic: 14

Summary Energy & Reactions E PE increases as bonds break PE decreases as bonds form ENDOTHERMIC Heat + Reactants Products Products have weaker bonds and a higher energy then Reactants.

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

16 Ex: Splitting Water 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 16

17 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 catalyst is a substance that speeds up the rate of a reaction and can be recovered unchanged. Catalysts lower Activation Energy, E a. 17

18 Catalysts The uncatalyzed reaction (higher E a ) is slower. The catalyzed reaction (lower E a ) is faster. H is the same for both reactions. 18

19 Catalysts: Photosystem II PQ + H2O --> PQH2 + O2 (g) The overall reaction of Photosystem II is the oxidation of water and the reduction of plastoquinone. 19

Equilibrium A reversible reaction can occur in either direction. The forward reaction proceeds to the right. CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) The reverse reaction proceeds to the left.

20 Equilibrium A reversible reaction can occur in either direction. The forward reaction proceeds to the right. CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) 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. 20

21 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 [products] = K = [reactants] = [C] c [D] d [A] a [B] b *Brackets, [ ], are used to symbolize concentration in moles per liter (mol/l). 21

22 Equilibrium N 2 (g) + O 2 (g) 2 NO(g) equilibrium constant = K = [NO] 2 [N 2 ] [O 2 ] *The coefficient becomes the exponent. 22

Equilibrium HOW TO Calculate the Equilibrium Constant for a Reaction Step [1] Write the expression for the equilibrium constant from the balanced equation.

23 Equilibrium HOW TO Calculate the Equilibrium Constant for a Reaction Step [1] Write the expression for the equilibrium constant from the balanced equation. A 2 + B 2 2 AB K = [AB] 2 [A 2 ][B 2 ] Step [2] Substitute the given concentrations in the equilibrium expression and calculate K. [AB] 2 K = = [A 2 ][B 2 ] [0.50] 2 [0.25][0.25] = =

24 Le Châtelier s Principle 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 24

25 Le Châtelier s Principle 25

1. The graph below represents the potential energy changes that occur in a chemical reaction. Which letter represents the activated complex?

1. The graph below represents the potential energy changes that occur in a chemical reaction. Which letter represents the activated complex? 4. According to the potential energy diagram shown above, the