A carbon compound ‘P’ on heating with excess conc. $H_2SO_4$ forms another carbon compound ‘Q’ which on addition of 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 and R and write chemical equations for the reactions involved.

Based on the chemical reactions and properties described:

• Compound P: Ethanol ($\text{CH}_3 ext{CH}_2 ext{OH}$ or $ext{C}_2 ext{H}_5 ext{OH}$)
• Compound Q: Ethene ($\text{CH}_2=\text{CH}_2$ or $ext{C}_2 ext{H}_4$)
• Compound R: Ethane ($\text{CH}_3-\text{CH}_3$ or $ext{C}_2 ext{H}_6$)

Chemical Equations Involved:

1. Dehydration of P to form Q:

When ethanol ('P') is heated with excess concentrated sulfuric acid at $443\text{ K}$, it undergoes dehydration to form ethene ('Q').

$$\text{CH}_3 ext{CH}_2 ext{OH} \xrightarrow{\text{Excess conc. } \text{H}_2\text{SO}_4, 443\text{ K}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$$

1. Hydrogenation of Q to form R:

Ethene ('Q') undergoes an addition reaction with hydrogen in the presence of a nickel catalyst to form ethane ('R').

$$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst, } \Delta} \text{CH}_3-\text{CH}_3$$

1. Combustion of R:

Ethane ('R') burns in the presence of oxygen to produce carbon dioxide and water. One molecule of ethane yields exactly two molecules of carbon dioxide and three molecules of water.

$$\text{C}_2\text{H}_6 + \frac{7}{2}\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$$

(Or expressed with whole-number coefficients:)

$$2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$$