CORROSION CHEMISTRY

CMT 552

EXPERIMENT 1

GALVANIC AND ELECTROLYTIC CELLS

Name :Siti Marzidah bt Md Mohtar 2009963439

Group Members :Nor Hazwani bt Zubairi2009659246

Umi Athirah bt Rosadey2009111729

Date experiment:3 February 2011

Due Date :17 February 2011

EXPERIMENT 1

Title : Galvanic and electrolytic cell

Objective

a) to distinguish between galvanic and electrolytic cells

b) to measure the relative reduction potentials for a number of redox couples in a galvanic cell

c) to develop an understanding of the movement of electrons, anions and cations in a galvanic cell

d) to identify the reactions occurring at the anode and cathode during the electrolysis of various aqueous salt solutions

Introduction

A Galvanic cell consists of two half-cells. In its simplest form, each half-cell consists of a

metal and a solution of a salt of the metal. The salt solution contains a cation of the metal and an

anion to balance the charge on the cation. In essence the half-cell contains the metal in two

oxidation states and the chemical reaction in the half-cell is an oxidation-reduction (redox)

reaction. In a galvanic cell one metal is able to reduce the cation of the other and, conversely, the

other cation can oxidize the first metal. The two half-cells must be physically separated so that

the solutions do not mix together. A salt bridge or porous plate is used to separate the two

solutions.The number of electron transferred in both directions must be the same, so the two

half-cells are combined to give the whole-cell electrochemical reaction.Anions must also be

transferred from one half-cell to the other. When a metal in one half-cell is oxidized, anions must

be transferred into that half-cell to balance the electrical charge of the cation produced. The

anions are released from the other half-cell where a cation is reduced to the metallic state. Thus,

the salt bridge or porous membrane serves both to keep the solutions apart and to allow the flow

of anions in the direction opposite to the flow of electrons in the wire connecting the

electrodes.The voltage of the Galvanic cell is the sum of the voltages of the two half-cells. It is

measured by connecting a voltmeter to the two electrodes. The voltmeter has very high

resistance, so the current flow is effectively negligible. When a device such as an electric motor

is attached to the electrodes, a current flows and redox reactions occur in both half-cells. This

will continue until the concentration of the cations that are being reduced goes to zero.For the

Daniell cell, depicted in the figure, the two metals are zinc and copper and the two salts are

sulfates of the respective metal. Zinc is the more reducing metal so when a device is connected to

the electrodes, the electrochemical reaction is

Zn + Cu2+ → Zn2+ + Cu

The zinc electrode is dissolved and copper is deposited on the copper electrode. By definition,

the cathode is the electrode where reduction (gain of electrons) takes place, so the copper

electrode is the cathode. The cathode attracts cations, so has a negative charge. In this case

copper is the cathode and zinc the anode.Galvanic cells are typically used as a source of

electrical power. By their nature they produce direct current. For example, a lead-acid battery

contains a number of galvanic cells. The two electrodes are effectively lead and lead oxide.The

Weston cell was adopted as an International Standard for voltage in 1911. The anode is a

cadmium mercury amalgam, the cathode is made of pure mercury, the electrolyte is a (saturated)

solution of cadmium sulfate and the depolarizer is a paste of mercurous sulfate. When the

electrolyte solution is saturated the voltage of the cell is very reproducible, hence its use as a

standard.

Procedure

A-Galvanic cell : reduction potentials of several redox couples

1. Collect the electrodes, solutions and equipment

a) Four small beaker is obtained and filled with 0.1M solutions

b) The strips of copper, zinc, magnesium and iron metal is polished with emery paper

and rinsed briefly with diluted 1M HNO3 and with deionized water

c) These polished strips is used as electrodes and put in beaker with its respective

solutions

d) These electrodes is connected to the voltmeter by using two electrical wires that

attached to alligator clips

2. Set up Copper/Zinc cell

a) A Cu strip is placed in Cu(No3)2 solution and Zn strip is placed in the Zn(No3)2

solution

b) A piece of filter paper is rolled and flattened and wet with 0.1M KNo3 solution

c) The end of the filter paper is folded and inserted into the solution that is in the beaker

and functions as salt bridge

d) One electrode is connected to the negative terminal of the voltmeter and the other to

the positive terminal

3. Repeat for the remaining cells

a) The cell potentials for all possible galvanic cells that can be constructed from the four

redox couples is determined . A new salt bridge is prepared for each galvanic cell

4. Determine the relative reduction potentials

a) The relative potentials of the all redox couple is determined

B-Electrolytic cell : Electrolysis of aqueous salts solutions

1. Set up the electrolysis apparatus

a) Two wire leads that attached to alligator clips is connected to a direct current power

supply

b) A u-tube glass is mounted on a ring stand or a clamp at a retort stand

c) The alligator clip is connected to the responding electrodes

2. Electrolyze the solutions

a) The u-tube is filled with a solution according to the table until three-forth full

b) Any evidence of reaction in the anode and cathode chambers is watched and recorded

Electrolytic cell

Solution Electrodes2g NaCl / 100ml Carbon (graphite)2g NaCl / 100ml Carbon (graphite)0.1 M CuSO4 Carbon (graphite)0.1 M CuSO4 Polished copper metal strips

Result

A-Galvanic cell

Galvanic Cell

Measured

Ecell

Anode Equation (Anode) Cathode Equation

(Cathode)

