Presentation on theme: "A very, very, very bad joke… A cation and an anion were walking down the street… …and the cation says to the anion, “Whoa, I think I lost an electron!”"— Presentation transcript:
A very, very, very bad joke… A cation and an anion were walking down the street… …and the cation says to the anion, “Whoa, I think I lost an electron!” The anion replies, “Are you sure?” …to which the cation replies, “Yes…I’m POSITIVE!”
Cations and Anions and Electrons, Oh My! Al 0 Al 3+ ____ protons____ electrons Al 0 has lost 3 electrons. + 3e - Al 0 has been oxidized: Its oxidation state has increased from 0 3. Al 3+ Al 0 ____ protons____ electrons Al 3+ has gained 3 electrons. + 3e - Al 3+ has been reduced: Its oxidation state has been reduced from 3 0.
Redox Reactions: A New Type of Reaction All chemical reactions can be categorized, roughly into: Acid-base reactions – depending on your definition Oxidation-Reduction (redox) reactions – exchange of electrons Al 0 Al e - I e - 2I - Displacement reactions – exchange of ions Acid-base reactions – exchange of H + or OH - NaBr (aq) + AgNO 3 (aq)NaNO 3 (aq) + AgBr (s)HBr (aq) + LiOH (aq)LiBr (aq) + H 2 O (l)
Redox Rules! Calculating Oxidation States 1. Oxidation states for elements are always zero. e.g. N 2 (g), Co(s), H 2 (g), C(gr), P 4 (s), Na(s), Ne(g) 2. Oxidation states for monatomic ions are the same as their charges. e.g. Al 3+, Cd 2+, Zn 2+, Ag +,and Group IA(1) and IIA(2) ions 3. The oxidation state of oxygen is usually 2-, except for peroxides (-1). e.g. Na 2 O, Al 2 O 3 ; compare with H 2 O 2, Na 2 O 2 – remember, O The oxidation state of hydrogen is usually 1+, except for hydrides (-1). e.g. H 2 O, OH - ; compare with NaH, LiAlH 4 – look for H bonded to metal 5. The oxidation state of halides are 1-, except when bonded to oxygen. e.g. HF, NaCl, MgBr 2 (F is always 1-); compare with ClO The oxidation numbers MUST add up to the molecule’s total charge. O H H
More Practice with Oxidation States Cr 2 O 7 2- Cr = ? O = -2 2(?) + 7(-2) = -2 Cr = +6 dichromate TiCl 4 4 Cl = 4(-1) Ti = +4 titanium(IV) chloride H2SO3H2SO3 S = +4 sulfurous acid BaO 2 2 O = 2(-1) barium peroxide P2O5P2O5 2 P = 2(+5) diphosphorus pentoxide
Back Again?! Aluminum and Iodine Al (s) + I 2 (s)AlI 3 (s) 0 2(0) 3(-1) +3 Al:0+3I:0-1 So, now that you know oxidation states… what is oxidized, and what is reduced?
Mnemonic Devices for Remembering Redox Rules LEO the lion goes GER OILOIL GER LEO Oxidation Is Loss RIGRIG Reduction Is Gain When an atom/molecule Loses Electrons, it is Oxidized. -its oxidation state becomes more positive -it is the reducing agent, or reductant When an atom/molecule Gains Electrons, it is Reduced. -its oxidation state becomes more negative -it is the oxidizing agent, or oxidant
Al (s) + I 2 (s)AlI 3 (s) Back For Good!! Aluminum and Iodine 0 2(0) 3(-1) +3 Al:0+3I:0-1 oxidation state increases loses electrons Al is oxidized Al is the reducing agent oxidation state decreases gains electrons I 2 is reduced I 2 is the oxidizing agent 2 Al (s) + 3 I 2 (s) 2 AlI 3 (s)
Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS! Oxidizing agents (oxidants) are reactants that are REDUCED. Reducing agents (reductants) are reactants that are OXIDIZED! Zn (s) + 2H + (aq)H 2 (g) + Zn 2+ (aq) 0 2(+1) 2(0) +2 Znloses/gains electrons Znis oxidized/reduced Znis the reductant/oxidant H+H+ loses/gains electrons H+H+ is oxidized/reduced H+H+ is the reductant/oxidant
Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS! Oxidizing agents (oxidants) are reactants that are REDUCED. Reducing agents (reductants) are reactants that are OXIDIZED! 2 H 2 O (l) + Al (s) + MnO 4 - (aq)Al(OH) 4 - (aq) + MnO 2 (s) What is oxidized? What is reduced? What is the oxidizing agent?What is the reducing agent? How many electrons are transferred? Al (s) MnO 4 - (aq) Al (s) 3 e -
Practical Applications: Electroplating AgNO 3 (s)Ag + (aq) + NO 3 - (aq) H2OH2O Ag + (aq) + e - Ag (s) Electroplating deposits coats of metal onto objects Bronze baby shoes Cu + (aq) + e - Cu (s) Recoating Tarnished objects
More Practical Applications: Photosynthesis Photosystem II 1. Chlorophyll in P680 molecule absorbs light. 2. P680 gives up electrons to a transport chain, becoming positive. 3. Electrons from 2 H 2 O molecules are given to P680 by a Mn 4 center. The Mn 4 center works because Mn 2+, Mn 3+, Mn 4+, and Mn 5+ are all accessible oxidation states. 2 H 2 O O H e - H 2 O is a reducing agent!
More Practical Applications: Thermite Reaction Fe 2 O 3 (s) + Al (s)Al 2 O 3 (s) + Fe (l) Railroad track repairs Underwater Welding 2 Fe (l) + 3H 2 O (l)Fe 2 O 3 (s) + 3H 2 (g)
More Practical Applications: Thermite Reaction Fe 2 O 3 (s) + Al (s)Al 2 O 3 (s) + Fe (l) 2(+3) iron(III) oxide 0 0 2(+3) Iron(III) oxide is the oxidizing agent. Aluminum oxide is the reducing agent.
Limits to Redox Reactions What is the oxidation number of S in the following compounds? H2SH2S S8S8 SCl 2 Na 2 SO 3 SO Which cannot be an oxidizing agent? Which cannot be a reducing agent? (reduced) (oxidized) H 2 S – cannot gain more electrons SO 4 2- – cannot lose more electrons
Disproportionation and Conproportionation These are special types of redox reactions: Disproportionation – One compound decomposes into two oxidation states H 2 O 2 (aq)H 2 O (l) + O 2 (g) 2(-1)-22(0) H 2 O 2 is BOTH oxidized and reduced! Conproportionation – Two oxidation states become one. catalyst Cr 1+ (aq) + Cr 3+ (aq)2 Cr 2+ (aq)