Chapter 8 Reduction and oxidation

Transcription

1 Chapter 8 Reductin and xidatin Redx reactins and xidatin states Reductin ptentials and Gibbs energy Nernst equatin Disprprtinatin Ptential diagrams Frst-Ebswrth diagrams Ellingham diagrams Oxidatin refers t gaining xygen, lsing hydrgen r lsing ne f mre electrns. Reductin refers t lsing xygen, gaining hydrgen r gaining ne r mre electrns. 2Mg + O 2 2MgO; (verall) Mg Mg e - O 2 + 4e - (xidatin) 2O 2- (reductin) Redx is the abbreviatin fr reductin-xidatin Given the fllwing reactin, write the xidatin and reductin half-reactins. Sn 2+ (aq) + 2Fe 3+ (aq) Sn 4+ (aq) + 2Fe 2+ (aq) Oxidatin: Reductin: Electrlytic cell passage f electrical current initiates a redx reactin. Galvanic (Vltaic) Cell a spntaneus redx reactin ccurs and generates an electrical current. 1

2 Example: In acidic slutin, write a balanced verall reactin between permanganate ins and xalate ins that frm divalent manganese ins and carbn dixide. Daniell cell In the left-hand cell, Cu 2 ins are reduced t cpper metal, and in the right-hand cell, zinc metal is xidized t Zn 2 ins. Zn s Zn 2 aq Cu 2 (aq) Cu(s). G zfecell F = 96,485 C ml-1 1V = 1J/C G ln K RT R = J K -1 ml -1 2

3 Example: In a Daniell cell Zn s Zn 2 aq Cu 2 (aq) Cu(s), the standard cell ptential at 25 C is 1.10 V. Calculate the crrespnding values f ΔG and K. Selected standard reductin ptentials (at 298 K); further data are listed in Appendix 11. The cncentratin f each substance in aqueus slutin is 1 ml dm 3 and the pressure f a gaseus cmpnent is 1 bar (10 5 Pa). 3

4 Standard Hydrgen Electrde By definitin, the reductin ptential fr hydrgen is 0 V: 2 H + (aq, 1M) + 2 e H 2 (g, 1 bar) E 0 cell = [E 0 reductin prcess] - [E 0 xidatin prcess] E 0 cell = E 0 red(cathde) - E 0 red(ande) E E RT ln zf Nernst equatin [reduced frm] [xidized frm] where R = mlar gas cnstant = J K -1 ml -1 T = temperature in K F = Faraday cnstant = 96,485 C ml -1 z = number f electrns transferred E E RT ln Q zf Q = reactin qutient E E 2.303RT lgq E zf V lgq z 4

5 Metal ins can be stabilized with respect t reductin by the frmatin f a precipitate r crdinatin cmplex. Ag + (aq) + e - Ag(s) E = V, G = -nfe = kj per ml Ag. As the cncentratin f Ag + is lwered, the reductin ptential becmes less psitive ( G is less negative), and the reductin f Ag + t Ag becmes less easy (Ag + is stabilized) Cmpare t the reductin f free Ag(I) versus Ag(I) in the frm f AgCl, Set up a thermchemical cycle that cmbines the equilibria: sl G (AgCl,s) fr prcess: AgCl(s) Ag + + Cl - (aq) AgCl(s) + e - Ag(s) + Cl - (aq) G (Ag + /Ag) fr prcess: Ag + + e - Ag(s) Ag + (aq) + Cl - (aq) + e - Apply Hess s Law: r G = sl G (AgCl,s) + G (Ag + /Ag) sl G (AgCl,s): AgCl has a K sp = 1.77x10-10 (use G = -RT ln K) AgCl(s) Ag + (aq) + Cl - (aq) G = kj per ml AgCl sl G (Ag +,Ag): has E = 0.80 V (use G = -zfe ) Ag + + e - Ag(s) G = kj per ml Ag r G = 55.6 kj/ml + (-77.2 kj/ml) = kj/ml rg E zf 21.6 kj/ml 0.22V 1*96,485 C/ml Disprprtinatin 2Cu + (aq) Cu 2+ (aq) + Cu(s) Using thermdynamic cncepts, shw that disprprtinatin f Cu + is favred. Determine the value f K fr the reactin abve. Cu 2+ (aq) + e - Cu + (aq) Cu 2+ (aq) + 2e - Cu(s) Cu + (aq) + e - Cu(s) E = +0.15V E = +0.34V E = +0.52V 5

6 Ptential Diagrams Latimer Diagrams The standard reductin ptential fr the reductin half-reactin invlving the tw species jined by the arrw is shwn abve the arrw. Latimer diagrams shw the redx infrmatin abut a series f species in a very cndensed frm. Frm these diagrams yu can predict the redx behavir f a given species. The mre psitive the standard reductin ptential, the mre readily the species n the left is reduced t the species n the right side f the arrw. Highly psitive standard reductin ptentials indicate the species n the left is a gd xidizing agent. Negative standard reductin ptentials indicate that the species t the right behaves as a reducing agent. Use the Latimer ptential diagram t calculate the value f E fr the reductin f Fe 3+ (aq) t elemental irn. E =? 6

7 Frst-Ebswrth Diagrams Graphical methd f summarizing redx relatinships fr species cntaining a given element in different xidatin states. Each pint represents a chemical species, ne can cnsider the relatinship between any pair f pints (nt just neighbring). The diagram prvides infrmatin abut the relative thermdynamic stability The lwest pint is the mst stable at ph = 0. A mve dwnwards is a thermdynamically favred prcess Any state n a cnvex pint is thermdynamically unstable with respect t disprprtinatin. Any state n a cncave pint is thermdynamically stable with respect t disprprtinatin. What cnclusins can be made frm this plt? What can we say abut the relative stabilities f the different xidatin states phsphrus in aqueus media at ph 0? 7

8 The half-reactin fr the reductin f M z ins t M, r H t H 2, can be cnsidered in terms f three cntributing steps fr which thermdynamic data may be determined independently. Ellingham Diagram Determine the prper reducing agent and cnditins cnsidering the Ellingham diagram as the temperature increases, each metal xide becmes less thermdynamically stable (less negative Δ f G ) CO becmes mre thermdynamically stable at higher temperatures (mre negative Δ f G ) The relative stabilities f the xides at any given temperature can be seen directly frm an Ellingham diagram. 8