Redox Reactions and Electrochemistry

Transcription

1 Redox Reactions and Electrochemistry Problem Set Chapter 5: 2126, Chapter 21: 1517, 32, 34, 43, 53, 72, 74 R Oxidation/Reduction & Electrochemistry Oxidation a reaction in which a substance gains oxygen atoms (e.g. the oxidation of a hydrocarbon) H C H H OH O O O 2 O 2 O 2 O 2 R C H C C R H R OH H O C O + H 2 O (i.e. this is equivalent to the combustion of a hydrocarbon) Reduction a reaction in which a substance loses oxygen atoms Oxidation/Reduction & Electrochemistry A different type of redox (i.e. reduction plus oxidation) reaction does not involve gain or loss of oxygen. Tarnishing of silver is a redox reaction that produces Ag 2 S. This spontaneous reaction can be reversed with a coupled reaction (add 80 g of baking soda and 80 g of table salt per litre of near boiling water in an aluminum pan to a depth covering the silver object). 3 Ag 2 S + 2 Al + 6 H 2 O 6 Ag + 2 Al(OH) H 2 S

2 Oxidation/Reduction & Electrochemistry Let s look at a simple case of this type of redox reaction: Cu +2 /Zn Cu /Zn +2 Cu +2 (aq) + Zn (s) Cu (s) + Zn +2 (aq) Redox Reactions Cu +2 (aq) + Zn (s) Cu (s) + Zn +2 (aq) The reaction can be represented by two halfreactions in which electrons are either gained or lost and the oxidation state of elements changes : Cu +2 (aq) + 2e Cu (s) oxidation state of Cu +2 0 Zn (s) Zn +2 (aq) + 2e oxidation state of Zn 0 +2 Reduction a process in which electrons are gained. (The oxidation state of an element decreases and electrons appear on the left side of the halfreaction.) Oxidation a process in which electrons are lost. (The oxidation state of an element increases and electrons appear on the right side of the halfreaction.) Oxidation States Review 1) Oxidation state of an atom in a free element is 0. 2) Total of the oxidation states of atoms in a molecule or ion is equal to the total charge on the molecule or ion. 3) Group 1A and Group 2A metals have oxidation state of +1 and +2 respectively. 4) F always has an oxidation state of 1. Cl also has oxidation state of 1 unless it is bonded to oxygen or fluorine. 5) H almost always has an oxidation state of +1. 6) O has oxidation state of 2 (unless bonded to itself or F). 7) When bound to metals, group 7A, 6A and 5A elements have oxidation states of 1, 2, 3 respectively.

3 ElectronegativitiesReview Example What is the formal oxidation state of P in: +1 2 H 3 PO 4? H 2 PO 4 HPO 4 : 3 x (+1) +? + 4 x (2) = 0 (charge on molecule) oxidation state of P = +5 : 2 x (+1) + (O.S. of P) + 4 x (2) = 1 (charge on ion) oxidation state of P = +5 : oxidation state of P = +5 If the oxidation state of elements do not change in a reaction, it is NOT a redox reaction! H 3 PO 4(aq) + 3 OH (aq) 3H 2 O + PO 4 3 (aq) acid/base reaction Balancing Redox Reactions Halfreaction method (not the same as method in textbook). 1) Identify species in which the oxidation state of an element is changing. Write the skeleton halfreactions including balancing of the redox atoms if necessary. 2) Identify oxidation state on both sides of equation for elements that have a change in oxidation state. 3) Add appropriate number of electrons to either left or right to balance oxidation states of redox atom(s). 4) Balance changes on left and right side of equation by adding H + (if in acidic solution) or OH (if in basic solution). 5) Add appropriate number of H 2 O s to left or right side of equation to balance atoms in the halfreaction.

