Ppt16a, Oxidation-Reduction Reactions 1.Overview, Importance 2.Definitions, Language Issues – Oxidation and “variants”: to get oxidized; to oxidize (somebody); oxidizing agent – Reduction and “variants”: to get reduced; to reduce (somebody); reducing agent 3.Concept of Oxidation Numbers (States) – For when species are not monatomic 4.How to Assign Oxidation Numbers 5.How to Use Oxidation Numbers to Assess Redox and determine the oxidizing agent and/or reducing agent 1 Ppt16a

Oxidation-Reduction (Redox) Reactions Electron-transfer reactions —One species gives electron(s) to another species —One species gains electrons and one loses —One species gets more negative and one gets more positive  Which is which? You should be able to figure this out; electrons are negative! Example from lab! Cu 2+ + Al → Cu + Al 3+ Answer the Q’s above for the reaction represented. Of these two species: 2 Ppt16a Cu 2+ (0 < +2) Cu 2+ Al (it gets more positive)

Importance? Redox reactions involve movement of electrons. – Movement of electrons = electric current! 3 Ppt16a – Redox reactions are what “drive” current in batteries! Makes our SmartPhones, mp3 players, etc. work! As a battery dies, reactants are getting used up and products are forming. Recharging a battery is making the reverse reaction occur! – No other kind of chemical reaction can be used to make a battery. Very important practically!

Definitions (Part I) OIL RIG – “Oxidation is loss (of electrons)” – “Reduction is gain (of electrons)” LEO goes GER also works – “Loss of electrons is oxidation” – “Gain of electrons is reduction” Oxidation and Reduction are both processes – If a species undergoes oxidation, it loses electrons (and therefore becomes more positive) – If a species undergoes reduction, it gains electrons (and therefore becomes more negative) 4

Back to Example Cu 2+ + Al → Cu + Al 3+ Which species underwent reduction? (i.e., which one gained electrons?) Ans. Cu 2+ ; it got more negative. Ans. Al; it got more positive. Which species underwent oxidation? (i.e., which one lost electrons?) 5 Ppt16a

Oxidation-Reduction Reactions— Language Issues (going a bit farther) 1b) A reducing agent is a species that reduces something else. 1a) To reduce (something) means to “give electron(s) to (it)” 1c) If A gets reduced (by B), then A has electrons given to it (by B) 1d) If A undergoes reduction (by B), then A has electrons given to it (by B) b/c that will reduce its charge “agent”: person or thing that does something (“acts”) 6Ppt16a

– The species that undergoes reduction (i.e., gets reduced) is the oxidizing agent It gains electrons, which means it takes them from somebody (who thus gets oxidized) NOTE! Based on the prior “language analysis”: – The species that undergoes oxidation (i.e., gets oxidized) is the reducing agent It loses electrons, which means it gives them to somebody (who thus gets reduced) 7 Ppt16a

One last time (all together) Cu 2+ + Al → Cu + Al 3+ Who got more positive (in going from being a R to a P) ? Did that species lose (give) or gain (take) electrons? Did that species reduce or oxidize somebody? Is that species the reducing agent or the oxidizing agent? Did that species undergo reduction or oxidation? 8 Ppt16a

Oxidation Numbers (States) and application If all redox reactions involved monatomic species, identification of redox reactions (and ox agent and red agent) would be fairly straightforward. – Figure out who got more positive and who more negative and do as in the prior example But many redox reactions involve molecules and/or polyatomic ions: CH 4 + O 2  CO 2 + H 2 O [this is redox!] How to tell? Use oxidation numbers (states) instead of actual charge 9 Ppt16a

Oxidation Numbers are assigned to EACH ATOM Each atom now gets its own “number” Look to see which atom type (not overall species) gets more positive or negative (look at oxidation number) If an atom in a species gets more positive, we say that that species undergoes oxidation (and is the reducing agent) – Vice versa for the oxidizing agent CH 4 + O 2  CO 2 + H 2 O (on board) 10 Ppt16a

How to Assign Oxidation Numbers (from handout) 11 Ppt16a

How to Assign Oxidation Numbers (from handout, continued) 12 Ppt16a

Examples Assign oxidation numbers (states) to each atom in each of the following (work on board) : C2H6C2H6 NH 3 H2H2 C 2 O 4 2- SO 4 2- SO 3 2- Ru 2 (CO 3 ) 3 Cr 2 O 7 2- PbS 2 P4P4 Rule 2, then 3Rule 1 Rule 2, then 3 Helpful Hint, then Rule 2, 3Helpful Hint, then Rule 3 Rule 2, then 3 Rule 1 13 Ppt16a Each H is +1  N must be -3 Element  each H is 0 Each H is +1  C must be -3Each O is -2  C must be +3 Each O is -2  S must be +6 Each O is -2  S must be +4 CO 3 2-  Ru is +3; rest as prior  S 2- & Pb 4+ Each O is -2  S must be +6 Element  each P is 0

Example State the oxidizing and reducing agents in the reaction represented by the following equation. Show work and give reasoning. NOTE: Ignore coefficients when assigning oxidation numbers! Coefficients are not a part of the formula of a subtance! 5 CO + I 2 O 5 → 5 CO 2 + I C goes from +2 to +4 (i.e., it gets more positive):  C loses e - ’s  gets oxidized/reduces somebody  CO is reducing agent I goes from +5 to 0 (i.e., it gets more negative):  I gains e - ’s  gets reduced/oxidizes somebody  I 2 O 5 is oxidizing agent 14 Ppt16a

Oxidation-Reduction Reactions Simplest redox reactions are between two elements: a metal reacting with a nonmetal. –Ca (s) + Cl 2(g)  CaCl 2(s) –2Ca (s) + O 2(g)  2CaO (s) –2Na (s) + Cl 2(g)  2NaCl (s 15Ppt16a Metals tend to give electrons; nonmetals [other than the noble gases] tend to gain electrons.

Another Common Class of Redox Rxns: Single Displacement Reactions metal + acid/salt  salt + hydrogen/metal – Mg (s) + HCl (aq)  MgCl 2(aq) + H 2(g) – 2Al (s) + 6HCl (aq)  2AlCl 3(aq) + 3H 2(g) – Mg (s) + Zn(NO 3 ) 2(aq)  Mg(NO 3 ) 2(aq) + Zn (s) 16Ppt16a Many other kinds…