Question

Which of the following reaction is not a redox reaction?

• A. (A) Combustion
• B. (B) Double displacement
• C. (C) Synthesis of two elements
• D. (D) Decomposition into elements

Answer 1

The correct option is B

(B) Double displacement

Explanation for correct option:

(A) Combustion

• A combustion reaction is a type of redox reaction that occurs when a substance combines with molecular oxygen to form an oxygen compound of another element in the reaction. An example is the burning of methane, the main component of natural gas.

$$\begin{gathered} {\mathrm{CH}}_4\left(\mathrm{g}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)⟶{\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right) \\ \left(\mathrm{Methane}\right)\left(\mathrm{Oxygen}\right)(\mathrm{Carbon}(\mathrm{Water}) \\ \mathrm{dioxide}) \end{gathered}$$

• Redox reaction is the reaction in which oxidation and reduction take place simultaneously. Oxidation is the addition of oxygen or loss of hydrogen or loss of electrons. Reduction is the opposite of oxidation. In the combustion of methane, the oxygen (O₂) is reduced and the other elements are oxidized.

Explaantion for incorrect options:

(B) Double displacement

• Double displacement is not a redox reaction. Double displacement is the reaction in which the ions are exchanged to form the product. Since the exchange of ions takes place neither reduction nor oxidation takes place.
• For example, silver nitrate reacts with sodium chloride to form silver chloride and sodium nitrate.

$$\begin{gathered} {\mathrm{AgNO}}_3\left(\mathrm{aq}\right)+\mathrm{NaCl}\left(\mathrm{aq}\right)\to \mathrm{AgCl}\left(\mathrm{s}\right)+{\mathrm{NaNO}}_3\left(\mathrm{aq}\right) \\ (\mathrm{Silver}(\mathrm{Sodium}(\mathrm{Silver}(\mathrm{Sodium} \\ \mathrm{nitrate})\mathrm{chloride})\mathrm{chloride})\mathrm{nitrate}) \end{gathered}$$

Here, ${\mathrm{NO}}_3^{-}$ in ${\mathrm{AgNO}}_3$ is replaced by ${\mathrm{Cl}}^{-}$ from NaCl and ${\mathrm{Cl}}^{-}$ in NaCl is replaced by ${\mathrm{NO}}_3^{-}$ from ${\mathrm{NaNO}}_3$. So oxidation or reduction does not take place.

(C) Synthesis of two elements

• A synthetic reaction is a type of redox reaction. The synthetic or direct combination is the reaction in which two or more reactants combine to form a single product.
• Let's look at an example of combining sodium and chlorine to make sodium chloride.

$$\begin{gathered} 2\mathrm{Na}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{NaCl}\left(\mathrm{s}\right) \\ \left(\mathrm{Sodium}\right)\left(\mathrm{Chlorine}\right)(\mathrm{Sodium}\mathrm{chloride}) \end{gathered}$$

This is clearly a synthetic reaction. However, note that this is clearly an oxidation/reduction reaction, as each Na loses an electron to become ${\mathrm{Na}}^{+}$ and each Cl acquires an electron to become ${\mathrm{Cl}}^{-}$.

(D) Decomposition into elements

• Decomposition is a redox reaction. Decomposition is the reaction in which one reactant is broken down to form two or more products. When decomposed into elements, the oxidation number of one component increases from a negative value to zero, and the oxidation state of the other atom decreases from a positive value to zero.
• Therefore, the decomposition into elements is a redox reaction.
• For example, the decomposition of silver chloride gives silver and bromine.

$$\begin{gathered} \mathrm{AgCl}\left(\mathrm{s}\right)\to \mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right) \\ (\mathrm{Silver}\mathrm{chlroide})\left(\mathrm{Silver}\right)\left(\mathrm{Chlorine}\right) \end{gathered}$$

Here, ${\mathrm{Ag}}^{+}$ reduces to Ag and ${\mathrm{Cl}}^{-}$ oxidized to ${\mathrm{Cl}}_2$.

Hence, option b is correct.