Question

A carbon compound 'P' on heating with excess conc. ${H_2}S{O_4}$ forms another carbon compound 'Q' which in addition to hydrogen in the presence of nickel catalyst forms a saturated carbon compound 'R'. One molecule of 'R' on combustion forms two molecules of carbon dioxide and three molecules of water. Identify P, Q, R and write chemical equations for the reactions involved.

Answer 1

Hint: R is a saturated carbon also called an alkane. And on combustion, R gives two molecules of carbon dioxide and three molecules of water; this means the alkane is of 2 carbon atoms and 6 hydrocarbons. Saturated carbons are formed by hydrogenation of unsaturated hydrocarbons. So Q is an unsaturated hydrocarbon with 2 carbon atoms. Find which compounds when heated in the presence of conc. ${H_2}S{O_4}$ gives unsaturated hydrocarbons. And this will decide our answers.

Complete step by step solution: 

We are given that P is heated in the presence of conc. ${H_2}S{O_4}$ to give Q and on adding hydrogen in the presence of nickel Q gives R. R is a saturated carbon compound and on combustion it gives carbon dioxide and water.
We have to find P, Q and R.
So we will solve this question using backtracking, which means we will start with R and then Q and then P.
R is a saturated carbon compound which on combustion gives two carbon dioxide molecules and water molecules.
$R + {O_2} \to 2C{O_2} + 3{H_2}O$
As we can see the carbon compound must contain 2 carbon atoms, a saturated carbon with 2 carbons is ethane. Therefore, R is ethane. Molecular formula of ethane is ${C_2}{H_6}\left( {C{H_3} - C{H_3}} \right)$
$C{H_3} - C{H_3} + \dfrac{7}{2}{O_2} \to 2C{O_2} + 3{H_2}O$
Hydrogenation of unsaturated carbon compounds gives saturated carbon compounds. Therefore, Q is unsaturated with 2 carbon atoms which will be ethene, ${C_2}{H_4}\left( {C{H_2} = C{H_2}} \right)$.
$
  Q + {H_2}\xrightarrow{{Ni}}C{H_3} - C{H_3} \\

  C{H_2} = C{H_2} + {H_2}\xrightarrow{{Ni}}C{H_3} - C{H_3} \\
 $
Alcohols when heated in the presence of conc. ${H_2}S{O_4}$ dehydrates and gives alkenes, here ethene. So the alcohol with 2 carbon atoms is ethyl alcohol or ethanol, ${C_2}{H_5}OH\left( {C{H_3} - C{H_2} - OH} \right)$
$
  P\xrightarrow[\Delta ]{{conc.{H_2}S{O_4}}}C{H_2} = C{H_2} \\

  C{H_3} - C{H_2} - OH\xrightarrow[\Delta ]{{conc.{H_2}S{O_4}}}C{H_2} = C{H_2} + {H_2}O \\
 $
Therefore, P is ethanol, Q is ethene and R is ethane.

Note: Alkanes are saturated hydrocarbons, Alkenes and Alkynes are unsaturated hydrocarbons because they contain carbon-carbon double and triple bonds. Do not confuse naming one with another as they have almost similar names. Unsaturated hydrocarbons lose their double and triple bonds to form saturated hydrocarbons with carbon-carbon single bonds. And combustion is heating or burning a compound in the presence of oxygen gas.