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Explain why electrolysis of aqueous solution of NaCl gives H2 at cathode and Cl2 at anode. Write overall reaction.


Sodium chloride and water ionize as follows:

Sodium chloride and water ionize as follows:At cathode: Both Na+ and
At cathode: Both Na+ and H+ ions are present near the cathode. But the discharge potential of H+ is lower than that of Na+ ion. So H+ ions are discharged in preference to Na+ ions.

Sodium chloride and water ionize as follows:At cathode: Both Na+ and
Thus H2 gas is liberated at the cathode and Na+ ions remain in the solution.
At the anode: Both Cl and OH ions are present near the anode. As the discharge potential of Cl ions is lower than that of oh" ions, so Cl ions are discharged in presence to OH ions.

Sodium chloride and water ionize as follows:At cathode: Both Na+ and
Thus Cl2 is liberated at anode and OH ions remain in the solution.
The overall reaction is:
NaCl(aq) + H2 O(l) → Na+ (aq) + OH– (aq)
plus 1 half straight H subscript 2 left parenthesis straight g right parenthesis plus 1 half Cl subscript 2 left parenthesis straight g right parenthesis

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Show that the electrical work obtainable from a galvanic cell is given by the expression.
G° = -nFE°cell

The cell potential is simply related to the free energy change for the reaction. In an electro chemical cell the system does work by transferring the electric charge through an external circuit. The free energy change G is equal to electrical work done.

G = Welectrical

For reaction occurring in a electrochemical cell whose electrodes differ in potential by Ecell, the work done when amount of charge nF is transferred is given by
                   Welect = -nFEcell
∴                   G = -nFEcell
Under standard state conditions
ΔG° = – n FE°cell.
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What is corrosion? Describe the electrochemical phenomenon of rusting of iron.


Corrosion is the process of slowly eating away of the metal due to attack of the atmospheric gases on the surface of the metal resulting into the formation of compounds such as oxides, sulphides, carbonates, etc.

The corrosion of iron is called rusting.
According to theory of rusting, impure iron surface behaves as a small electrochemical cell in the presence of water containing dissolved oxygen or CO2.
The pure iron acts as anode and impure surface as cathode.
At Anode : Iron atom undergo oxidation spontaneously forming Fe2+ ion.
Fe → Fe2+ (aq) + 2e-cell = – 0.44 V
Fe2+ ions move into solution and electrons into cathodic area where they are picked up by H+ ions of the solution.
At cathode:
             2H++12O2+2e-  H2O   E°red = 1.23 V
H+ ions are produced by secondary reaction either from H2O or from H2CO3 (CO2 + H2O)
H2O ---->H+ + OH

H2CO3------> H+ + HCO3

The overall reaction of the corrosion cell may be represented as:
Fe(s)+2H+(aq)+12O2 Fe2+(aq)+H2O                                                                    E0cell = 1.67 V                  

The Fe2+ ions move through water and come at the surface where these are further oxidized into Fe3+ ions by atmospheric oxygen to form hydrate ferric oxide known as rust, Fe2O3.xH2O.
2Fe2++12O2+2H2O  Fe2O3+4H+        Fe2O3+xH2O  Fe2O3.xH2O                                            Rust


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What are fuel cells? Write reaction of a oxygen-hydrogen fuel cell. Write two advantages of the use of a hydrogen-oxygen fuel cell.

A device to convert chemical energy of fuel into electrical energy is called fuel cell.

A device to convert chemical energy of fuel into electrical energy is
Hydrogen-oxygen fuel cell: The cell consists of three compartments separated from one another by porous electrode. The hydrogen gas is fed into one compartment and the oxygen gas is fed into another compartment. These gases then diffuse slowly through the electrodes and react with an electrolyte that is in the central compartment. The electrodes are made of a conducting material, such as graphite, with a sprinkling of platinum to act as a catalyst, and the electrolyte is an aqueous solution of a base. The reactions are

A device to convert chemical energy of fuel into electrical energy is

Advantages: (i) Fuel cells are efficient and free from pollution.
(ii) The only product in the reaction of fuel cell is water which can be removed and the astronauts of a spacecraft can drink it.


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Define the terms equivalent and molar conductivity. What are their physical signific-ance?

Equivalent conductivity : The conductivity of a volume (V) of a solution containing one equivalent of electrolyte placed between two electrodes separated by unit distance apart and of large enough area of cross-section to hold the entire volume (V) is called equivalent conductance. It is denoted by Λeq.
Λeq = k x V
where k = Specific conductance
V = Volume of solution containing one equivalent of electrolyte.

Molar conductivity : It is the conductivity of volume (V) of a solution containing 1 mole of a dissolved electrolyte place between two electrodes separated by unit distance apart and of enough area of cross-section to hold the entire volume V. It is denoted by Λm.
Λm= k x V = k / V where V = Volume of solution Containing 1 mole of electrolyte C = molarity of solution k = Specific conductivity.

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