Experiment 12

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Experiment

#

12

Introduction

An electrochemical system is under standard conditions when the concentrations of all

reactants and products in solution are 1.0 M, the partial pressures of all gases involved in

the process are 1.0 atm, and the temperature of the environment in which the reaction

takes place is 25oC. The Nernst equation, shown below, is used when the

electrochemical cells are not under standard conditions.

E = Eo

-{no

nF

of

E = Eo

-0'0592

he

n at25"C

In the first part of this experiment, you

will

measure the cell potentials produced by six different voltaic cells.

All

of the voltaic cells

will

be constructed with a 1.0 Msolution

of

Zrf*

atthe anode, but they

will

have different concentrations of Cu2* at the cathode.

Because the concentrations of Cu2* for these voltaic cells are not all 7.0 M, we

will

make

use of the Nernst equation to carry out the necessary calculations. To

simpliff

calculations and data collection, it

will

be assumed that the temperature of the solutions is

exactly

25'C.

This

will

allow you to use the second version of the Nemst equations,

which can be rearranged to produce the equation for a straight line:

y

:

mx

*

6.

With the results from this experiment, you

will

plot a graph of .E verses

lnQ.

You

will

then use the slope to calculate the value of n.

In the second and third parts of the experiment, you

will

construct electrolytic cells. In

the second part, you

will

make approximate measurements of the gases produced at the

anode and cathode during the electrolysis of a sodium sulfate solution. In the third part of the experiment you

will

electroplate copper on the surface of another metal object. By

measuring the mass of the object before and after this process, you

will

be able to

determine the mass of copper plated to the object. From this mass, you

will

make several other quantitative calculations pertaining to the reaction.

Purnose:

o

To examine the effects of changing the concentration in the half cell where the

reduction occurs on cell potential in a voltaic cell

o

To determine the number of moles of electrons transferred in the balanced

chemical equation for a redox reaction through experimental means

e

To collect and compare the relative volumes of the gases produced during the

electrolysis of a sodium sulfate solution

o

To electroplate an object with copper, and determine the amount of current that

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www,apchemsol utions.com Safetv

.

Wear safety goggles, alab coat, and latex gloves at all times during this lab.

.

Make sure that there are no open flames in the room when you are collecting

gases in the second part of the experiment.

,

Zinc Sulfate, ZnSOa, and Sodium Sulfate, NaSO+

o

totlot$r"r:

Flush eye with plenty of water for 10 minutes.

call

for

medical help is irritation persists.

.

Skin: Rinse off with water.

.

_Injection:

If

a large amount is swallowed, call for medical help.

.

Copper

(II)

Sulfate, CuSOa, and Potassium Nitrate, KNO:

o

KNO3 can cause a fire when in contact with combustible materials.

o

First

ot$r"r'

Flush eye with plenty of water for 10 minutes.

call

for

medical help is irritation persists.

.

Skin: Rinse off with water.

.

_Injection:_{Rinse out}_{mouth with}water. Call for medical help.

Materials

.

Seven droppers.

'

Seven beakers (one must be at least 250 mL)

o

The above can be substituted for seven dropper bottles and a 250 mL beaker.

.

10 mL graduated cylinder

.

_{Two identical}test tubes

.

Spot plate

.

Voltmeter

.

6 volt battery

.

Two wires with alligator clips that

will

hook up to the voltmeter

.

_Twoinsulated copper wires (-12 cm each)

.

One bare copper wire/electrode

(-

7 cm)

.

Steel wool or sandpaper

.

Paper towel

.

Funnel

'

One sandwich size Tupperware dish (a test tube must be able to lay flat on the

bottom)

.

One twist tie and a wood pencil or plastic pen

.

Ruler

r

Stopwatch

.

_Watersoluble marker and/or masking tape

.

(Potassium nitrate, filter paper, scissors, stir stick, fweezers, and distilled water) or

(a salt bridge)

.

Distilled water

.

A five cent coin, a key, or another metallic object that can be electroplated.

.

2 cm copper wire and 1.0 MCuSO+

.

2 cmzinc strip and 1.0 MZuSO+

'

-1.0 MNazSO+

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www.apchemsolutions.com Procedures

Voltaic Cells and the Nernst Equation

1.

Pour about 10 mL

of

1.0 MCuSO+ into a clean and dry beaker. Label the

beaker. Take the 1.0 MCuSO+ solution and make a

lll0

dilution. To do this,

add 1.0 mL of 1.0 MCuSOa to a 10 mL graduated cylinder.

Fill

the graduated

cylinder to about the 9 mL mark with distilled water poured from a beaker.

