Energy from Electron Transfer (ET) Batteries Fuel cells (H2) Hybrid vehicles Cellular Electrochemistry (ATP production) Batteries are a collection of several galvanic cells bundled together System for direct conversion of chemical energy to electrical energy (spontaneous) Opposite of electrolytic cell (we will look at this at the end of the unit)

LEO the lion says GER Galvanic cells produce useful energy from redox reactions (electron transfer) the substances that loses e- or donates the e- is oxidized and is the reducing agent the substance that gains e- is reduced and is the oxidizing agent the overall redox reaction can be represented with two balanced half-reactions

Corrosion: Common Redox Rxns Natural redox process that oxidizes metals to their oxides and sulfides (like Fe, Ag, Cu) Rust 2Fe(s) + 3/2O2(g) + nH2O → Fe2O3·nH2O(s) Tarnish develops over Cu, Brass (Cu/Zn alloy), Ag, Al. slower than rust. caused by chemicals in the air like S. Bleaching

Decide which species is oxidized, which is reduced. (NiCad battery): 2 Ni3+ + Cd → 2 Ni2+ + Cd2+ 2 H2O (l) + Al (s) + MnO4- (aq) → Al(OH)4- (aq) + MnO2 (s) Balance the second rxn (half reaction method).

Construction of Galvanic/Voltaic Cell DEMO: Zn (s) in Cu2+ (aq) What is happening? There is ET, but does the system generate electrical energy? If not, how to make it conduct? How to construct electrochemical cell?

Separation of half reactions is the essential idea behind a galvanic/voltaic cell The two cells are joined by the circuit which consists of a wire and salt bridge transfer of electrons through an external circuit produces electricity e- flow is driven by a difference in potential energy, this creates voltage

Electrochemical Cells Both galvanic and electrolytic cells have 3 main parts, 2 electrodes and electrolyte Electrodes: the objects that conduct the electricity between the cell and the surroundings Electrolyte: a mixture of ions (usually aq) that are involved in the reaction or that carry the charge. The electrodes are dipped into the electrolyte

Electrodes Anode: the electrode where oxidation takes place (AUTO) Cathode: the electrode where reduction takes place (CAR)

Active vs. Inactive Electrodes In galvanic cell, e- flows from anode to cathode (e- flow creates voltage)

Consider the 2 ½ rxns Zn → Zn2+ + 2e- ClO3- + 6 H+ + 6 e- → Cl- + 3 H2O Indicate which reaction occurs at the anode and which at the cathode? Which electrode is consumed? Inactive or active?

Cell Voltage vs. Current depends primarily on which elements and compounds participate in the reaction does NOT depend on size of cell, amount of material it contains, or size of the electrodes DEMO-different size alkaline batteries Current is the rate of electron flow through an external circuit, measured in amperes (amps, A) larger cells contain more materials and can sustain ET over a longer period of time

How to determine anode material vs. cathode? Ans: electromotive series Overall cell potential, standard emf, voltage = Eo Eocell = Eocath – Eoanode What’s V for Zn/Cu cell?

Al3+ + Mg (s) → Al (s) + Mg2+ Write a balanced eqn. Determine cell potential. Do you see trend from electromotive series? (more neg is easier to oxidize, anode)

Strength Questions If make a voltaic Cell from the following, which is anode? cathode? Eocell? Cd2+ + 2e- → Cd (-0.40 V) Sn2+ + 2e- → Sn (-0.14 V) 2. Rank following in order of increasing ability to get reduced NO3- (Ered = 0.96 V), Ag+ (0.80 V), Cr2O72- (1.33 V)

Can H2 reduce Ag+ to Ag? Pick a reagent that can oxidize Mn to Mn2++ but not oxidize Ni to Ni2+.

Thermodynamics Eocell tells something about spontaneity Recall: rxn is spontaneous if products are favored and energy is released Determined by H, S, and G (∆G = ∆H -T∆S) What do you think about sign for Eocell? ∆H ∆S Spont? - + Always low Temp High Temp never

Cell potential indicates spontaneity If Eocell is positive, redox is spont. Spont. reactions can be used to do work (related to ∆Go, where -∆Go is spont.) Eocell = V = J/C (work/charge) If system does work, w is negative At constant T and P, ∆Go = w Eocell = -w/charge = - ∆Go/charge 1 mol e- can transfer 96485 C (Faraday’s constant, F) Eocell = - ∆Go/nF ∆Go = -nFEo

Is rxn spont? 2 NaClO2 (aq) + Cl2 (g) → 2 ClO2 (g) + 2 NaCl (aq)

Eocell = - ∆Go/nF ∆Go = -RT lnK Eo = RT lnK = RT lnK nF nF Prac Ex pg 849 For the reaction, what is the value of n? Calc ∆Go and K at 298 K. 3 Ni2+ + 2 Cr(OH)3 + 10 OH-→3 Ni + 2 CrO42- + 8 H2O Eo = RT lnK = RT lnK nF nF