4 Balancing Redox Reactions, cont d At this point, both halfreactions should be balanced. The next step is to combine the two halfreactions to form an overall equation. 6) Multiply through each halfreactions by appropriate coefficients to match electrons in each halfreaction. (i.e. number of electrons lost by the oxidized species must equal the number gained by the reduced one) 7) Add halfreactions and cancel electrons and other common species on left and right sides of the equation. 8) Check Reaction! It should be balanced in terms of oxidation states, charge and atoms. IF NOT, YOU HAVE MADE A MISTAKE! Examples Determining sulfite in wastewater. Sulfite is reacted with permanganate to produce sulfate and Mn(II) ion in acidic solution. Balance the redox reaction. SO 3 + MnO 4 SO 4 + Mn 2+ skeleton reaction SO 3 SO 4 identify oxidation states SO 3 SO 4 + 2e balance O.S. with electrons SO 3 SO 4 + 2e + 2H + balance charges with H + H 2 O + SO 3 SO 4 + 2e + 2H + balance atoms with H 2 O Cont d, Mn halfreaction MnO 4 Mn 2+ identify oxidation states MnO 4 + 5e Mn 2+ balance O.S. with electrons 8 H + + MnO 4 + 5e Mn 2+ balance charges with H + 8 H + + MnO 4 + 5e Mn H 2 O balance atoms with H 2 O Balanced HalfReactions H 2 O + SO 3 SO 4 + 2e + 2H + 8 H + + MnO 4 + 5e Mn H 2 O x 5 x 2 to balance e s

5 Balancing full equation Balanced full reaction: 5H 2 O + 5SO 3 5SO e + 10H H + + 2MnO e 2Mn H 2 O 5SO 3 + 2MnO H + 5SO 4 + 2Mn H 2 O Check atom balance. OK Try example 5.7 using this approach, use OH to balance charge in basic solution. Much easier. This method forces you to know oxidation states. Another Example Write the half reaction for Cr 2 O 7 Cr +3 Cr 2 O 7 (aq) Cr +3 (aq) skeleton (in acidic solution), Cr 2 O 7 (aq) 2 Cr +3 (aq) balance redox atoms Cr 2 O 7 (aq) 2 Cr +3 (aq) determine O.S. of redox atoms Cr 2 O 7 (aq) + 2(3e ) 2 Cr +3 (aq) balance O.S. with e s Cr 2 O 7 (aq) + 6e 2 Cr +3 (aq) 14 H + + Cr 2 O 7 (aq) + 6e 2 Cr +3 (aq) balance charges with H+ 14 H + + Cr 2 O 7 (aq) + 6e 2 Cr +3 (aq) + 7H 2 O balance atoms with H 2 O Extra Practise Balancing Redox Reactions (solutions on web site) #1) Cl 2 ClO + Cl in basic solution #2) I + IO 3 I 2 in acidic solution #3) H 2 O 2 (aq) O 2 (g) (in either acidic or basic solution)

6 Disproportionation Reactions A disproportionation reaction occurs when an element in a substance is both oxidized and reduced. Example: H O O H Hydrogen peroxide: antiseptic agent, O 2 acts as germicide H 2 O 2(aq) 2 H 2 O (l) + O 2(aq) Ox/Red Agents, cont d Oxidizing Agent a chemical substance that oxidizes (removes electrons from) other substances in a chemical reaction. In the process of oxidizing something, the oxidant becomes reduced; it s oxidation state decreases. Reducing Agent a chemical substance that reduces (loses electrons to) other substances. In the process of reducing, the reductant becomes oxidized; it s oxidation state increases. Oxidizing and Reducing Agents Removes electrons Oxidation States of Nitrogen Loses electrons (best oxidizing agent) (best reducing agent)

7 Oxidizing Agents O 2 Probably the most common and most important oxidant known to us. Ubiquitous. Organic Oxidation Schemes (Example: methane) O 2 CH 4 H 3 C OH O 2 H 2 C O formaldehyde methane methanol (aldehyde) O 2 +2 (alkane) (alcohol) O HC OH formic acid (carboxylic acid) O 2 +4 O C O carbon dioxide (inorganic carbon) Other oxidizing agents Oxides in their highest oxidation state are frequently strong oxidizing agents NO 3 HNO 3 NO 2 NO N 2 O N 2 strong oxidizing agents weaker oxidizing agents HClO 4 ClO 3 HClO 2 HOCl HNO 3 and HClO 4 are oxidizing acids. Nonoxidizing acids HCl, HBr, HI, acids for which the only possible reduction halfreaction is: 2H + (aq) + 2e H 2(g) Oxidizing Agents, cont d HNO 3 is a much stronger oxidizing agent than H +. Metals that dissolve in dilute H + to produce H 2 Li, Na, K (1A metals) Mg, Ca (2A metals) Al, Zn Fe, Sn, Pb Metals that will not dissolve Cu, Ag, Au, Hg Practice problem Cu will dissolve in HNO 3 producing Cu +2 in solution and the brown gas NO 2. Write a balanced equation for this process.

