2 Oxidation NumbersWe’ve talked about oxidation nos. before. What is theDefinition of an atoms oxidation number?The number of electrons an atom tends to give up or gain in aChemical reactionWhere can we find an elements oxidation number?What are the oxidation nos. for O and S in the compound SO2 ?Looking at the reference table what is oxygen’s oxidation no.If Oxygen is -2 Sulfur must be +4 (Why?)
3 Oxidation NumbersBut what if we had looked up the oxidation no. of Sulfur first?The table say’s sulfurs most common oxidation no. is -2 soOxygen would be +1 (why).But that can’t be. There is no +1 for oxygen.So how do we figure this out?There are a few RULES we need to learn when assigningOxidation Numbers to atoms in compounds.
4 Ox. Number Rules Rule 1: Uncombined atoms are always 0 Rule 2: Group 1 atoms are alwaysRule 3: Group 2 atoms are always +2Rule 4: Oxygen is alwaysexception: Peroxides like H2O2, Na2O2 oxygen is -1In OF2 oxygen is (know these) +2Rule 5: Fluorine is alwaysRule 6: The other Halogens are “usually” - 1Rule 7: Hydrogen combined with non-metals +1example: HBrRule 8: Hydrogen combined with metals (hydrides) - 1example: CaH2Rule 9: Total of the charges for all atoms in a compound is zero (0)Rule 10: Total of the charges for all atoms in a polyatomic is equalto the charge of the polyatomic ion
5 Oxidation NumbersExample: What is the oxidation number of each element in thefollowing compound.SrCl2Sr is group 2 so it always has an oxidation no. of +2The sum of all the charges of a compound is zero sothe 2 Cl must be -2.Therefore each of the Cl has an Ox. Number of -1
6 Oxidation NumbersExample: What is the oxidation number of each element in thefollowing compound.PbCrO4First you need to recognize that CrO4 is a polyatomic witha charge of -2Since Oxygen is always -2, Cr must be +6 (why?)Since CrO4 has a charge of -2, Pb must be +2
7 Oxidation NumbersProblem: What is the oxidation number of each element inNH4NO3In this compound there are 2 polyatomics (NH4+, NO3-)NH4+:H +1 (Always)N - 3 why?NO3-:O - 2 (Always)N +5 why?
8 Find the oxidation #’s for each element in the compound H2S2O7 H is always +1 when bonded to a non-metalO is always -2 (except in H2O2 and OF2)To find Sulfur, the sum of charges must equal 02 (+1) + 2X + 7 (-2) = 0X = 02X = 02X = 12X = 6Sulfur = +6
9 RedoxWe’re now going to discuss a topic that involves something calledOxidation & Reduction- These reactions are commonly known asREDOX REACTIONSSO … what is oxidation and what is reduction?Originally oxidation referred to any reaction in whichOxygen was one of the reactants.Ex: 4Fe + 3O2 2Fe2O32Cu + O2 2CuOCH4 + 2O2 CO2 + 2H2O
10 Redox Opposite of Oxidation is Reduction . Originally, this referred to any reaction in which Oxygen was removed from a reactant.Ex: 2Fe2O C 4Fe + 3CO2CuO + H2 Cu + H2OToday, oxidation & Reduction still refer to these type ofreactions but both have taken on a much broader meaningRecall that Oxygen is a very electronegative element- EN = 3.5 on a scale of 0 - 4- This means it very strongly draws electrons to itself(how many electrons does it want ?)Right… 2e- : O + 2e- O-2
11 Redox Since Oxygen wants electrons, substances that react with oxygen Have to lose electrons- Cu: + O Cu+2O-2From this comes the broader definition of Oxidation/ReductionOxidation is the process by which substances lose 1 or more e-Reduction is the process by which substances gain 1 or more e-HOW CAN WE REMEMBER THIS?- Remember the phrase OIL RIGOxidation Is Lose of electrons / Reduction Is Gain of electrons
12 Redox NOTE: Oxidation and reduction ALWAYS occur together Electrons can’t just be lost they need to be accepted by someOther substance.Metals are typically oxidized (lose electrons)Na Na+ +1e-Non-metals are typically reduced (gain electrons)Cl + 1e- Cl-
13 Half-cell reactionsThe Equation that show how an element either gains or losesElectrons is called a HALF CELL reactionFor example what is oxidized and what is reduced in thefollowing equation and what are the 2 half cell reactions?Fe + S FeSsulfur is reduced (why?)S0 + 2e- S-2 This is known as the REDUCTION half celliron is oxidized (why?)Fe0 Fe+2 + 2e- This is known as the OXIDATION half cell
14 Half cell rxnsOxidation half cell Rxns lose electrons (they shows up on the product side)2 F- F2 (g) + 2 e - Mn+2 (s) Mn e –Reduction half cell Rxns gain electrons (they show up on the reactant side)F2(g) + 2 e - 2 F- Mn e - Mn+2 (s)Oxidation OxidationReduction Reduction
15 Reducing Agent When something is Oxidized: It’s called the Reducing AgentWhen something is oxidized itLoses electrons (“OIL”)Something that accepts theseElectrons is reduced (“RIG”)Therefore what is oxidizedFacilitates the reduction ofSomething elsemetalsNon-metalsSo … It’s the Reducing AGENT!
16 Reducing agent Let’s look at an example of a reducing agent at work Fe + O2 Fe2O3What is the Reducing Agent? And what is Reduced?The first question to ask is which is “WHAT’S OXIDIZED?”Iron is losing electrons: Fe0 Fe+3 so Fe is oxidized.But Fe has to give it’s electrons to something else. What is it?Oxygen. So O2 is reduced. Therefore, Fe is the Reducing Agent
17 Oxidizing AgentWe said that when something is Oxidized it’s the Reducing Agent.So… When something is Reduced It’s called the Oxidizing Agent.We know that when something GAINS electrons it’s reducedThose electrons must come from another Atom or Ion.The atom or ion that lost those electrons is therefore oxidized.For example: 2H2 + O2 H20 (what is the Oxidizing agent?)Oxidation half cell: H0 H e-Reduction half cell: O0 + 2e- O-2Since Oxygen is reduced; it’s the “agent” that oxidizes Hydrogen,so it’s the Oxidizing agent.
18 Problem: for the following Reaction C(s) + Cl2(g) CCl4 (l)A) What is reduced?B) What gets oxidized?C) What is the reducing agent?D) What is the oxidizing agent?Problem: for the following Reaction2 MgO 2 Mg + O2What is the oxidizing agent?Write the Oxidation and Reduction Half cell reactions.Cl0C0Mg+2O-2
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