Electrochemistry: Important Questions

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Very Short Answer Question [1 Marks Question]

Ques. Corrosion of automobiles is more of a concern in the winter when salts are used to melt ice and snow on the roadways. Why?

Ans. When two metals are brought together under the surface of an electrolyte, a short-circuited cell is generated. Corrosion is a bigger problem in the winter since cars contain metals like lead and chromium, and the salt used to melt ice is NaCl, which works as an electrolyte and forms a short circuit cell.

Ques. What does the negative symbol in \(E^{\circ} _{Zn^{2+} /Zn}\) = – 0.76 V stand for?

Ans. It suggests that Zn is more reactive than hydrogen or that it is a more powerful reducing agent. Zinc will be oxidized to Zn²⁺ ions in a cell with a zinc electrode and a conventional hydrogen electrode present in two half cells, whereas H⁺ ions will be reduced to hydrogen.

Ques. What is the meaning of electrode potential?

Ans. The transport of charged species across the interface causes electrode potential to develop at the interface between an electrode and an electrolyte.

Ques. When brine (NaCl solution) is electrolyzed, how will the pH be affected?

Ans. NaCl is a salt that has both a strong acid and a strong base. It's a neutral color. The NaCl solution has a pH of 7. However, when the aqueous solution is electrolyzed, Na⁺ and OH^– ions are produced. The pH of the solution rises as it gets basic.

Ques. What are the benefits of fuel cells against main and secondary batteries?

Ans. Primary batteries or cells have a finite number of reactants that must be used before they may be discharged. Recharging secondary batteries or cells is possible, but it takes a long time. Fuel cells can operate indefinitely if the reactants (fuel) are available.

Ques. What is the definition of an electrochemical series? How does it determine whether a redox reaction is feasible?

Ans. The electrochemical series is the arrangement of metals and ions in ascending order of their electrode potential values.

The reduction half-reaction with a lower reduction potential will operate as an anode, while the one with a higher value will act as a cathode. There will be no reversal of the situation.

Ques. Is it possible for an electrochemical cell to function as an electrolytic cell? How?

Ans. Yes, by supplying an external opposing potential higher than its internal electrical potential, an electrochemical cell may be transformed into an electrolytic cell.

Ques. The potential of a single electrode cannot be established. Why?

Ans. Because reduction and oxidation occur concurrently, a single half-cell does not exist. As a result, single electrode potential cannot be measured.

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Short Answer Question [2 Marks Question]

Ques. What are the factors that influence an electrolyte's conductivity?

Ans. An electrolyte's conductivity is determined by :

• The ion sizes created
• The solvent's nature and viscosity.
• Electrolyte concentration
• temperature

Ques. Metal conductivity decreases as temperature rises, but electrolyte conductivity rises. Why?

Ans. Higher temperature raises the K.E. of metal cations, which obstructs the free passage of electrons and reduces metal conductivity, however, increased temperature improves the mobility of ions in electrolytes, which enhances ion conductance.

Ques. What are the electrolysis laws of Faraday?

Ans. Faraday's Laws of Electrolysis:

First Law: The amount of chemical reaction that happens at any electrode during current electrolysis is proportional to the amount of electricity carried through the electrolyte.

Second Law: The chemical equivalent weights of various compounds freed by the same amount of power going through the electrolytic solution are proportionate.

Ques. Why does a solution's conductivity drop when it is diluted?

Ans. A solution's conductivity is defined as the conductance of ions in a unit volume of the solution. When a solution is diluted, the quantity of ions (which are responsible for transporting current) falls. As a result, as a solution is diluted, its conductivity drops.

Ques. What is the difference between a main and secondary battery?

Ans. The reaction occurs only once in primary batteries, and after a period of usage, the battery turns dead and cannot be reused. A secondary battery may be recharged by flowing current through it in the opposite direction after it has been used, allowing it to be used again.

Ques. Is it possible to keep copper sulfate solutions in a zinc pot?

Ans. Copper is more reactive than zinc. As a result, zinc can remove copper from its salt solution. If copper sulfate solution is kept in a zinc pot, zinc will eat away at the copper.

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Long Answer Question [3 Marks Question]

Ques. Define conductivity and molar conductivity in an electrolyte solution. Why does dilution reduce the conductivity of a solution?

Ans. The conductance of one unit volume of solution held between two platinum electrodes with the unit area of cross-section and at unit length is the conductivity of a solution at any given concentration. The conductance of volume V of a solution containing one mole of electrolyte held between two electrodes with an area of cross-section A and unit length is the molar conductivity of a solution at a certain concentration.

