Oxidation-Reduction Reactions 20.1 The meaning of Oxidation and Reduction 20.2 Oxidation Numbers 20.3 Balancing Redox Reactions Updated 6/26/2019 Created by C. Ippolito July 2007

The Meaning of Oxidation and Reduction Objectives Define oxidation and reduction in terms of loss or gain of oxygen and the loss or gain of electrons State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent Describe what happens to iron when it corrodes Updated 6/26/2019 Created by C. Ippolito July 2007

What Are Oxidation and Reduction? gaining oxygen in a reaction loss of electrons in a reaction OIL oxidation is loss Mg  Mg+2 + 2e- substance losing electrons is the Reducing Agent Reduction losing oxygen in a reaction gain of electrons in reaction RIG reduction is gain S + 2e-  S-2 substance gaining electrons is the Oxidizing Agent Updated 6/26/2019 Created by C. Ippolito July 2007

Created by C. Ippolito July 2007 Corrosion Iron corrodes by being oxidized to iron ions in a reaction with oxygen 2Fe(s) + O2(g) + H2O(ℓ)  2Fe(OH)2(s) Resistance to Corrosion Aluminum corrodes to form aluminum oxide which is “close” forms a protective layer stopping further corrosion Controlling Corrosion Coat Surface painting, oiling, plastic coating Sacrifice a More Active Metal magnesium or zinc on hulls of boats magnesium or zinc on underground pipelines Updated 6/26/2019 Created by C. Ippolito July 2007

Created by C. Ippolito July 2007 Oxidation Numbers Objectives Determine the oxidation number of an atom of any element in a pure substance Define oxidation and reduction in terms of a change in oxidation number and identify atoms being oxidized or reduced in redox reactions Updated 6/26/2019 Created by C. Ippolito July 2007

Assigning Oxidation Numbers positive or negative number assigned to an atom to designate its degree of oxidation or reduction generally the charge possessed if electrons in a bond are assigned to the more electronegative element Updated 6/26/2019 Created by C. Ippolito July 2007

Rules for Assigning Oxidation Numbers In monatomic ions it is equal in magnitude and sign to its ionic charge. Hydrogen is usually +1 except in metal hydrides (NaH) where it is -1 Oxygen is usually -2 except in peroxides (H2O2) where it is -1 or with fluorine where it is positive Elemental atoms (uncombined) are 0, such as Na, N2, O2 In all compounds the sum of oxidation numbers of its atoms must equal 0 In polyatomic ions the sum of oxidation numbers of its atoms must equal the ionic charge of the ion Updated 6/26/2019 Created by C. Ippolito July 2007

Oxidation-Number Changes in Chemical Reactions An increase in oxidation number indicates OXIDATION A decrease in oxidation number indicates REDUCTION Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Reactions Objectives Describe how oxidation numbers are used to identify redox reactions Balance a redox equation using oxidation-number-change method Balance a redox equation by breaking the equation into oxidation and reduction half-reactions, and then using the half-reaction method Updated 6/26/2019 Created by C. Ippolito July 2007

Identifying Redox Reactions electrons are transferred from one species to another Single Replacement Reactions Direct Combination (Synthesis) Reactions Decomposition Reactions Combustion Reactions Non Redox Reactions electrons are NOT transferred from one species to another Double Replacement Reactions Acid-Base (Neutralization) Reactions Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Oxidation-Number Change Method Fe2O3(s) + CO(g)  Fe(s) + CO2(g) Assign oxidation numbers to all the atoms in the equation. Identify which atoms are oxidized and which are reduced. Use one bracketing line to connect atoms that are oxidized and another line to connect those that are reduced. Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Oxidation-Number Change Method Make the total increase in oxidation number equal to the total decrease in oxidation number using appropriate coefficients Make sure the equation is balanced for both atoms and charge Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Using Half Reactions equation showing just the oxidation or just the reduction that takes place in a redox reaction Write the unbalanced equation in ionic form Write separate half-reactions for the oxidation and reduction processes Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Using Half Reactions Write separate half-reactions for the oxidation and reduction processes Oxidation: Reduction: Balance the atoms in the half-reactions Add enough electrons to balance the charges Multiply each half reaction to make number of electrons in each equal Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Using Half Reactions Add the balanced half-reactions to show the overall reaction and subtract terms on both sides Updated 6/26/2019 Created by C. Ippolito July 2007

Balancing Redox Equations: Using Half Reactions Add the spectator ions and balance the equation Updated 6/26/2019 Created by C. Ippolito July 2007