Experiment 17: Oxidation Reduction

14/9/2018 6:51 AM

Experiment 17: Oxidation – Reduction (http://intro.chem.okstate.edu/html/sexp17.htm)

The following preparatory questions should be answered before coming to class. They are intended to introduce you to several ideas important to aspects of the experiment. You must have finished your pre-lab, before you can start your lab.

Pre-lab Questions:

1. Define the following term.

(a) oxidizing agent

(b) reducing agent

(c) oxidation

(c) reduction

2. Complete the table for the given reactions:

Species Species Reducing Reaction Oxidized Reduced Agent ( a ) 2Ag+ + Co(s)  Co2 +(aq) + 2Ag(s) ( b ) Ni2+(aq) + Co(s)  Co2+ (aq) + Ni(s) ( c ) 2Ag+(aq) + Ni(s)  Ni2+ (aq) + 2Ag(s)

3. Rank the strength of the oxidizing and reducing agents identified in the table

Strength Oxidizing Agent Reducing Agent Strongest Intermediate Weakest

Experiment : Oxidation – Reduction

Equipment:

24-well plate…………….1 24/9/2018 6:51 AM steel wool…………a little bit forceps…………………..1 paper towels………several

Part I: Constructing a Qualitative Potential Series (work in groups)

Using a well plate, half fill two wells each with Cu2+, Pb 2+ and Zn2+ solutions. Be sure to note the location (well number) of each solution. To each of the wells add a shiny piece of Cu, Pb or Zn metal to form all of the combinations listed in Table I. (If necessary, shine the metal pieces with steel wool or sand paper.) Complete the table by describing your observations and writing balanced chemical equations for each reaction you observe. If no reaction occurs, write NR.

Table I Metal Solution # Observations Reaction Of ions Cu Pb2+ Cu(s) + Pb2+ (aq)  Zn2+ Cu(s) + Zn2+ (aq)  Pb Cu2+ Pb(s) + Cu2+(aq)  Zn2+ Pb(s) + Zn2+(aq)  Zn Cu2+ Zn(s) + Cu2+(aq)  Pb2+ Zn(s) + Pb2+(aq) 

Based on your observation answer the following questions. Remember to always show the charge on the ions.

I. (a) Prepare a potential series for Cu, Pb and Zn and their ions.

Strongest reducing agent ____________Weakest oxidizing agent __________________

(b) Which metal ion is the strongest oxidizing agent?

(c) Which metal ion is the weakest oxidizing agent?

(d) Which metal is the strongest reducing agent?

(e) Which metal is the weakest reducing agent?

Using clean wells on the well plate, half fill three wells with 6 M HCl. Scrub pieces of Cu, Pb and Zn metal carefully with steel wool. Do not touch the clean metal surfaces with your hands. Use forceps or a paper towel. To each of the three wells add a different metal piece. Complete the Table II by describing your observations and writing balanced chemical equations for each reaction you observe. If no reaction occurs, write NR. (Note: 34/9/2018 6:51 AM

Some of the reactions may occur very slowly. Allow at least five minutes before deciding that no reaction has occurred.)+

Table II

Metal Solution of Ions Observations Reaction

+ (g) Position H2 – H in the potential series prepared in I (a).

Strongest reducing agent ____________Weakest oxidizing agent ________________________

Experiment : Oxidation – Reduction

Equipment:

Filter paper……………………..3 sheets steel wool or sand paper…… a little bit Forceps………………………………...1 scissors……………………………….1 Styrofoam coffee cups………………...4 tape……………………about 12 inches Sheet of card board or stiff paper……..1 Voltmeter with leads…………………1

PART II: Semi-Micro Voltaic Cells

Cut four Styrofoam coffee cups about 2 cm from the bottom to form shallow cups. 44/9/2018 6:51 AM

Figure I

Tape the bottom of one of the cups to the center of a piece of cardboard or stiff paper. Tape the remaining three cups around the center cup as shown in Figure II.

Figure II

Fill the center cup about half full of 1.0 M NH4NO3 solution (5 to 10 mL). Be careful not to splash any solution into the surrounding cups. Fill one of the surrounding cups about half full of 0.1 M CuSO4. Half fill another with 0.1 M PbSO4 and the third with 0.1 M ZnSO4. Label the cups by writing the name of each solution on the cardboard sheet.

Using sandpaper or steel wool, polish copper, zinc and iron electrodes until they shine. Do not handle the electrodes with your fingers, use forceps or paper towels.

Carefully dip one end of the copper electrode in the copper (II) sulfate solution in the cup. Bend the electrode or use small pieces of tape as necessary to secure the electrode to the cup so that one end is below the surface of the solution and the other extends out of the cup (see Figure III). 54/9/2018 6:51 AM

Figure III

Repeat the process placing the zinc electrode in the zinc sulfate solution and the lead electrode in the lead sulfate solution. Label each cup by writing on the cardboard.

Fold three sheets of filter paper into strips about 1 cm wide and several layers thick.

Bend one of the strips into a ‘U’ shape. Invert the ‘U’ and submerge one end of the strip in the ammonium nitrate solution in the center cup and the other end in the solution in one of the surrounding cups. Repeat the procedure with the remaining two surrounding cups so that each of the three surrounding solutions is connected to the center solution by a strip of filter paper. These connections form a salt bridge between any two of the surrounding cups.

Connect one of the leads of the voltmeter to the copper electrode with an alligator clip. Connect the other lead to the zinc electrode. Read the voltage on the voltmeter scale. ( Turn on the calculator, hit “apps”, go to datamate and find differential voltage program, only if your calculator does not show the voltage sensor already attached). 64/9/2018 6:51 AM

If the deflection is negative, remove the clips and attach them to the opposite electrodes. Wait a few seconds for the reading to stabilize then record the potential. Obs. #1

The cathode will be attached to the red pole of the voltmeter and anode to the black.

Which electrode is the anode?

Obs. #2

Which electrode is the cathode?

Obs. #3

Write the half reaction occurring at the anode. Is this oxidation or reduction?

Obs. #4

Write the half reaction occurring at the cathode. Is this oxidation or reduction?

Obs. #5

Write the overall reaction for the cell. Calculate the cell potential and compare it your observed value.

Obs. #6

Record the information (Obs. #1 - #6) for the zinc and copper, and lead and zinc cells as well on the next page. Measure the potential of each voltaic cell by attaching the voltmeter lead clips to each combination of electrodes producing a positive voltage.

E0 Cell for Cu/Zn Cell = E0 Cell for Cu/Pb Cell = E0 Cell for Pb/Zn Cell = 74/9/2018 6:51 AM