Week of 4/8

Monday: Lecture using powerpoint. First half was a review of electrochemical cell potential, the added on gibbs free energy and nernst equation.

Tuesday: Two demos: copper plating a car key and electrolysis of aqueous sodium iodide. Had students draw the set-ups, label anode, cathode, negative positive electrodes, direction of electron flow, half reactions.

Wednesday: Practice balancing redox reactions (learning products of common strong oxidizing and reducing agents).  Then Avogadro’s lab. This was not graded…1/2 class on music trip.

Thursday: Review day.  Nothing formal, but I did provide some practice exam questions.  Most students worked on homework.

In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe(s) and Cl2(g).

  1. (a) Write the equation for the reaction that occurs at the anode.
  2. (b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution.
  3. (c) Write the balanced equation for the overall reaction that occurs in the cell.
  4. (d) How many liters of Cl2(g), measured at 25 °C and 750 mmHg, are produced when the cell operates as described in part (b)?
  5. (e) Calculate the current that would produce chlorine gas at a rate of 3.00 grams per hour.

Answer:

  1. a) 2Cl- –> Cl2 + 2 e
  2. b)FeCl2 (find moles of electrons and moles of copper, then find ratio, should be 2 mole e-/1 mole Fe)
  3. c) Fe2+ + 2Cl- –> Cl2 + Fe
  4. d) 0.231 L
  5. e) 2.27 A

Problem 2

  1. Sketch a voltaic cell of chromium (Cr3+) and copper (Cu2+). Write a balanced net ionic reaction for the cell.
  2. identify oxidizing and reducing agent
  3. which metal is the cathode?
  4. explain why you need a salt bridge
  5. What is the cell potential under standard conditions?
  6. How does the cell potential change if the concentration of the chromium electrolyte is 3 M instead of 1 M?

Answer:

  1. can’t do online…
  2. Cr3+ is reducing agent b/c it gets oxidized since it has a lower reduction potential than Cu.  Cu2+ is oxidizing agent b/c it gets reduced.
  3. Reduction happens at cathode, so Copper is the cathode.
  4. You need a salt bridge to prevent positive charge build up…as the Chromium is oxidized, chromium ions are formed increasing the positive charge in the anode cell.  If this is allowed to build up w/0 being balanced by enough negative ions, the current will stop flowing. (Same argument could be made for cathode cell…too much build up of negative charge)
  5. 1.07 V
  6. Cell potential decreases.  Use nernst equation to see how Q will be > 1 when concentration of chromium ions is increased.

Friday: Test.

Week of 4/1

Monday: teacher work day

Tuesday: Redox review using CA 48.  Should have done more practice with predicting redox reactions (advanced ones), but was too inefficient!!

Wednesday:  Warm-up using green sheet activity series.  Build a wet cell, electrochemical cell tutorial (worksheet)

Thursday: CA 50 and 51 to learn Ecell and SHE

Friday: Lab analysis using microvoltaic cells lab