Befre we can talk reactins ELECTROCHEMISTRY Oxidatin State (r Oxidatin Number): real r apparent charge f an atm in a cmpund; used fr keeping track f electrns in a redx reactin *In an inic cmpund, the charges are real (actual inic charges). In a mlecular cmpund, the charges are apparent (charges that atms wuld have if electrns were cmpletely transferred; f curse, they are nt!). Rules fr Assigning Oxidatin Numbers: 1. In free elements, each atm has an xidatin number f 0. 2. Fr mnatmic ins, the xidatin number is equal t the charge f the in. 3. The xidatin number f xygen in mst cmpunds is 2. Hwever, in hydrgen perxide (H 2O 2) and perxide in (O 2 2-), the xidatin number f xygen is 1. 4. The xidatin number f hydrgen is +1, except when it is bnded t metals f binary cmpunds (in which it has an xidatin number f 1). 5. Flurine has an xidatin number f 1 in all its cmpunds. When Cl, Br, and I ccur as halide ins, they als have xidatin numbers f 1. 6. In a neutral mlecule, the sum f the xidatin numbers f all the atms must be zer. In a plyatmic in, the sum f the xidatin numbers f all the atms must be equal t the net charge f the in. Oxidatin State Examples: N 2, P 4, Fe 3+, H 2O, H 2O 2, CH 4, NiH 2, KCl, CO 2, NO 3, MnO 4 Electrchem Ling Oxidatin: lss f electrns by an atm in a chemical reactin Reductin: gain f electrns by an atm in a chemical reactin LEO says GER Redx Reactin: chemical reactin in which electrns are transferred frm ne atm t anther (number f electrns lst number f electrns gained) Oxidizing Agent: substance that causes xidatin; it s the stuff that gets reduced! Reducing Agent: substance that causes reductin; it s the stuff that gets xidized! Nw let s apply ur understanding f redx terminlgy t the chemical reactin that ccurs in alkaline batteries. Which substance was xidized? Which substance was reduced? Which is the xidizing agent? Which is the reducing agent? Zn (s) + 2 MnO 2 (s) ZnO (s) + Mn 2 O 3 (s)
Practice Redx Reactins: Identify the elements that underg xidatin and reductin in each reactin. Which substance is the xidizing agent? The reducing agent? Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) 4Al (s) + 3 O 2 (g) 2 Al 2O 3 (s) Cu (s) + 2 AgNO 3 (aq) Cu(NO 3) 2 (aq) + 2 Ag (s) CH 4 (s) + 2 O 2 (g) CO 2 (g) + 2 H 2O (g) 4 I (aq) + 2 Cu 2+ (aq) I 2 (s) + 2 CuI (aq) Balancing Redx Reactins It s nt always pssible t balance redx reactins by inspectin. Take, fr example, the reactin belw: Au 3+ (aq) + I (aq) Au (s) + I 2 (s) At first glance, it seems that this equatin can be balanced by placing a 2 in frnt f the I. Au 3+ (aq) + 2 I (aq) Au (s) + I 2 (s) Nte, hwever, that althugh the atms are nw balanced, the charge is nt. The sum f the charges n the left is +1, and the sum f the charges n the right is zer, as if the prducts culd smehw have ne mre electrn than the reactants. T crrectly balance this equatin, it helps t lk mre clsely at the xidatin and reductin that ccur in the reactin. The idine atms are changing their xidatin number frm -1 t 0, s each idide in must be lsing ne electrn. The Au 3+ is changing t Au, s each gld(iii) catin must be gaining three electrns. The unbalanced half-reactins are: Oxidatin: 2 I (aq) I 2 (s) + 2e - Reductin: Au 3+ (aq) + 3e- Au (s) We knw that in redx reactins, the number f electrns lst by the reducing agent must be equal t the number f electrns gained by the xidizing agent; thus, fr each Au 3+ that gains three electrns, there must be three I - ins that each lse ne electrn. Therefre, the balanced redx reactin is 2 Au 3+ (aq) + 6 I (aq) 2 Au (s) + 3 I 2 (s)
Hw t Balance Redx Reactins: The Half-Reactin Methd Step 1: Separate reactin int tw half reactins (xidatin and reductin). Step 2: Balance the atms in each half reactin. First, balance all atms except H and O. Next, in acidic slutin, add H 2O t balance O atms and then add H + t balance H atms. (In basic slutin, use OH - and H 2O t balance H and O atms.) Step 3: Use electrns t balance the charges in each half reactin. Step 4: Ensure that bth half reactins have the same number f electrns. If nt, multiply by the equatin(s) by the apprpriate cefficient(s). Step 5: Recmbine the half reactins. Practice Balancing Redx Reactins: Nw yu try it! Cu + Al 3+ Cu 2+ + Al Fe 3+ + Sn 2+ Fe 2+ + Sn 4+ MnO 4 - + I - I 2 + Mn 2+ (Assume an acidic slutin)
Predicting the Spntaneity f Redx Reactins As yu already knw, remving electrns frm any atm/in requires energy. Adding electrns t any atm/in releases energy. We quantify the energy absrbed/released by individual xidatin/reductin prcesses relative t the standard hydrgen electrde (SHE): See table f Standard Reductin Ptentials. 2 H + (1.00 M) + 2e H 2 (1 atm) E 0 V E standard half-cell vltage; measures likelihd f reactin t ccur spntaneusly in the directin it is written at standard cnditins (25 C, 1 atm, 1 M cncentratins fr all aqueus slutins) E cell E x + E red Nte: Half-cell ptentials are intensive variables and thus d NOT depend n the quantity f substance xidized/reduced. 1 Vlt 1 Jule/Culmb If E cell If E cell > 0, the reactin is spntaneus in the directin written. < 0, the reactin is nt spntaneus in the directin written. Practice Prblems: Predict whether the fllwing reactins are spntaneus. Cu + 2 Ag + 2 Ag + Cu 2+ Oxidatin: Reductin: E x E red E cell 3 Cu + 2 Al 3+ 3 Cu 2+ + 2 Al Oxidatin: Reductin: E x E red E cell Yes, indeed! E and ΔG are related! ΔG nfe cell n number f mles f electrns exchanged F Faraday s cnstant 96,487 C/ml e r 96,487 J/(V ml e) *A Culmb is a unit f electrical charge. The charge carried by ne electrn is 1.60 X 10-19 C.
Electrchemical Cells Separating a redx reactin int separate reductin and xidatin parts is nt purely a theretical exercise. In real life, tw half reactins can be carried ut in separate cmpartments (as in the alkaline battery) r beakers (as we used in lab) and cupled s that electrns flw thrugh an external circuit. Ande: attracts anins (site f xidatin) Cathde: attracts catins (site f reductin) A. Galvanic/Vltaic Cells r Batteries A redx reactin ccurring in a galvanic cell has a negative ΔG and is therefre a spntaneus reactin. These reactins push electrns and are used t d wrk. Oxidatin: Zn (s) Zn 2+ (aq) + 2e E x 0.76 V Reductin: Cu 2+ (aq) + 2e- Cu (s) E red 0.34 V Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) E cell 1.10 V B. Electrlytic Cells A redx reactin ccurring in an electrlytic cell has a psitive ΔG and is therefre a nnspntaneus reactin. In electrlysis, electrical energy is required t induce reactin; i.e., we need a surce f electrical pwer (a battery!) t frce a reactin t take place. Net Reactin: 2 H 2O (l) 2 H 2 (g) + O 2 (g) What are the tw half-reactins? Hw can yu ratinalize these reactins with yur bservatins?