8 Electrochemistry (a) Cu (s) / Ag + (aq) (b) Cu (s) / Zn 2+ (aq) Cu 2+ (aq) / Ag (s) Spontaneous! ( G < 0) No reaction! Not spontaneous! ( G > 0) Ag + (aq) + e Ag (s) reduction Cu (s) Cu 2+ (aq) + 2 e oxidation Electrochemical Cells Flow of electrons (current) can do work. We can connect halfreactions in separate containers through an electrical circuit. This will produce a current (electron flow) and voltage according to the spontaneity of the reactions. Atomic view of a Voltaic (galvanic) cell Salt bridge (e.g. KNO 3 ) maintains neutrality Anode oxidation Cathode reduction

9 Cell Diagrams anode (oxidation) is placed at left side of diagram cathode (reduction) is placed on right side of diagram boundary line,, indicates a boundary between different phase (i.e. solution solid) a double boundary line indicates a boundary (i.e. salt bridge) between the two halfcell compartments anode (oxidation) Zn (s) Zn 2+ (aq) Cu 2+ (aq) Cu (s) halfcell salt bridge halfcell cathode (reduction) Voltages and Current anode cathode + Electromotive Force (EMF) The voltage difference between two solutions provides a measure of the driving force of the electron transfer reaction. Standard Electrode Potentials In electronics and electricity theory, a voltage is a measurement of the potential to do electrical work measured between two points in a circuit. Absolute measurements of potential (voltage) at a single point are meaningless, UNLESS, they are measured against some known reference. In electricity, that reference is known as ground. In electrochemistry, that reference is the standard hydrogen electrode (SHE). A Standard Electrode potential, E o, measures the tendency for the reduction process to occur at an electrode, when all species have unit activity (substances in solution are ~ 1.0 M or, if gases, are at 1 bar {~1 atm} pressure).

10 Standard Hydrogen Electrode (SHE) a H2 = 1.0 ~ P H2 = 1.0 bar ~ 1.0 atm a H3O+ = a H+ = 1.0 ~ [H + ] = 1M H + (aq) (1M) H 2(g) (1 atm) Pt 2H + (aq) + 2e H 2(g) E o = 0.00 V frequently written as: E o H+(aq)/H2(g) Easy to reduce, hard to oxidize (good oxidizing agents) Hard to reduce, easy to oxidize (good reducing agents) Standard Electrode (reduction) Potentials The potential of an electrochemical cell under standard conditions may be calculated by E o cell = Eo cathode Eo anode where the E o s are standard reduction potentials taken from a table. The cathode is the electrode at which reduction occurs (electrons on left side of equation, oxidation state decreasing). The anode is the electrode at which oxidation occurs (electrons on right side of equation, oxidation state increasing). Also for a spontaneous reaction, E o cell > 0, as we will see shortly.

11 Example 212 A new battery system currently under study for possible use in electric vehicles is the ZnCl 2 battery. Reaction: Zn (s) + Cl 2(g) ZnCl 2(aq) What is the standard potential of the cell, E o. Zn 2+ (aq) + 2e Zn (s) E o Zn2+/Zn = 0.763V oxidation potential Zn (s) Zn 2+ (aq) + 2e E o Zn/Zn2+ = Eo Zn2+/Zn = (0.763V) = V Cl 2(g) + 2e 2Cl (aq) E o Cl2/Cl = V Zn (s) + Cl 2(g) ZnCl 2(aq) E o cell = V OR E o cell =Eo cathode Eo anode = ( 0.763)V = V Spontaneous change in a Cell Previously, it was said E cell > 0 for a spontaneous reaction. Where did this come from? Electrical work: W electrical = Q V Q = charge, V = voltage If Q in coloumbs, V in volts, W in joules Related: P = iv {P = power i = current (charge/time), V = voltage} If i coloumbs/sec (Amp), V volts, P joules/sec = watts In an electrochemical cell, Q = n x F n = moles of electrons F = charge/mole of electrons = Faraday F = C/ mole of electrons V = E cell Spontaneous change, cont d W electrical = Q V = nfe cell This applies to a reversible process (implying that the reaction is carried out slowly enough that the system maintains equilibrium). Previously it was argued that the amount of work we can extract from a chemical process is equal G (pg 796, Petrucci Are You Wondering box). G = W electrical G = nfe cell G o = nfe o cell If E o cell > 0, Go < 0 and the reaction is spontaneous If E o cell < 0, Go > 0 and reaction is nonspontaneous