For both weak and strong electrolytes, conductivity diminishes as concentration decreases. This is due to a reduction in the number of ions per unit volume that transport current in the solution.

Ques. Define conductivity and molar conductivity in an electrolyte solution. Discuss how they vary as the temperature changes.

Ans.  The conductivity of a solution is defined as the conductance of a solution with a length of 1 cm and a cross-section area of 1 sq. cm. It is symbolized by the letter K.

S cm^-1 is the unit of measurement.

The conductance of all the ions produced from one mole of the electrolyte dissolved in V cm³ of the solution when the electrodes are 1 cm apart and the area of the electrodes is so big that the whole solution is contained between them is known as molar conductivity.

S cm² mol^-1 is its unit.

The conductivity and molar conductivity of electrolytes increases with increasing temperature.

Ques. What exactly is corrosion? Explain the electrochemical theory of iron rusting and list the reactions that occur throughout the rusting process.

Ans. Corrosion: Corrosion is the degradation of a material because of its response to its surroundings. Corrosion is a result of an electrochemical reaction. Oxidation occurs at a specific location on an iron item, and that location acts as an anode, initiating the process.

At the Anode: 2Fe → 2Fe²⁺ + 4e^–

In the presence of H+, electrons released at the anodic point migrate through the metal and to another place on the metal, reducing oxygen. This location acts as a cathode.

At the cathode : O₂ + 4H⁺ + 4e^–

Overall reaction: 2Fe + O₂ + 4H⁺ → 2Fe²⁺ + 2H₂O

Ques. Why does an electrochemical cell eventually stop working? An electrode's reduction potential is determined by the concentration of the fluid with which it comes into contact.

Ans. The concentration of reactants drops as the cell functions. The equilibrium will then change in the other way, according to Le Chatelier's principle. On the other hand, if the concentration is higher on the reactant side, the equilibrium will be shifted ahead. The concentration in an anodic compartment in the cathodic compartment drops as the cell operates, and so the E°_cathode lowers.

E°_cell = E°_cathode – E°_anode is now the cell's EMF.

A decrease in E°_cathode and a corresponding increase in E°_anode will mean that EMF of the cell will decrease and will ultimately become zero i.e., cell stops working after some time.

Ques. State the independent ion migration Kohlrausch law. Why does a solution's conductivity drop when it is diluted?

Ans. At infinite dilution, when the dissociation is complete, Kohlrausch's law of independent migration of ions asserts that each ion contributes a certain amount to the molar conductance of the electrolyte, regardless of the type of the other ion with which it is coupled. Kohlrausch's law also states that an electrolyte limiting molar conductivity is equal to the total of the individual contributions of the electrolyte's ions.

Because the quantity of ions per unit volume supplied by an electrolyte reduces with dilution, the conductivity (K) of the electrolyte solution drops.

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Very Long Answer Question [5 Marks Question]

Ques. 

1. Explain the following:
   1. CO₂ is always present in natural water. Explain its effect (increases, stops, or no effect) on rusting of iron.
   2. Rusting of iron is quicker in saline water than in ordinary water. Explain.
2. Discuss the use of electrical protection to keep iron pipes in subterranean water from rusting.

Ans. (i)

(a) Presence of CO₂ in natural water increases rusting of iron. It dissolves in water to form H₂CO₃ which gives H⁺ ions. The H⁺ ions accelerate the process of corrosion.

In rusting of iron, Fe oxidizes to Fe²⁺ ions

Fe (s) → Fe²⁺ (aq) + 2e^– (anode)

The released electrons go to the cathode and reduce oxygen in the presence of H⁺ ions (obtained from H₂CO₃). The reaction occurs at the cathode. Thus, CO₂ increases rusting.

O₂ (g) + 4H⁺(aq) + 4e^– → 2H₂O (l)

(b) In saline water (saltwater), iron rusts faster than in regular water. This is because saline water enhances the electrical conductivity of the electrolyte solution generated on the metal surface. As a result, if saltwater is available, corrosion becomes a more critical issue.

(ii) This approach prevents corrosion of iron goods in contact with water, such as subterranean water pipelines. Iron is associated with more active metals such as magnesium and zinc. This reduces the likelihood for it to lose electrons, preventing corrosion. The cathodes of magnesium and zinc can be attached to the surface of iron or burnt in subsurface water near the pipes.

