Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 1 Chapter 6 Chemical Reactions 6.5 Oxidation-Reduction Reactions

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 2 Oxidation and reduction Are an important type of reaction. Provide us with energy from food. Provide electrical energy in batteries. Occur when iron rusts. 4Fe + 3O 2 2Fe 2 O 3 Oxidation and Reduction

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 3 An oxidation-reduction reaction involves the transfer of electrons from one reactant to another. In oxidation, electrons are lost. Zn Zn e - (loss of electrons) In reduction, electrons are gained. Cu e - Cu (gain of electrons) Electron Loss and Gain

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 4 Half-Reactions for Oxidation- Reduction In the oxidation-reduction reaction of zinc and copper(II) sulfate, the zinc is oxidized and the Cu 2+ (from Cu 2+ SO 4 2- ) is reduced. Zn Zn e - oxidation Cu e - Cureduction

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 5 Identify each of the following as an 1) oxidation or a 2) reduction: __A. Sn Sn e- __B. Fe e - Fe 2+ __C. Cl 2 + 2e - 2Cl - Learning Check

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 6 Identify each of the following as an 1) oxidation or a 2) reduction: 1 A. Sn Sn e- 2 B. Fe e - Fe 2+ 2 C. Cl 2 + 2e - 2Cl - Solution

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 7 In a balanced oxidation-reduction equation, the loss of electrons is equal to the gain of electrons. Zn + Cu 2+ Zn 2+ + Cu The loss and gain of two electrons is shown in the separate oxidation and reduction reactions. Zn Zn e - oxidation Cu e - Cu reduction Balanced Red-Ox Equations

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 8 In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction. uv light Ag + + Cl - Ag + Cl A. Which reactant is oxidized? 1) Ag + 2) Cl - 3) Ag B. Which reactant is reduced? 1) Ag + 2) Cl - 3) Cl Learning Check

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 9 In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction. uv light Ag + + Cl – Ag + Cl A. Which reactant is oxidized 2) Cl – Cl – Cl + e – B. Which reactant is reduced? 1) Ag + Ag + + e – Ag Solution

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 10 Write the separate oxidation and reduction reactions for the following equation. 2Cs + F 2 2CsF Learning Check

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 11 Write the separate oxidation and reduction reactions for the following equation. 2Cs + F 2 2CsF Cs Cs + + 1e – oxidation F + 1e – F - reduction Solution

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 12 An early definition of oxidation is the addition of oxygen O 2 to a reactant. A metal or nonmetal is oxidized while the O 2 is reduced to O 2-. 4K + O 2 2K 2 O C + O 2 CO 2 2SO 2 + O 2 2SO 3 Oxidation with Oxygen

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 13 In organic and biological reactions, oxidation involves the loss of hydrogen atoms and reduction involves a gain of hydrogen atoms. oxidation = Loss of H reduction = Gain of H CH 3 OH H 2 CO + 2H (loss of H) Methanol Formaldehyde Gain and Loss of Hydrogen

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 14 Summary

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 15 Learning Check Identify the substances that are oxidized and reduced in the following reactions. A. 4Fe + 3O 2 2Fe 2 O 3 B. 6Na + N 2 2Na 3 N C. 2K + I 2 2KI

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 16 Solution A. 4Fe + 3O 2 2Fe 2 O 3 Fe oxidized; O 2 reduced B. 6Na + N 2 2Na 3 N Na oxidized: N 2 reduced C. 2K + I 2 2KI K oxidized: I 2 reduced

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 17 Chapter 6 Chemical Reactions 6.6 Energy in Chemical Reactions 6.7 Rate of Reaction

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 18 Reaction Conditions A chemical reaction occurs when the reacting molecules collide. Collisions between molecules must have sufficient energy to break the bonds in the reactants. Once the bonds between atoms of the reactants are broken, new bonds can form to give the product.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 19 Chemical Reactions In the reaction H 2 + I 2 2 HI, the bonds of H 2 and I 2 must break, and bonds for HI must form. H 2 + I 2 collision bonds break 2HI

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 20 Activation Energy The activation energy is the minimum energy needed for a reaction to take place. When a collision has the energy that is equal to or greater than the activation energy, reaction can occur.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 21 Exothermic Reactions The heat of reaction is the difference in the energy of the reactants and the products. An exothermic reaction releases heat because the energy of the products is less that the reactants.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 22 Endothermic Reactions In an endothermic reaction, heat is absorbed because the energy of the products is greater that that of the reactants.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 23 Learning Check Identify each reaction as 1) exothermic or 2) endothermic A. N 2 + 3H 2 2NH kcal B. CaCO kcal CaO + CO 2 C. 2SO 2 + O 2 2SO 3 + heat

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 24 Solution Identify each reaction as 1) exothermic or 2) endothermic 1 A. N 2 + 3H 2 2NH kcal 2 B. CaCO kcal CaO + CO 2 1 C. 2SO 2 + O 2 2SO 3 + heat

