Oxidation & Reduction Ch 20

Why do we call it “oxidation” ? Intro to Redox Why do we call it “oxidation” ? In the past scientists believed there was a gain or loss of oxygen. This is FALSE!

Definition of Redox Reaction Oxidation Reduction Reaction (Redox): A reaction in which electrons are transferred from one substance to another.

Definition of Redox Reaction Oxidation cannot occur without reduction Definition of Oxidized: 1. Losing of electrons (LEO) 2. Increasing the ox. number, ie: 0 to +1 or -2 to -1 Definition of Reduced: 1. Gaining of electrons (GER) 2. Reducing the ox. number, ie: -1 to -2 or 1 to 0

Redox Rules to memorize! The Statue of Liberty’s outer layer of copper has undergone redox

Oxidation Rule #1 Oxidation number of a neutral atom, molecule, or compound = zero i.e. atom: Mg, H2, Na i.e. All H2 O2 N2 Cl2 Br2 I2 F2 i.e. compounds: (the total ox#s adds up to 0 NH3 SiCl4 K3PO4 H2O

Rule #2 Oxidation number of an ion = charge of ion i.e. monotomic: Mg+2 , O–2, Al +3 , Br – i.e. Polyatomic: NH4+, CO3–2, SO4–2, NO3 –

Rule #3 The oxidation number for H is +1 Exception: when bonded to a less electronegative element (metal), then it is -1. i.e. LiH, NaH, MgH2 i.e. HF, HCl +1 -1 +1 -1 +2 -1 +1 -1 +1 -1

Rule#4 Two exceptions: peroxides and fluorine in H2O2 with OF2 Oxygen’s oxidation number is -2, in a compound Two exceptions: peroxides and fluorine in H2O2 with OF2 +1 ? = -1 where O is -1 2(+1) + 2( ?) = 0 +2 -1 where O is +2 ? + = 0 2(-1) fluorine (more electronegative)

Rule #5 The most electronegative element fluorine ALWAYS has an oxidation number of -1 when it is bonded to another element. i.e. HF

Question Time Assign oxidation numbers to each element Cl2 Cl – MgCl2 NaH HCl K3PO4 CaSO4 KF

Classwork Redox Rules Oxidation numbers of the nitrogen N2O b. NO c. N2O4 d. N2O5 e. NO2- f. NO3- 2. Oxidation number of the sulfur atom: H2S b. S c. H2SO4 d. S2- e. HS- f. SO2 g. SO3 3. Indicate the oxidation number of phosphorus in each of the following compounds: a. HPO3 b. H3PO2 c. H3PO3 d. H3PO4 e. H4P2O7 f. H5P3O10

Half Reactions

Example of a complete electron transfer reaction 2Mg + O2  2MgO Mg atom transfers two electrons to each oxygen atom. As a result Mg atoms become Mg2+ and oxygen atoms become O2- Mg loses two electrons O gains two electrons

Example of an electron transfer reaction 2Mg + O2  2MgO Mg atom transfers two electrons to each oxygen atom. As a result 2 Mg atoms become Mg2+ and two oxygen atoms become O2- Mg loses two electrons O gains two electrons

General examples of Oxidation ½ Reaction When an electron is lost (An electron is found on the product side) there is oxidation X0  X+1 + e – When an electron is gained (An electron is found on the reactant side) there is reduction e – + Y+1  Y0

Using the Rules Oxidation Number If oxidation number goes up it is oxidized. If oxidation number goes down it is reduced.

Two ways to remember oxidation/reduction (redox) L E O goes G E R O I L R I G X I D AT I O N O S E L E C T R O N S X I D AT I O N A I N L E C T R O N S E D U C T I O N S E D U C T I O N O S I N G S A I N I N G

½ Reaction Examples: start finish Ex1 Na0  Na+1 +1e – oxidized Fill in the electrons, then tell if it is oxidation or reduction. start finish Ex1 Na0  Na+1 Ex2 Fe+3  Fe+2 Ex3 N+1  N+2 Ex4 Mg+2 Mg0 +1e – oxidized 1e – + reduced +1e – oxidized 2e – + reduced start homework

Using the Rules-Last Lesson for CP Balance the reactions, write the oxidation numbers for each atom. Ex5 HCl + Zn  ZnCl2 + H2 Loss of electron oxidation (LEO) +1 -1 +2 -1 2 Gain of electron reduction (GER)

Using the Rules (last lesson this year for CP) Balance the reactions, then write oxidation # for each atom. Ex6 Mg + N2  Mg3N2 Ex7 H2S + Cl2  HCl + S Ex8 Fe + O2  Fe2O3

Summary of Oxidation/Reduction There are two definitions of oxidation and reduction: Oxidation Reduction 1._______________ 1._______________ 2._______________ 2._______________

Classwork with ½ reactions Assign oxidation numbers for each element. Then, for the following balanced redox reactions answer the following questions: 1) AgNO3 + Cu  CuNO3 + Ag Write all the oxidation numbers. Identify what charges change. Write the ½ reactions on the lines. _______________________ ________________________ (oxidation or reduction) (oxidation or reduction) Add the electrons to each ½ reaction. Identify each ½ reaction as oxidation or reduction. AgNO3 + 1e –  Ag Cu  CuNO3 + 1e – Finish classwork, then start homework.

2) Zn(s) + HCl(aq)  H2(g) + ZnCl2(aq) Write all the oxidation numbers. Write the ½ reactions. __________________ ___________________ (oxidation or reduction)(oxidation or reduction) Add the electrons to each ½ reaction. Identify each ½ reaction as oxidation or reduction.

3) SbCl5 + 2 KI  SbCl3 + I2 + 2 KCl Write all the oxidation numbers. Write the ½ reactions. __________________ ___________________ (oxidation or reduction)(oxidation or reduction) Add the electrons to each ½ reaction. Identify each ½ reaction as oxidation or reduction.

H Balancing Redox Reactions using Half-Reaction method in Acidic Conditions Assign oxidation numbers to each atom Write the 2 half reactions Balance all elements except O & H Balance O by adding H2O Balance H by adding H+ (Acidic solution) Add OH- ions to each side of the equation to neutralize H+ ions. Cancel H2O molecules. (Basic solution) Balance charge by adding e- Balance e- by multiplying by a factor Add the ½ reactions, cancel e- Check that atoms and charges balance

Ex 1: MnO4 – + Fe+2 → Fe+3 + Mn+2

Cl2 → ClO – + Cl –

Cl2 + S2O32– → Cl – + SO42–

MnO4 – + SO32– → Mn+2 + SO42–