Ques. State and explain Faraday's electrochemical laws. What is the equivalence in electrochemistry?

Ans. The First Law of Electrolysis is as follows: The amount of mass deposited on an electrode during electrolysis is exactly proportional to the amount of electricity going through it.

zct = W

Where W is the mass deposited on the electrode, c is the current, and t is the duration in seconds.

The deposited mass of material on the electrode is equal to its electrochemical equivalent if a 1-amp current is conducted through a solution in one second. The electrochemical equivalent is z in this case.

Electrolysis' Second Law: When the same amount of power is transmitted through the various electrolytic cells, the deposited masses on the electrodes are precisely proportional to their chemical equivalents.

Ques. What exactly do you mean when you say, "electrolytic conduction"? What are the variables that influence electrolyte conduction? What role does temperature play in electrolytic conduction? How does electrolytic conductance become measured in the lab? Explain what regular hydrogen electrodes are and what they're used for.

Ans. Electrolytic conductance is the ability of electrolytic solutions to enable current to travel through them when an electric current passes through them.

Electrolytic conductance is influenced by a variety of factors:

• Ion concentrations
• The electrolyte's composition
• Temperature

Temperature effect: the electrolyte dissolves in the solution as the temperature changes. As a result, as the temperature rises, the solubility of electrolytes rises, and electrolytic conduction rises.

The conductivity of electrolytes will be determined by the resistance between two nodes. The electrolyte solution dissociates into positive and negative ions when electricity flows through it. Electrolyte conductivity can be measured using galvanic cells or the electrolysis technique.

A normal hydrogen electrode, also known as a standard hydrogen electrode, is an electrode that is used as a reference for all half-cell potential processes.

The potential of the typical hydrogen electrode is zero, which is used to compute the potentials of different half cells and concentrations.

Ques. The chemistry of iron corrosion is mostly an electrochemical process. Explain the processes that occur when the iron is corroded in the atmosphere.

Ans. The chemistry of iron corrosion is based on an electrochemical hypothesis including oxidation and reduction processes. According to this hypothesis, in the presence of water containing dissolved oxygen or carbon dioxide, the non-uniform surface of metals or impurities present in iron acts like microscopic electric cells (called corrosion couples). At various points on the metal surface, a coating of moisture containing dissolved CO₂ functions as an electrolytic solution. The anode in these miniature electrolytic cells is pure iron, whereas the cathodes are impure parts. The following steps are involved in the total rusting process:

Oxidation occurs at the anode of each electrochemical cell. Therefore, at each anode iron is oxidized to Fe²⁺ ions.

At anode:

Fe (s) → Fe²⁺ (aq) + 2e^– …… (i)

Thus, the metal atoms in the lattice pass into the solution as ions, leaving electrons on the metal itself. These electrons move towards the cathode region through the metal.

At the cathode of each cell, the electrons are taken up by hydrogen ions (reduction takes place). The H⁺ ions are obtained either from water or from the acidic substance in water:

H₂O → H⁺ + OH^– …… (ii)

or CO₂ + H₂O → H⁺ + H⁺CO₃^– …… (iii)

At cathode:

H⁺ + e^– → H⁺ …… (iv)

Thus, hydrogen atoms on the iron surface reduce dissolved oxygen.

4H + O₂ → 2H₂O …….(v)

Therefore, the overall reaction at the cathode of different electrochemical cells may be written as:

4H⁺(aq) + O₂ (g) + 4e^– → 2H₂O (l) ……. (vi)

The overall redox reaction may be written by multiplying the reaction at anode Eq. (i) by 2 and adding the reaction at cathode Eq. (iv) to equalize a few electrons lost and gained, i.e., Oxidation half-reaction:

Fe (s) → Fe²⁺ (aq) + 2e^– × 2

Reduction half reaction:

4H⁺(aq) + O₂ (g) + 4e^– → 2H₂O (l)

Overall cell reaction:

2Fe (s) + 4H⁺(aq) + O₂ (g) → 2Fe²⁺(aq) + 2H₂O (l)

The ferrous ions are oxidized further by atmospheric oxygen to Fe³⁺ (as Fe₂O₃) and form rust

4Fe²⁺ + O₂ (g) + 4H₂O → 2Fe₂O₃ + 8H⁺

and Fe₂O₃ + xH₂O → Fe₂O₃. xH₂O

The H⁺ ions produced above are also used for reaction (iv).

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