Presentation on theme: "Introduction to Oxidation-Reduction Reactions"— Presentation transcript:
1 Introduction to Oxidation-Reduction Reactions Electron Transfer Reactions
2 Types 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.
3 Oxidation/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?
4 Oxidation/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)?
5 REDOX REACTIONSRedox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor.
6 REDOX REACTIONSTransfer leads to—increase in oxidation number of some element = OXIDATION2. decrease in oxidation number of some element = REDUCTION
7 Electron Transfer in Redox Reactions OxidationLoss of electronsGain in oxygenReductionGain of electronsLoss of oxygen“LEO the lion goes Ger”
8 Mg S Mg 2+ S2- Example The reaction of a metal and non-metal All the electrons must be accounted for!MgSMg 2+S2-+→+
9 Oxidation-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.
10 Assigning Oxidation States An Oxidation-reduction reaction involves the transfer of electrons.You should memorize these rules
11 Rules 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
12 Assigning 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.
13 Rules 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.
14 Practice in Oxidation States Assign the oxidation states to each element in the following.K2SO4NO3-H2SO4Fe2O3Fe3O4
15 Identify the Oxidizing agent Reducing agent Substance oxidized Substance reducedOn the worksheet
16 Types of Oxidation-Reduction Reactions Combination ReactionA + B C+4-2S + O SO2Decomposition ReactionC A + B+1+5-2+1-12KClO KCl + 3O2
17 Types 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
18 The Activity Series for Metals Hydrogen Displacement ReactionM + BC AC + BM is metalBC is acid or H2OB is H2Ca + 2H2O Ca(OH)2 + H2
19 Copper Demonstration Copper Pennies reacting with nitric acid. Can you figure out the equation?
20 Types of Oxidation-Reduction Reactions Disproportionation ReactionElement is simultaneously oxidized and reduced.+1-1Cl2 + 2OH ClO- + Cl- + H2OChlorine Chemistry
21 Classify the following reactions. Ca2+ + CO CaCO3NH3 + H NH4+Zn + 2HCl ZnCl2 + H2Ca + F CaF2
22 Half-ReactionsAll redox reactions can be thought of as happening in two halves.One produces electrons - Oxidation half.The other requires electrons - Reduction half.
23 Half-Reactions Write the half reactions for the following. Na + Cl2 → Na+ + Cl-SO3- + H+ + MnO4- → SO4- + H2O + Mn+2
24 Balancing 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.
25 Balancing Redox Equations Write separate half reactionsFor each half reaction balance all reactants except H and OBalance O using H2O
26 Acidic SolutionBalance H using H+Balance charge using e-
27 Acidic Solution Multiply equations to make electrons equal Add equations and cancel identical speciesCheck that charges and elements are balanced.
28 Practice Balance the following reactions: Sn 2+ (aq) + 2Fe 3+ → Sn 4+ (aq) + 2Fe 2+MnO4- (aq) + C2O4-2 (aq) → Mn2+ (aq) + CO2 (g)
30 Now for a tough oneFe(CN)6-4 + MnO4- ® Mn+2 + Fe+3 + CO2 + NO3-
31 Basic 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
32 Redox 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.