Polarities Electronegativities: H 2.1, C & S 2.5, N & Cl 3.0, O 3.5, F 4.0 NH 3 - is the N-H bond polar? Molecule polar? SO 2 – is the S-O bond polar? Molecule polar? CS2 – is the C-S bond polar? Molecule polar? 1

Chemical Reactions Every chemical reaction contains what 3 parts, as a minimum…. Write the balanced chemical reaction of the combustion of propane, C 3 H 8 If g C 3 H 8, how many moles of H 2 O ? If g C 3 H 8, and g O 2, a) what is the limiting reactant? b) how many g of CO 2 form? c) If actual yield of CO 2 is 115 g, what is the % yield? 2

3 5.9 Oxidation and Reduction A. General Features Oxidation is the loss of electrons from an atom. Reduction is the gain of electrons by an atom. Both processes occur together in a single reaction called an oxidation−reduction or redox reaction. Thus, a redox reaction always has two components, one that is oxidized and one that is reduced. A redox reaction involves the transfer of electrons from one element to another.

4 5.9 Oxidation and Reduction A. General Features Zn + Cu 2+ Zn 2+ + Cu Zn loses 2 e – Cu 2+ gains 2 e − Zn loses 2 e − to form Zn 2+, so Zn is oxidized. Cu 2+ gains 2 e − to form Cu, so Cu 2+ is reduced.

5 5.9 Oxidation and Reduction A. General Features Zn + Cu 2+ Zn 2+ + Cu Zn loses 2 e – Cu 2+ gains 2 e − Zn loses 2 e − to form Zn 2+, so Zn is oxidized. Cu 2+ gains 2 e − to form Cu, so Cu 2+ is reduced.

6 5.9 Oxidation and Reduction A. General Features Oxidation half reaction:ZnZn e − Each of these processes can be written as an individual half reaction: Zn + Cu 2+ Zn 2+ + Cu Zn loses 2 e – Cu 2+ gains 2 e − loss of e − Reduction half reaction:Cu e − Cu gain of e −

7 5.9 Oxidation and Reduction A. General Features Zn + Cu 2+ Zn 2+ + Cu Zn acts as a reducing agent because it causes Cu 2+ to gain electrons and become reduced. A compound that is oxidized while causing another compound to be reduced is called a reducing agent. oxidizedreduced

8 5.9 Oxidation and Reduction A. General Features Zn + Cu 2+ Zn 2+ + Cu A compound that is reduced while causing another compound to be oxidized is called an oxidizing agent. Cu 2+ acts as an oxidizing agent because it causes Zn to lose electrons and become oxidized. oxidizedreduced

9 5.9 Oxidation and Reduction A. General Features

Oxidation and Reduction B. Examples of Oxidation–Reduction Reactions Iron Rusting 4 Fe(s) + 3 O 2 (g) 2 Fe 2 O 3 (s) Fe 3+ O 2– neutral Feneutral O Fe loses e – and is oxidized. O gains e – and is reduced.

Oxidation and Reduction B. Examples of Oxidation–Reduction Reactions Inside an Alkaline Battery Zn + 2 MnO 2 ZnO + Mn 2 O 3 neutral ZnMn 4+ Zn 2+ Mn 3+ Zn loses e − and is oxidized. Mn 4+ gains e − and is reduced.

Oxidation and Reduction B. Examples of Oxidation–Reduction Reactions Zn + 2 MnO 2 ZnO + Mn 2 O 3

Oxidation and Reduction B. Examples of Oxidation–Reduction Reactions Oxidation results in the:Reduction results in the: Gain of oxygen atoms Loss of hydrogen atoms Loss of oxygen atoms Gain of hydrogen atoms

14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Lecture Outline Prepared by Andrea D. Leonard University of Louisiana at Lafayette

6.1Energy 15 Energy is the capacity to ______ _________ 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. _________ is the energy of motion.

6.1 Energy 16 Chemical bonds store potential energy. Reactions that form products having lower potential energy than the reactants are favored. A compound with lower potential energy is more stable than a compound with higher potential energy.

6.1Energy A. The Units of Energy 17 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

6.2Energy Changes in Reactions 18 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. Cl To cleave this bond, 58 kcal/mol must be added. To form this bond, 58 kcal/mol is released.

6.2Energy Changes in Reactions A. Bond Dissociation Energy 19  H is the energy absorbed or released in a reaction; it is called the heat of reaction or the enthalpy change. When energy is absorbed, the reaction is said to be endothermic and  H is positive (+). When energy is released, the reaction is said to be exothermic and  H is negative (−). Cl To form this bond,  H = −58 kcal/mol. To cleave this bond,  H = +58 kcal/mol.

6.2Energy Changes in Reactions B. Calculations Involving  H Values 20  H indicates the relative strength of the bonds broken and formed in a reaction. More energy is released in forming bonds than is needed to break the bonds. When  H is negative: The bonds formed in the products are stronger than the bonds broken in the reactants. CH 4 (g) + 2 O 2 (g)CO 2 (g) + 2 H 2 O(l)  H = −213 kcal/mol Heat is released

6.2 Energy Changes in Reactions B. Calculations Involving  H Values 21 More energy is needed to break bonds than is released in the formation of new bonds. When  H is positive: The bonds broken in the reactants are stronger than the bonds formed in the products. 6 CO 2 (g) + 6 H 2 O(l)C 6 H 12 O 6 (aq) + 6 O 2 (g) ΔH = +678 kcal/mol Heat is absorbed

6.3Energy Diagrams 22 E a, the energy of activation, is the difference in energy between the reactants and the transition state.

6.3Energy Diagrams 23 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.

6.3Energy Diagrams 24 The difference in energy between the reactants and the products is the  H. If  H is negative, the reaction is exothermic:

6.3Energy Diagrams 25 If  H is positive, the reaction is endothermic:

6.4Reaction Rates A. How Concentration and Temperature Affect Reaction Rate 26 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

6.4Reaction Rates B. Catalysts 27 A catalyst is a substance that speeds up the rate of a reaction. A catalyst is recovered unchanged in a reaction, and does not appear in the product. Catalysts accelerate a reaction by lowering E a without affecting  H.

6.4Reaction Rates B. Catalysts 28 The uncatalyzed reaction (higher E a ) is slower. The catalyzed reaction (lower E a ) is faster.   H is the same for both reactions.

6.4Reaction Rates C. Focus on the Human Body: Biological Catalysts 29 Enzymes (usually protein molecules) are biological catalysts held together in a very specific three- dimensional shape. The enzyme lactase converts the carbohydrate lactose into the two sugars glucose and galactose. People who lack adequate amounts of lactase suffer from abdominal cramping and diarrhea because they cannot digest lactose when it is ingested. The active site binds a reactant, which then under- goes a very specific reaction with an enhanced rate.

6.5Equilibrium 30 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.

6.5Equilibrium A. The Equilibrium Constant 31 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). For the reaction: a A + b Bc C + d D equilibrium constant =K = [C] c [D] d [A] a [B] b = [products] [reactants]

6.5Equilibrium B. The Magnitude of the Equilibrium Constant 32 For the reaction: 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.

6.6Le Châtelier’s Principle 33 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

6.6Le Châtelier’s Principle A. Concentration Changes 34 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.

6.6Le Châtelier’s Principle A. Concentration Changes 35 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.

6.6Le Châtelier’s Principle A. Concentration Changes 36 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.