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 25 Rate of Reaction The rate of a reaction is the speed at which product forms. Adding more of the reactants speeds up a reaction by increasing the number of collisions that occur. Raising the temperature speeds up a reaction by providing the energy of activation to more colliding molecules.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 26 Factors that Increase Rate

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 27 Effect of Catalysts A catalyst speeds up the rate of a reaction by lowering the energy of activation. Then more collisions can result in reaction and the formation of products. A catalyst is not used up during the reaction.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 28 Learning Check State the effect of each on the rate of reaction. 1) increases rate2) decreases rate 3) does not change the rate A. Increasing the temperature. B. Removing some of the reactants. C. Adding a catalyst. D. Placing the reaction flask in ice.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 29 Solution State the effect of each on the rate of reaction. 1) increases rate2) decreases rate 3) does not change the rate 1 A. Increasing the temperature 2 B. Removing some of the reactants 1 C. Adding a catalyst 2 D. Placing the reaction flask in ice

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 30 Chapter 6 Chemical Reactions 6.8Chemical Equilibrium

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 31 Reversible Reactions A reversible reaction has both forward and reverse reactions. If SO 2 and O 2 are present initially, they collide and the forward reaction begins. 2SO 2 + O 2 2SO 3 As the amount of product increases, SO 3 molecules collide and form the reactants. This is shown as a double arrow. forward 2SO 2 + O 2 2SO 3 reverse

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 32 Chemical Equilibrium In chemical equilibrium The rate of the forward reaction becomes equal to the rate of the reverse reaction. There is no further change in the amounts of reactant and product. Reactions continue at equal rates in both directions.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 33 Reaching Chemical Equilibrium A container filled with SO 2 and O 2 or only SO 3 eventually contains mostly SO 3 and small amounts of O 2 and SO 3. Equilibrium is reached in both situations.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 34 Equilibrium can Favor Product If equilibrium is reached after most of the forward reaction has occurred, the system favors the product.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 35 Equilibrium Can Favor Reactant If equilibrium is reached when very little of the forward reaction has occurred, the system favors the reactants.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 36 LeChâtelier’s Principle For a system at equilibrium, a change in the amounts of reactants or products or the temperature causes stress. LeChâtelier’s principle states that the equilibrium will shift to relieve the stress. That means that the rate of the forward and reverse reaction will change until they are equal again.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 37 Effect of Adding Reactant Consider the equilibrium system for the reaction H 2 (g) + F 2 (g) 2HF(g) If one of the reactants (H 2 or F 2 ) is added, there is an increase in the number of collisions. To remove this stress, the rate of the forward reaction increases and forms more HF product. Because more HF is produced, the effect of adding a reactant shifts the equilibrium towards the products.

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 38 Effect of Adding Product If more of the product HF is added, there is an increase in collisions of HF molecules. To remove this stress, the rate of the reverse reaction increases and forms more H 2 and F 2 reactants. The effect of adding a product shifts the equilibrium towards the reactants. H 2 (g) + F 2 (g) 2HF(g) Adding HF Adding H 2

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 39 Effect of Removing Reactant If some reactant, H 2 or F 2, is removed, there are fewer collisions between reacting molecules. The rate of the forward reaction decreases. Removing a reactant shifts the equilibrium towards the reactants. H 2 (g) + F 2 (g) 2HF(g) Removing H 2 or F 2

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 40 Effect of Removing Product If some product HF is removed, the rate of the reverse reaction decreases. Removing some product shifts the equilibrium towards the products. H 2 (g) + F 2 (g) 2HF(g) Removing HF

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 41 Effect of Temperature When the temperature for an exothermic reaction is lowered (heat removed), the equilibrium shifts towards the products. When the temperature of an exothermic reaction is raised (heat added), the equilibrium shifts towards the reactants. N 2 (g) + 3H 2 (g) 2NH 3 (g) + 22 kcal Adding heat Removing heat

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 42 Effect of Temperature When the temperature is lowered (heat removed) for an endothermic reaction, the equilibrium shifts towards the reactants. When the temperature is raised (heat added) for an endothermic reaction, the equilibrium shifts towards the products. CaCO kcal CaO + CO 2 Removing heat Adding heat

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 43 Summary of Changes on Equilibrium

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 44 Learning Check Indicate the effects of the changes on equilibrium for N 2 (g) + O 2 (g) + heat 2 NO(g) 1) Shifts towards the products 2) Shifts towards the reactants A. Adding NO B. Adding N 2 C. Raising the temperature D. Removing O 2

Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 45 Solution Indicate the effects of the changes on equilibrium for N 2 (g) + O 2 (g) + heat 2 NO(g) 1) Shifts towards the products 2) Shifts towards the reactants 2 A. Adding NO 1 B. Adding N 2 1 C. Raising the temperature 2 D. Removing O 2