Topic 19 Oxidation and Reduction

1)What is the oxidation number of P in PO 4 -3 ? 2)If Cu and Zn and connected, which is the anode? 3)What reaction (oxidation or reduction) occurs at the cathode? 4)What is the sign of the anode in a voltaic cell? 5)Which type of cell is spontaneous, voltaic or electrolytic? 6)Balance: S 2 O OCl -  Cl - + S 2 O 5 -2

Describe, using a diagram, the essential components of a voltaic cell

Explain how a redox reaction is used to produce electricity in a voltaic cell [ ] Voltaic cells spontaneously produce electricity Electrons flow from oxidized to reduced Anode = negative electrode, oxidation Cathode = positive electrode, reduction – An ox, red cat Cations from salt bridge flow to cathode, anions to anode Recall two half cells are needed for voltaic cell

Voltaic cells [337] Electrons spontaneously flow from anode to cathode due to differences in potential energy – Spontaneously produces electricity Potential energy of the anode is higher than potential of the cathode The potential energy difference (EMF)between electrode is measured in volts (V) EMF of a cell is also called cell potential or cell voltage

Voltaic cells [338] Any spontaneous redox cell (voltaic) will have a positive cell EMF or E cell A E cell measured at standard states, is E  cell Standard state = 25  C, 1 atm, all species in their standard state at these conditions, and all concentrations are 1 M

Standard Cell Potential Refer to the figure in slide 4 of the voltaic cell Zn + Cu +2  Zn +2 + Cu E  cell = +1.10V A voltaic cell is made of two parts Anode and cathode E  cell = E  red cathode - E  red anode Values of of E  anode and E  cathode are found in tables of standard reduction potentials

Standard Reduction Potentials [341] All reactions written as reductions Recall that positive voltage indicates spontaneous reaction Reactions with negative voltage indicate that those species will not spontaneously be reduced, but indeed want to be oxidized – strongest reducing agents have negative reduction potentials

Standard Hydrogen Electrode [338] Because voltaic cells consist of two half cells, it is not possible to directly measure the E  anode or E  cathode Reduction potentials are relative to the standard hydrogen electron (SHE) – 2H + + 2e -  H 2 – SHE is assigned a value of 0.00 V

Standard Hydrogen Electrode [338]

Voltaic cell questions Exercises page 345

Voltaic vs. electrolytic voltaicelectrolytic spontaneousnonspontaneous AnodeOxidationNegativeOxidationPositive CathodeReductionPositiveReductionNegative

Describe, using a diagram, the essential components of an electrolytic cell [ ] Diagram needs to include – Source of electric current – Conductors – Positive and negative electrodes – Electrolyte Oxidation occurs at the anode which is positive Reduction occurs at the cathode which is negative

Diagram of electrolytic cell

Electrolytic Example [ ] 1)Identify all species present in the electrolyte and determine which will migrate to which electrode. 2)Determine which species will be discharged. 3)Balance electrons lost and gained to write the equation for the net reaction 4)Consider what changes would be observed in the cell as a result of the redox reaction.

Electrolysis of molten PbBr 2 [ ]

Electrolysis questions Exercises page 349

Electrolysis of aqueous solutions [ ] Trickier situation Water can be both oxidized and reduced Reduction of water = H 2 O + e -  ½H 2 + OH V Oxidation of water = H 2 O  ½O 2 + 2H + + 2e V

Electrolysis of aqueous solutions [ ] BUT both will be written as reductions H 2 O + e -  ½H 2 + OH V ½O 2 + 2H + + 2e - → H 2 O V Will the ions of the salt or the water be oxidized or reduced? (selective discharge)

Electrolysis of NaCl(aq) [351] List species present in NaCl(aq) What can be oxidized? (most negative E  red ) What can be reduced? (most positive E  red ) Looking at the E  red, who what will be oxidized and reduced? Observations?

Electrolysis of CuSO 4 (aq) [352] List species present in CuSO 4 (aq) What can be oxidized? What can be reduced? Looking at the E  red, who what will be oxidized and reduced?

Determine the relative amounts of the products formed during electrolysis [ ] Factors to determine amount of metal produced are 1)charge on ion, 2)current, and 3)time Charge = current x time (C = A x s) Moles of electrons = charge ÷ Units – Charge = coulomb (C) – Current = ampere (A)

Determine the relative amounts of the products formed during electrolysis A Cr +3 solution is electrolyzed, using a current of 7.60 A. What mass of Cr is plated out after 2.00 days? Exercises page 356

Determine the relative amounts of the products formed during electrolysis If a current of 2.00 amps is passed through a solution of AgNO 3 for 10 minutes, moles of Ag are formed. a)How much would form if a current of 1.00 amp were passed through the same solution for 30 minutes? b)How much Cu would form is the quantity of electricity in (a) were passed through a solution of CuSO 4 ?

Describe the use of electrolysis in electroplating [ ] gold plating copper plating chrome bumpers