Energy Changes, Reaction Rates and Equilibrium. Thermodynamics: study of energy, work and heat. Kinetic energy: energy of motion

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1 Energy Changes, Reaction Rates and Equilibrium Thermodynamics: study of energy, work and heat Kinetic energy: energy of motion Potential energy: energy of position, stored energy Chemical reactions involve changes in energy. Types of energy include: Heat, sound, electricity, light, motion, etc. Example: 2H + O 2 2H 2 O + energy Energy Law of conservation of energy: the total energy in a system does not change. (Energy cannot be created or destroyed during chemical reactions.) Chemical bonds store potential energy. A compound with lower potential energy is more stable than a compound with higher potential energy. Reactions that form products having lower potential energy than the reactants are favored. The Units of Energy calorie (cal): the amount of energy needed to raise the temperature of 1 g of water by 1 o C. 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

2 Energy Changes in Reactions When molecules come together and react, bonds are broken in the reactants and new bonds are formed in the products. Bond breaking always requires an input of energy. Bond formation always releases energy. To cleave this bond, 58 kcal/mol must be added. Cl Cl To form this bond, 58 kcal/mol is released. Enthalpy change (ΔH): Energy Changes in Reactions the energy absorbed or released in a reaction; it is also called the heat of reaction When energy is absorbed, the reaction is endothermic; ΔH is positive (+). When energy is released, the reaction is exothermic; ΔH is negative ( ). To cleave this bond, ΔH = +58 kcal/mol. Cl Cl To form this bond, ΔH = 58 kcal/mol. Energy Diagrams The difference in energy between reactants and the products is ΔH. If ΔH is negative, the reaction is exothermic: 2

3 If ΔH is positive, the reaction is endothermic: 7 Exothermic Reaction: Reaction that releases heat to surroundings Endothermic Reaction: Reaction that absorbs heat from surroundings Exothermic Rxn ΔH is negative ( ) Endothermic Rxn ΔH is positive (+) Summary of Energy Changes in Reactions 3

4 Practice: Identify each reaction as exothermic or endothermic, and indicate the if the ΔH is positive or negative. A. N 2 + 3H 2 2NH kcal B. CaCO kcal CaO + CO 2 Exo, -ΔH Endo, +ΔH C. 2SO 2 + O 2 2SO 3 + heat Exo, -ΔH Energy of activation (E a ): the minimum amount of energy necessary for a reaction to occur Note the Activation Energy (Ea) in Exothermic and Endothermic Rxns Exothermic Rxn Endothermic Rxn 4

5 Activation Energy (E a ) The E a is the minimum amount of energy that the reactants must possess for a reaction to occur. E a is called the energy barrier and the height of the barrier determines the reaction rate. 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. Factors that Influence Reaction Rates Temperature of Reactants Increasing the temperature increases the kinetic energy of the particles, allowing more collision to occur Concentration of Reactants The greater the concentration of reactants, the more collisions leading to a reaction will occur Presence of Catalysts Catalyst: Substance that increases rate of a reaction without being used up in the reaction Catalysts provide alternate way for reaction to occur, with a lower activation energy than the normal way 5

6 Effect of Catalyst on Activation Energy Without Catalyst (High Ea) With Catalyst (Lower Ea) The uncatalyzed reaction (higher E a ) is slower. The catalyzed reaction (lower E a ) is faster. ΔH is the same for both reactions. Chemical Equilibrium Chemical reactions can go both directions (forward and reverse) H 2 + I 2 2HI Equilibrium: Condition when rate of forward reaction equals rate of reverse reaction Equilibrium Concentrations: Unchanging concentrations of products and reactants in a reaction that is at equilibrium 6

7 The Equilibrium Constant Equilibrium constant, K: relationship between concentration of products and concentration of the reactants; concentration of products divided by concentration of reactants a A + b B c C + d D equilibrium constant = K = [products] [reactants] = [C]c [D] d [A] a [B] b Note: The coefficient becomes the exponent! N 2 + O 2 2 NO equilibrium constant = K = [NO] 2 [N 2 ] [O 2 ] What does the Equilibrium Constant Tell Us? When K is much greater than 1 (K > 1): [products] [reactants] The numerator is larger. Equilibrium favors the the products and lies to to the the left. When K is much less than 1 (K < 1): [products] [reactants] The denominator is larger. Equilibrium favors the the reactants and lies to to the the right. 7

8 When K is around 1 (0.01 < K < 100): [products] [reactants] Both are similar in magnitude. Both reactants and products are are present. 2 H 2 (g) + O 2 (g) 2 H 2 O(g) K = 2.9 x The product is favored because K > 1. The equilibrium lies to the right. Equilibrium 23 Calculating the Equilibrium Constant Example: Calculate K for the reaction between the general reactants A 2 and B 2. The equilibrium concentrations are as follows: [A 2 ] = 0.25 M [B 2 ] = 0.25 M [AB] = 0.50 M A 2 + B 2 2 AB K = [AB] 2 [A 2 ][B 2 ] 8

9 [AB] 2 K = = [A 2 ][B 2 ] [0.50] 2 [0.25][0.25] = = 4.0 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: concentration changes temperature changes pressure changes Le Châtelier s Principle: Concentration Changes 2 CO(g) + O 2 (g) 2 CO 2 (g) What happens if [CO(g)] is increased? The concentration of O 2 (g) will decrease. The concentration of CO 2 (g) will increase. 9

10 2 CO(g) + O 2 (g) 2 CO 2 (g) What happens if [CO 2 (g)] is increased? The concentration of CO(g) will increase. The concentration of O 2 (g) will increase. What happens if a product is removed? The concentration of ethanol will decrease. The concentration of the other product (C 2 H 4 ) will increase. Le Châtelier s Principle: Temperature Changes When the temperature is increased, the reaction that absorbs heat is favored. An endothermic reaction absorbs heat, so increasing the temperature favors the forward reaction. 10

11 An exothermic reaction releases heat, so increasing the temperature favors the reverse reaction. Conversely, when the temperature is decreased, the reaction that adds heat is favored. Le Châtelier s Principle: Pressure Changes When pressure increases, equilibrium shifts in the direction that decreases the number of moles in order to decrease pressure. When pressure decreases, equilibrium shifts in the direction that increases the number of moles in order to increase pressure. 11

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