Chapter 4 · Carbon and its Compounds

NCERT Class 10 Science Chapter 4 Intext Questions (Page 71)

All 2 questions of NCERT Class 10 Science Chapter 4 Intext Questions (Page 71) — Carbon and its Compounds — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Why ​is ​the ​conversion ​of ethanol to ethanoic ​acid ​an oxidation reaction?
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   Ethanol gains one oxygen atom and loses two hydrogen atoms on becoming ethanoic acid, and gain of oxygen is oxidation.

   Because in this conversion ethanol gains oxygen (and loses hydrogen): ${\text{CH}}_3{\text{CH}}_2\text{OH}$ (${\text{C}}_2{\text{H}}_6\text{O}$) becomes ${\text{CH}}_3\text{COOH}$ (${\text{C}}_2{\text{H}}_4{\text{O}}_2$) — one oxygen atom is added and two hydrogen atoms are removed. Addition of oxygen to a substance (or removal of hydrogen from it) is oxidation, so ethanol is oxidised to ethanoic acid. The oxygen is supplied by an oxidising agent, alkaline potassium permanganate or acidified potassium dichromate, on heating: ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\to {\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$.

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2. Q2
   A mixture of oxygen ​and ​ethyne is burnt for welding. Can ​you tell why a ​mixture ​of ethyne and air is not ​used?
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   In air ethyne burns incompletely with a cooler, sooty flame; in pure oxygen it burns completely with a very hot flame that melts metals.

   Ethyne is an unsaturated hydrocarbon with a high proportion of carbon, and its complete combustion needs a large amount of oxygen: $2{\text{C}}_2{\text{H}}_2+5{\text{O}}_2\to 4{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{heat and light}$. Air contains only about one-fifth oxygen (the rest is mostly nitrogen), so in air ethyne burns incompletely, giving a yellow, sooty flame that produces much less heat, while the nitrogen absorbs heat and cools the flame further; the soot would also spoil the metal being welded. In pure oxygen ethyne burns completely with a clean, extremely hot flame (about $3000{}^{\circ }\text{C}$), which is what is needed to melt metals for welding. That is why a mixture of ethyne and oxygen, and not ethyne and air, is used.

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