12 Behavior of Metals Previously we said that experimental evidence shows the following: Metals that dissolve in dilute H + to produce H 2 Li, Na, K (1A metals) Mg, Ca (2A metals) Al, Zn Fe, Sn, Pb Metals that will not dissolve Now we can better understand this: M (s) M n+ (aq) + n e oxidation 2 H + (aq) + 2 e H 2(g) reduction E o = 0.00V E o cell = Eo cathode Eo anode = 0 Eo M+/M If E o M+/M < 0, Eo cell > 0, the process is spontaneous. If E o M+/M > 0, a stronger oxidizing agent than H+ is required (i.e. HNO 3, HClO 4 ). Cu, Ag, Au, Hg Nernst Equation Previously, we talked about standard electrode potentials in which everything was in its standard state. Very rarely are things in standard state! G = G o + RT ln Q R = gas constant T = temperature (K) Q = reaction quotient nfe cell = nfe o cell + RT ln Q E cell = E o cell RT/nFlnQ = Eo cell RT/ (2.303 nf) log Q o o E cell= Ecell logq, for T = 25 C n Nernst Equation Applications of the Nernst Equation 1) Draw the condensed cell diagram for the voltaic cell pictured at right. 2) Calculate the value of E cell. 1) Pt Fe 2+ (0.1M), Fe 3+ (0.2M) Ag + (1.0M) Ag (s) 2) The cell is in nonstandard conditions so we need to apply the Nernst equation we will need to find E o cell, n, and Q.

13 What is Q? cont d From table of Standard Reduction Potentials: Fe 3+ + e Fe 2+ E o Fe3+/Fe2+ = V Ag + + e Ag E o Ag+/Ag = V Overall reaction: Cathode: Ag + + e Ag (s) after we combine Anode: Fe 2+ Fe 3+ + e halfreactions, n = 1 What is E o cell? Ag + (aq) + Fe2+ (aq) Fe 3+ (aq) + Ag (s) 3+ [Fe ] Q = 2+ + [Fe ][Ag ] E o cell = E o cathode Eo anode = Eo Ag+/Ag Eo Fe3+/Fe2+ = 0.800V 0.771V = 0.029V Cont d o E cell= Ecell log Q n [Fe ] = V log [Fe ][Ag ] (0.20M) = V log = 0.011V (0.10M)(1.0M) Example A: Calculate E cell for the following cell Al Al 3+ (0.36M) Sn 4+ (0.086 M), Sn 2+ (0.54 M) Pt Al e Al E o Al3+/Al = V Sn e Sn 2+ E o Sn4+/Sn2+ = V Cathode: Sn e Sn 2+ x 3 Anode: Al Al e x 2 cont d 3 Sn 4+ (aq) + 2 Al (s) 3 Sn 2+ (aq) + 2 Al3+ (aq) [Sn ] [Al ] Q = 4+ 3 [Sn ] after we combine halfreactions, n = 6 E o cell = E o cathode Eo anode = Eo Sn4+/Sn2+ Eo Al3+/Al = (1.676)V = 1.830V o E cell= Ecell log Q n [Sn ] [Al ] = V log [Sn ] (0.54M) (0.36M) = V log = 1.815V 3 6 (0.086)

14 Change in E cell with Conditions Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) E cell E logq = 1.103V log [Zn 2+ ] o = cell 2+ n 2 [Cu ] Slope = 59/2 mv per decade change in log{[zn 2+ ]/[Cu 2+ ]} If we let cell reaction proceed, reaction shifts to right, [Zn 2+ ] increases, [Cu 2+ ] decreases and E cell decreases. When does it stop? It stops at equilibrium, E cell = 0.00V o = Ecell logq K eq!! eq n o necell Keq = 10 We can calculate K eq from E o values! For above reaction, K eq = 1.5 x10 37 Concentration Cells Both halfcells are the same chemical system, just different concentrations. The driving force (i.e. the EMF) is provided by the difference in concentrations. Pt H 2 (g, 1.0 atm) H + (x M) H + (1 M) H 2 (g,1.0 atm) Pt (s) Concentration Cell Cathode: 2 H + (1M) + 2e H 2(g) Anode: H 2(g) 2 H + (xm) + 2e E o cell = E o cathode Eo anode = = 0.00V + 2 o [H ] E cell= Ecell logq =0.00V log + 2 n 2 [H ] X = log = ( 2logX ) = logX Since ph = log X, E cell = ph anode cathode This concentration cell behaves as a ph meter! Other concentration cells can be used to measure unknown concentrations of other species (i.e. potentiometry).

