Question

When formic acid is heated with concentrated ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ , the gas evolved is :
A) only ${\text{C}}{{\text{O}}_{\text{2}}}$
B) only ${\text{CO}}$
C) a mixture of ${\text{CO}}$ and ${\text{C}}{{\text{O}}_{\text{2}}}$
D) a mixture of ${\text{S}}{{\text{O}}_2}$ and ${\text{C}}{{\text{O}}_{\text{2}}}$

Answer 1

Let us find out if there is any change in the oxidation number of carbon during the reaction. In other words, determine if the reaction between formic acid and concentrated sulphuric acid is a redox reaction or it is simply a dehydration reaction.

Complete answer:
When you heat formic acid in presence of concentric sulphuric acid, you get a mixture of carbon monoxide gas and water vapors.
${\text{HCOOH }}\xrightarrow[\Delta ]{{{\text{conc }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_4}}}{\text{ CO + }}{{\text{H}}_2}{\text{O}}$

In the above reaction, the concentrated sulphuric acid acts as a dehydrating agent. It removes a molecule of water from formic acid and carbon monoxide gas is left behind. The reaction of formic acid with concentric sulphuric acid is not a redox reaction as neither oxidation, nor reduction occurs during the reaction. There is no change in the oxidation number of carbon during the reaction. We can call the reaction of formic acid with concentric sulphuric acid a dehydration reaction as a water molecule is eliminated during the reaction.

Note: Concentrated sulphuric acid is a powerful dehydrating agent as it has strong affinity for water. Sulphuric acid removes water from several organic compounds. A black charred mass is usually left behind.