CORROSION CHEMISTRY CMT 552
EXPERIMENT 1
GALVANIC AND ELECTROLYTIC CELLS
:Siti Marzidah bt Md Mohtar Name 2009963439 Group Members :Nor Hazwani bt Zubairi 2009659246 Umi Athirah bt Rosadey 2009111729 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 halfcells 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 2g NaCl / 100ml 2g NaCl / 100ml 0.1 M CuSO4 0.1 M CuSO4
Electrodes Carbon (graphite) Carbon (graphite) Carbon (graphite) Polished copper metal strips
Result A-Galvanic cell Galvanic Cell
Measured
Anode
Equation (Anode)
Cathode
Ecell
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
Reduction potential
potential(measured)
(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
NaCl
C(gr)
No changes
Gas
Yes at anode observe bubble
Equations
Anode: 2 Cl-
Cl2 + 2 e-
Cathode: 2Na+ + 2e-
2Na
Overall: 2 Na+ + 2Cl- (l ) Cl2( g )
2 Na(l ) +
NaBr
C(gr)
No changes
Yes at anode observe bubble
Anode: 2Br -
Br 2+ 2 e-
Cathode: 2Na+ +2 e-
2Na
Overall: 2N a+ + 2Br - (l )
2 Na(l ) +
Br 2(g) CuSO4
C(gr)
Blue to red
No gas,
Anode: 4OH-
At anode
Cathode: 2Cu2+ + 4 e-
precipitate At cathode-
2H2O + O2 +4e2Cu
Overall: 2Cu2+ +4OH- (l )
2Cu(l ) +
2H2O + O2 (g)
corroded
CuSO4
Cu(S)
No changes
Cu2+ + 2 e-
No gas,
Anode: Cu
At anode
Cathode: Cu2+ + 2 e-
precipitate
Overall: Cu
Cu
Cu
At cathode corroded
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
