Question

Which of the following is a redox reaction?

• A. ${CaCO}_3\to CaO+{CO}_2$
• B. $H_2+{Cl}_2\to 2HCl$
• C. $CaO+2HCl\to {CaCl}_2+H_{2}O$
• D. $NaOH+HCl\to NaCl+H_{2}O$

Answer 1

The correct option is B

${\mathrm{H}}_2+{\mathrm{Cl}}_2\to 2\mathrm{HCl}$

Explanation for correct option:

(B):

• A reaction in which oxidation and reduction take place simultaneously is called a redox reaction.
• Oxidation refers to the loss of electrons while reduction refers to the gain of electrons.
• The given equation is:
• ${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{HCl}\left(\mathrm{g}\right)$
• In this reaction, the oxidation state of Chlorine ($\mathrm{Cl}$) changes from zero in Chlorine gas (${\mathrm{Cl}}_2$) to -1 in Hydrochloric acid ($\mathrm{HCl}$). Hence, it undergoes reduction.
• The oxidation state of Hydrogen ($\mathrm{H}$) changes from zero in Hydrogen gas (${\mathrm{H}}_2$) to +1 in Hydrochloric acid ($\mathrm{HCl}$). Hence, it undergoes oxidation.
• Since reduction and oxidation take place simultaneously, thus, it is a redox reaction.

Explanation for incorrect options:

(A)

• The given equation is:
• ${\mathrm{CaCO}}_3\left(\mathrm{s}\right)\to \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$
• The oxidation states of Calcium ($\mathrm{Ca}$) is +2 in Calcium carbonate (${\mathrm{CaCO}}_3$) and +2 in Calcium oxide ($\mathrm{CaO}$) as well.
• Similarly, the oxidation states of Carbon ($\mathrm{C}$) and Oxygen ($\mathrm{O}$) remain the same on the reactant and product side. Hence, it is not a redox reaction.

(C)

• The given equation is:
• $\mathrm{CaO}\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
• The oxidation states of Calcium ($\mathrm{Ca}$) is +2 in Calcium oxide ($\mathrm{CaO}$) and +2 in Calcium chloride (${\mathrm{CaCl}}_2$) as well.
• Similarly, the oxidation states of Chlorine ($\mathrm{Cl}$) and Oxygen ($\mathrm{O}$) remain the same on the reactant and product side. Hence, it is not a redox reaction.

(D)

• The give equation is:
• $\mathrm{NaOH}\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)\to \mathrm{NaCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
• The oxidation states of Sodium ($\mathrm{Na}$) is +1 in Sodium hydroxide ($\mathrm{NaOH}$) and +1 in Sodium chloride ($\mathrm{NaCl}$) as well.
• Similarly, the oxidation states of Hydrogen ($\mathrm{H}$), Chlorine ($\mathrm{Cl}$) and Oxygen ($\mathrm{O}$) remain the same on the reactant and product side. Hence, it is not a redox reaction.

Final Answer:

Hence, option (B) is correct.