Presentation is loading. Please wait.

Presentation is loading. Please wait.

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!”

Similar presentations


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:

1 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!”

2 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.

3 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)

4 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

5 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

6 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?

7 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

8 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)

9 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

10 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 -

11 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

12 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!

13 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)

14 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. 

15 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

16 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)


Download ppt "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!”"

Similar presentations


Ads by Google