Presentation on theme: "Introduction to Oxidation-Reduction Reactions"— Presentation transcript:
1Introduction to Oxidation-Reduction Reactions Electron Transfer Reactions
2Types of Chemical Reactions There are four types of chemical reactions:Acid/BasePrecipitation/SolubilityComplex Formation/Complex DissociationOxidation/ReductionAny chemical reaction consists of one (or more) of these basic categories.
3Oxidation/Reduction Reactions Acid/Base reactionsinvolve a donation /acceptance of protonsPrecipitation/ Solubility reactionsinvolve a donation/ acceptance of negative chargewhat is being donated and accepted in a redox reaction?
4Oxidation/Reduction Reactions Electrons!Consider the reaction taking place in a disposable battery: Zn + 3MnO2 Mn3O4 + 2ZnO How can you tell that electrons are being donated and accepted? Which species is donating electron( s) and which is accepting electron (s)?
5REDOX REACTIONSRedox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor.
6REDOX REACTIONSTransfer leads to—increase in oxidation number of some element = OXIDATION2. decrease in oxidation number of some element = REDUCTION
7Electron Transfer in Redox Reactions OxidationLoss of electronsGain in oxygenReductionGain of electronsLoss of oxygen“LEO the lion goes Ger”
8Mg S Mg 2+ S2- Example The reaction of a metal and non-metal All the electrons must be accounted for!MgSMg 2+S2-+→+
9Oxidation-ReductionOxidation means an increase in oxidation state - lose electrons.Reduction means a decrease in oxidation state - gain electrons.The substance that is oxidized is called the reducing agent.The substance that is reduced is called the oxidizing agent.
10Assigning Oxidation States An Oxidation-reduction reaction involves the transfer of electrons.You should memorize these rules
11Rules for Oxidation States The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.The oxidation state of elements in their standard states is zero.Example: Na, Be, K, Pb, H2, O2, P4 = 0
12Assigning Oxidation States Oxidation state for monatomic ions are the same as their charge.Example: Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2Oxygen is assigned an oxidation state of -2 in its covalent compounds except as a peroxide.
13Rules for Oxidation States The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1.Group IA metals are +1, IIA metals are +2 and fluorine is always –1.The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.
14Practice in Oxidation States Assign the oxidation states to each element in the following.K2SO4NO3-H2SO4Fe2O3Fe3O4
15Identify the Oxidizing agent Reducing agent Substance oxidized Substance reducedOn the worksheet
16Types of Oxidation-Reduction Reactions Combination ReactionA + B C+4-2S + O SO2Decomposition ReactionC A + B+1+5-2+1-12KClO KCl + 3O2
17Types of Oxidation-Reduction Reactions Displacement Reaction a.k.a Single ReplacementA + BC AC + B+1+2Sr + 2H2O Sr(OH)2 + H2Hydrogen Displacement+4+2TiCl4 + 2Mg Ti + 2MgCl2Metal Displacement-1-1Cl2 + 2KBr KCl + Br2Halogen Displacement
18The Activity Series for Metals Hydrogen Displacement ReactionM + BC AC + BM is metalBC is acid or H2OB is H2Ca + 2H2O Ca(OH)2 + H2
19Copper Demonstration Copper Pennies reacting with nitric acid. Can you figure out the equation?
20Types of Oxidation-Reduction Reactions Disproportionation ReactionElement is simultaneously oxidized and reduced.+1-1Cl2 + 2OH ClO- + Cl- + H2OChlorine Chemistry
21Classify the following reactions. Ca2+ + CO CaCO3NH3 + H NH4+Zn + 2HCl ZnCl2 + H2Ca + F CaF2
22Half-ReactionsAll redox reactions can be thought of as happening in two halves.One produces electrons - Oxidation half.The other requires electrons - Reduction half.
23Half-Reactions Write the half reactions for the following. Na + Cl2 → Na+ + Cl-SO3- + H+ + MnO4- → SO4- + H2O + Mn+2
24Balancing Redox Equations In aqueous solutions the key is the number of electrons produced must be the same as those required.For reactions in acidic solution an 8 step procedure.
25Balancing Redox Equations Write separate half reactionsFor each half reaction balance all reactants except H and OBalance O using H2O
26Acidic SolutionBalance H using H+Balance charge using e-
27Acidic Solution Multiply equations to make electrons equal Add equations and cancel identical speciesCheck that charges and elements are balanced.
28Practice Balance the following reactions: Sn 2+ (aq) + 2Fe 3+ → Sn 4+ (aq) + 2Fe 2+MnO4- (aq) + C2O4-2 (aq) → Mn2+ (aq) + CO2 (g)
30Now for a tough oneFe(CN)6-4 + MnO4- ® Mn+2 + Fe+3 + CO2 + NO3-
31Basic SolutionDo everything you would with acid, but add one more step.Add enough OH- to both sides to neutralize the H+CrI3 + Cl2 ® CrO4- + IO4- + Cl-CN MnO4- → CNO- + MnO2
32Redox Titrations Same as any other titration. the permanganate ion is used often because it is its own indicator. MnO4- is purple, Mn+2 is colorless. When reaction solution remains clear, MnO4- is gone.Chromate ion is also useful, but color change, orangish yellow to green, is harder to detect.