Slowly add distilled water using a dropper until the meniscus is level with the

10.0 mL mark. Add this 1/10 dilution to a clean and dry beaker. Label the beaker 0.1 MCuSOa. Use the 0.1 MCuSO+ to make 10 mL

of

1

x

10-2

M

CUSOa by following the procedures above. Make three more successive 1/10

dilutions. The final dilution

will

give you a 1

x

10-5 MCuSO+ solution.

2.

Obtain droppers for each of the six copper sulfate solutions. Label each

dropper with the concenkation of the solution it

will

be transferring.

3.

Pour about 10 mL of 1 .0 M ZnSO+ into a clean and dry beaker. Label the

beaker.

4.

Obtain and label the dropper that

will

be used to transfer the Zn2* solution.

5.

Obtain

a2

cmpiece of copper wire anda2.0 cm strip of solid zinc. Clean the

oxidation off of the metal strips using the steel wool or sand paper.

6.

If

you are making your salt bridges out of filter paper and potassium nitrate, pour about 5 mL of distilled water into a beaker. Add a small amount

of

potassium nitrate to the water and stir until it dissolves. Continue adding potassium nitrate and stirring until you produce a saturated solution. Do not waist the potassium nitrate. The solution is saturated as soon as there are a

few grains of solid that do not dissolve. Wash your hands before you touch

the fiiter paper. Handle the filter with the tweezers as much as possible, as

you cut it into six strips which are about 1.0 cm wide and 2.5 cm long. They

must be of a sufficient length to allow each end to be about 0.5 cm below the surface of the solutions that

will

be held in adjacent divots. Keep in mind that

the divots

will

be about2l3 full of

liquid.

Place all the cut strips of filter paper

in the saturated solution of potassium nitrate.

If

they are floating, hold them

on the bottom with the tweezers for a minute or

two.

They need to soak up as

much solution as possible.

7.

Make a plan for adding solutions to your spot plate. To make a voltaic cell with a ZnJZn2* half cell and a Cu/Cu2* halicen the solutions must be in

adjacent divots. You have six different Cu2* solutions, which you

will

use to

make six different voltaic cells. Each voltaic ce1lwill have a different concentration of Cu2* in the half cell of the anode, and all

will

have 1.0

M

Zn2* inhalf cell at the cathode. You can use the same ZnJZn2* half cell for

more than one voltaic cell, as its concentration does not change. When you

have decided on a layout for you spot plate, label the divots using a water

soluble marker or masking tape and a pen.

8.

Transfer each solution to its divot on the spot plate using its associated

dropper. Add 16 drops of solution to each divot.

9.

AtIach the alligator clips to the voltmeter. As the reduction

will

always occur tn a Cu/Cl2* half cell, the solid copper strip

will

always be the cathode.

Attach the positive wire from the voltmeter to the top of the coppff wire with

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the alligator

clip.

As the oxidation

will

always occur in a Zn/Znz* half cell,

the solid zinc strip

will

always be the anode. Attach the negative wire from

the voltmeter to the top of the zinc strip with the alligator clip.

10. Measure the cell potential for each of the six voltaic cells. Record the values

in your data table.

The Electrolysis of a Sodium Sulfate Solution

I 1 . Strip about 3.0 cm of insulation from both ends of the two copper wires. Bend each of the wires 180" (in half) about 4.0 cm from one of their ends.

12.Ptut on a pair of latex gloves, and keep them on for the remainder of this

section,

ifyou

have not already done so.

13. Pour about 180 mL of the sodium sulfate solution into the sandwich size

Tupperware

dish.

The surface of the solution should be about 3.0 cm from

the bottom of the dish. Place the two test tubes in the solution. Adjust the

positioning of the test tubes so that all of the air bubbles escape. Invert each

of the test tubes being careful not to let any air get

in. If

one of the test tubes contains air bubbles, lay it flat to get rid of the bubbles and invert it again. Do not allow the mouth of either test tube to pass the surface of the solution, as

this

will

cause air to rush in and your solution to rush out.

14. Carefully place the bent end of one of the wires under the mouth of one of the test tubes. The rim of the test tube

will

sit on the bend. Place the bent end

of

the other wire under the rim of the other test tube. Continue to hold the test tubes so that no air gets in.

15. Attach the other end of one of the wires to the negative post on the battery,

and attach the other end of the other wire to the positive post on the battery.

16. Gases

will

form at both electrodes as soon as the battery is hooked

up.

The test tubes

will

collect these gases, allowing you to determine the relative volume of each gas produced. You must continue holding the test tubes until

the level of the solution has dropped by at least 3.0 cm in one of the test tubes. 17. While one partner is holding the test tubes, the other person

will

draw a

diagram of the apparatus. Make sure that the positive and negative battery

posts are correctly identified in the diagram. Switch roles when the first

diagram is complete.

18. Measure the height of gas _{collected in}each test tube. Record this data on your diagram. Make sure that you know which volume of gas is associated with

the positive electrode, and which volume of gas is associated with the negative electrode.

