Electrochemistry - Practice Questions & MCQ

ELECTROCHEMISTRY
Electrochemistry is the branch of the science that deals with the transformation of chemical energy into electrical energy and vice versa or it deals with the relationship between electrical and chemical energy produced in a redox reaction.

Galvanic Cell (or Voltanic Cell)
Consider the following redox reaction:

$\mathrm{Zn}(\mathrm{s})+\mathrm{Cu}^{2+}(\mathrm{aq}) \longrightarrow \mathrm{Cu}(\mathrm{s})+\mathrm{Zn}^{2+}(\mathrm{aq})$

In the above reaction, Zn displaces copper ions (Cu²⁺) from aqueous solution. This reaction can be achieved very easily in practice. Put a Zn rod into a solution of CuSO₄ (containing Cu²⁺ ions). It is observed that the blue colour of CuSO₄ solution disappears after some time. In this situation, Zn loses 2 electrons per atom and Cu²⁺ ions in the solution accepts them. Cu²⁺ ions from solution in this manner are deposited out in the form of solid Cu and Zn goes into the solution as Zn²⁺ (colourless). The reaction can well be understood in terms of two half-reactions:

Oxidation: $\quad \mathrm{Zn}(\mathrm{s}) \longrightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}$
Reduction: $\quad \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})$

Now, we can make the same reaction take place even if the copper ions and zinc rod are not in direct contact. If we put the Cu²⁺ ions and Zn rod in two separate containers and connect the two by a conducting metallic wire and introduce an inverted U shape instrument (called as salt-bridge), then electrons will still be transferred through the connecting wires. The electrons from Zn rod travel to Cu²⁺ ions through the connecting wires and the same reaction takes place. This flow of electrons through the wire generate electricity.

Electrolysis

It is a process by which an electric current is passed through a substance to effect a chemical change. The chemical change is one in which the substance loses or gains an electron (oxidation or reduction). The process is carried out in an electrolytic cell, an apparatus consisting of positive and negative electrodes held apart and dipped into a solution containing positively and negatively charged ions. The substance to be transformed may form the electrode, may constitute the solution or may be dissolved in the solution. Electric current enters through the negatively charged electrode (cathode); positively charged components of the solution travel to this electrode, combine with the electrons and are transformed to neutral elements or molecules. The negatively charged components of the solution travel to the other electrode (anode), give up their electrons and are transformed into neutral elements or molecules. If the substance to be transformed is the electrode, the reaction is generally one in which the electrode dissolves by giving up electrons.