15 Determination of K sp (see Example 2110) From measured E cell, determine K sp. Solution: Set up Nernst equation with Ag + (xm) at anode, 0.1M at cathode. Solve Nernst equation to get x. x = [Ag + ] = S, [I ] = S K sp = S 2 Ag (s) Ag + (sat. AgI) Ag + (0.1M) Ag (s) Electrolysis The use of an externally applied voltage to force an electrochemical reaction, even if it is naturally nonspontaneous. Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) Spontaneous! E o cell = V What about the reverse process? Nonspontaneous! Zn 2+ (aq) + Cu (s) Zn (s) + Cu 2+ (aq) E o cell = 1.10 V But if we apply a potential > 1.10 V across the cell, we overcome the natural negative voltage, thus providing the driving force to make the reaction proceed. Current is in opposite direction of voltaic, or galvanic, cell. Galvanic and Electrolytic Cells electron flow reversed External energy (voltage) source Galvanic Cell Electrolytic Cell Regardless of the cell type, anode and cathode always defined by the process: oxidation at the anode, reduction at the cathode.

16 Zn/Cu 2+ electrolysis example continued... The amount of current that flows in the electrolytic cell tells us how much Zn has been produced or how much Cu 2+ has dissolved. Faraday s Law of Electrolysis: The number of moles of product formed in an electrolysis cell by an electric current is chemically equivalent to the number of moles of electrons supplied. or charge (coulombs) Q = nf = it moles of electrons current (amperes) time (seconds) Note: 1 A = 1C/s Faradays constant = C /mole of e Example 2112 Electrodeposition of Cu can be used to determine Cu 2+ content of sample. Cathode: Cu e Cu (s) Anode: 2H 2 O O 2(g) + 4H + (aq) + 4e What mass of Cu is deposited in 1 hr if current = 1.62A? Solution: Find moles of electrons, then find moles of Cu, then find mass of Cu. Mole of e = 1.62 A (C/s) x 3600 sec x 1/(96485 C/mole e ) Mole of Cu = mole e x 1 mole Cu / 2 mole e Mass Cu = moles Cu x g Cu/mole Cu Answer = 1.92 g of Cu deposited in 1 hour Cont d Example B: How long will it take to produce 2.62 L of O 2(g) at 26.2 o C and 738 mmhg at a Pt anode with a constant current of 2.13A? Solution: Find moles of O 2, then find moles of electrons, then find charge, then find time. 738 Mole of O 2 : n PV atm 2.62L 760 = = 1 1 RT Latmmol K K (recall anode reaction: 2H 2 O O 2 (g) + 4H + (aq) + 4e ) Mole of electrons = moles of O 2 x 4 mole electrons/ mole O 2 Charge = moles of electrons x F (C/mole of electrons) Time = Charge (C)/Current (C/s) Answer = sec = 5.23 hr = 5 hr & 14min

17 2 3 3 o ChlorAlkali Process Electrolysis of NaCl solutions 2 Cl (aq) Cl 2(g) + 2 e anode E o Cl2/Cl = V 2 H 2 O + 2 e H 2(g) + 2 OH (aq) cathode Eo H2O/H2 = V 2 H 2 O + 2 Cl (aq) Cl 2(g) + H 2(g) + 2OH (aq) E o cell = 2.19 V Cl 2 produced at anode H 2 and NaOH (aq) produced at cathode membrane allows Na + movement 11% NaOH and 15% NaCl is concentrated, NaCl crystallized and removed final product, 50% NaOH (1% NaCl impurity), Cl 2, H 2 Threeyear and Fouryear Degrees with Special Focus on Analytical Chemistry Honours Major Chemistry & Minor Biology with Special Focus on Biological Chemistry Specialized Honours Chemistry with Special Focus on Materials Chemistry CO 2, H2O o OH, NO, O, h ν 2 hydrocarbons oxygenated Particulate O, NO matter hydrocarbons sources 1 Wet and dry deposition 4Year Specialized Honours Programme Specialized Honours Chemistry, Stream in Atmospheric Chemistry 3Year Bachelor of Science Degree in Chemistry Chemistry Honours Major/Minor Degrees Major or Minor in Chemistry, combine with Major or Minor in Biology, Physics, EATS or any other science Major or Minor in Chemistry, combine with Major or Minor in Arts, Fine Arts, Environmental Studies Honours Double Major Degrees Major in Chemistry, combine with another Major in Science