19. Remove your latex gloves and wash you hands up to you elbows, then put on

an new pair of latex gloves.

Electroplating

20. Pour the rest of your 1 .0 M CuSOa into a clean 250 mL beaker. Add addition

1.0 MCUSO+ until the beaker contains about 200 mL of the solution. 21. Attach one of the wires with alligator clips to the negative terminal of the

battery. Attach the other wire with alligator clips to the positive terminal

of

the battery. There must be free alligator clips at the far ends of these wires.

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22. Clean the bare copper wire with steel wool or sandpaper to get rid of any copper oxides. Place the bare copper wire in the solution. Clip the positive

lead from the battery to the top of the copper wire.

23.Prepare the object to be electroplated by polishing it with steel

wool.

This

will

remove any oxides, dirt, or finger grease, giving the copper a clean surface to adhere

to.

Handle the object with a clean piece of paper after it has been polished, as you do not want to get any oils from your fingers on it. 24.Measure and record the mass of the polished object.

25. Clip the negative lead from the battery to the object.

26. Place a wood pencil or plastic pen on top of the beaker across its diameter.

Lower the object to be electroplated into the solution by the wire, being careful not the let the alligator clip touch the solution. Start the stopwatch as

soon as the object touches the solution. Secure the wire to the pencil or pen

with the twist tie, making sure that most of the object is in the solution. The alligator clip must not be touching the solution.

27. Observe the electroplating process for several minutes. When you are ready

to stop electroplating, pull the coin out of the solution, stop the stopwatch, and record the time in your data table.

28. Do not touch the copper plated object with your hands! Dry the object with

paper towel and then allow it to

at

dry for several minutes. Measure and record the mass of the dry copper plated object.

29. Clean all glassware. Ask your teacher for instructions on disposal of the

chemicals. Wash your hands.

Data

Voltaic Cells and the Nernst Equation Voltaic

Cell # [C,rt*l at the Cathode Cell

Potential,8

"1

(9

1 1.0

M

2

0.1M

3

4

5

6

Height of gas collected over the positive electrode (cm) Height of gas collected over the negative electrode (cm)

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Electroplating

Mass of polished object (g)

Mass of object after plated

with

copper (g) Mass of copper plated to object (g)

Time (s)

Calculations and Granhs

1)

Use your results from this part of the experiment to plot a graph of the cell potential, Ess1, vorsos _{log $znz"ll}

lcut*]).

2)

Calculate the value of n, from the Nernst equation using the slope of the line from

the graph plotted in question (1).

3)

Write the balanced net ionic equation for the spontaneous reaction that took place

in the voltaic cells.

Electroplating

4)

_Quantiffthe charge, in coulombs, that passed though the circuit.

5)

Calculate the amount of current that was running through the solution during the

electroplating process.

6)

Calculate the number of copper atoms that were deposited on the object.

7)

Calculate the number of moles of electrons transferred during the entire

electroplating process.

Analvsis

1)

How does a decrease in the concentration of the active cation in the half cell

where the reduction occurs change the overall cell potential of a voltaic cell?

Justiff your answer using the Nernst Equation.

2)

How does a decrease in the concentration of the active cation in the half cell

where the oxidation occurs change the overall cell potential of a voltaic cell? Justify your answer using the Nemst Equation.

3)

Is it Ppssible to create a spontaneous reaction that

will

generate voltage with

Ni/Ni2* in the oxidation half cell andNi/Ni2* in the reduction half cell of a

galvanic cell? Justify your answer.

4)

Write the balanced equation for the half reaction that took place atthe anode.

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5)

Write the balanced equation for the half reaction that took place at the cathode.

6)

White the balanced net ionic equation for the overall reaction that took place in

the electrolytic cell.

7)

What was the approximate ratio of gas volumes collected (anode : cathode)?

8)

Explain why the volumes of gas collected were different.

Electroplating

9)

Write the balanced equation for the half reaction that took place at the anode.

10) Write the balanced equation for the half reaction that took place at the cathode.

11) Write the balanced net ionic equation for the overall reaction that took place in the

electrolytic cell.

Pre-Lab Ouestions

i)

A student starts with a 2.0M CtSOa solution and makes three successive 1/10

dilutions. What is the concentration of each of the three solutions that are

formed?

How many successive 1/10 dilutions of 1.0MCuSOa are required to produce a

I x

l0-7 UCusOosolution?

Write the equation for the half reaction that takes place at the anode tnthe Voltaic

Cells ond the Nernst Equstion section of this lab.

write the equation for the half reaction that takes place at the cathode in the

Voltaic Cells and the Nernst Equation section of this lab.

Tutorials to assist you with this material are available online at www.apchemsolutions.com.

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