Cu-Zn 0.84 Zn Zn(s) → Zn 2+ (aq)+ 2e- Cu Cu2+(aq) + 2e- → Cu(s)

Cu-Mg 1.45 Mg Mg(s) → Mg2+ (aq)+ 2e- Cu Cu2+(aq) + 2e-→ Cu(s)

Cu-Fe 0.64 Fe Fe(s) → Fe2+ (aq)+ 2e- Cu Cu2+(aq) + 2e-→ Cu(s)

Zn- Mg 0.68 Mg Mg(s) → Mg2+ (aq)+ 2e- Zn Zn2+(aq) + 2e-→ Zn(s)

Fe -Mg 0.80 Mg Mg(s) → Mg2+ (aq)+ 2e- Fe Fe2+(aq) + 2e-→ Fe(s)

Zn - Fe 0.20 Zn Zn(s) → Zn2+ (aq)+ 2e- Fe Fe 2+(aq) + 2e-→ Fe(s)

The overall equations for above reactions:

1. Zn(s) + Cu2+(aq) → Zn 2+ (aq)+ Cu(s)

2. Mg(s) + Cu2+(aq) → Mg2+ (aq)+ Cu(s)

3. Fe(s) + Cu2+(aq) → Fe2+ (aq)+ Cu(s)

4. Mg(s) + Zn2+(aq) → Mg2+ (aq)+ Zn(s)

5. Mg(s) + Fe2+(aq) → Mg2+ (aq)+ Fe(s)

6. Zn(s) + Fe2+(aq) → Zn 2+ (aq)+ Fe(s)

2. Compare the sum of the Zn-Mg and Cu-Mg cell potentials with the Zn- Cu cell potential.

The sum of the Zn-Mg and Cu-Mg cell potentials is 2.13 where as the Zn- Cu cell

potential is 0.84

3. Compare the sum of the Zn-Fe and Zn-Mg cell potentials with the Fe-Mg cell potential.

The sum of the Zn-Fe and Zn-Mg cell potentials is 0.88 where as the Fe-Mg cell potential

is 0.80.

Arrange the four redox couples in order of decreasing (measured) reduction potentials.

Redox couple Redox

potential(measured)

Reduction potential

(calculated)

% Error

Cu/Mg 1.45 2.19 33.78%

Fe –Mg 0.80 1.816 55.95%

Cu-Zn 0.84 1.10 23.64%

Zn- Mg 0.68 1.09 37.61%

B-Electrolytic cell

1.

Solution Electrodes Litmus test Gas Equations

NaCl C(gr) No changes Yes at anode

observe bubble

Anode: 2 Cl- Cl2 + 2 e-

Cathode: 2Na+ + 2e- 2Na

Overall: 2 Na+ + 2Cl- (l) 2 Na(l) +

Cl2(g)

NaBr C(gr) No changes Yes at anode

observe bubble

Anode: 2Br- Br2+ 2 e-

Cathode: 2Na+ +2 e- 2Na

Overall: 2N a+ + 2Br- (l) 2 Na(l) +

Br2(g)

CuSO4 C(gr) Blue to red No gas,

At anode

precipitate

At cathode-

corroded

Anode: 4OH- 2H2O + O2 +4e-

Cathode: 2Cu2+ + 4 e- 2Cu

Overall: 2Cu2+ +4OH- (l) 2Cu(l) +

2H2O + O2 (g)

CuSO4 Cu(S) No changes No gas,

At anode

precipitate

At cathode

corroded

Anode: Cu Cu2+ + 2 e-

Cathode: Cu2+ + 2 e- Cu

Overall: Cu Cu

2. Actually it may be different because of the ion of Cu2+ will oxidized in the solvent.

3. The electrode may have small amount of metal of the anode, it showing that oxidation state happened.

Discussion

In this experiment we are able to distinguish between galvanic cell and electrolytic cell.

A galvanic cell is composed to two half-cells connected by an external circuit and a salt bridge.

The oxidation and reduction half-reactions will take place in the cells. The redox reaction in a

galvanic cell is a spontaneous reaction. An electrolytic cell is one where a non-spontaneous

reaction occurs due to electrical charge that supplied was used to induce the electrolysis reaction.

A salt bridge is used in galvanic cell where a filter paper was soaked with a relatively inert

electrolyte, in this experiment; potassium nitrate was used because they are chemically inert. It

allows the flow of ions to maintain a balance in charge between the oxidation and reduction

vessels while keeping the contents of each separate. With the charge difference balanced,

electrons can flow once again, and the reduction and oxidation reactions can proceed. To identify

the reactions occurring at the anode and cathode, NaCl and NaBr were in the catogeries of pure

molten salt. So, in the experiment, cation will be reduced and anion will be oxidized. The

movement of ion occurred because they are attracted by the oppositely charge electrodes. The

cation will flows toward the cathode and anion will flow toward the anode. Copper (II) sulphate

that was used was one type of aqueous salt solution.

Conclusion

From this experiment, we had successfully distinguish between galvanic cell and

electrolytic cell and measure the reduction potentials for the redox couples in the galvanic cell.

We also had develop an understanding on the movement of electrons, anions and cation in

galvanic cell and also identify the reaction that occurring at the anode and cathode during the

electrolysis. Objective is succeeded.

References

1. Textbook: Chemistry 9th Edition; Raymond Chang

2. www.electrochemistry.com

3. www.wikipedia/.com