Given : (i) C (graphite) $+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta \mathrm{rH}^{\circ}=\mathrm{x} \mathrm{kJ} \mathrm{mol}^{-1}$ (ii) C (graphite) $+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{CO}(\mathrm{g}) ; \Delta \mathrm{rH}^{\circ}=\mathrm{y} \mathrm{kJ} \mathrm{mol}^{-1}$ (iii) $\mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta \mathrm{rH}^{\circ}=\mathrm{z} \mathrm{kJ} \mathrm{mol}^{-1}$ Based on the above thermochemical equations, find out which one of the following algebraic relationships is correct?

JEE Main · 2019 · 12 Jan 2019 · Shift 2 · Chemical Thermodynamics

Given :
(i) C (graphite)

+ O_{2} (g) \to CO_{2} (g); Δ rH^{\circ} = x kJ mol^{- 1}

(ii) C (graphite)

+ \frac{1}{2} O_{2} (g) \to CO (g); Δ rH^{\circ} = y kJ mol^{- 1}

(iii)

CO (g) + \frac{1}{2} O_{2} (g) \to CO_{2} (g); Δ rH^{\circ} = z kJ mol^{- 1}

Based on the above thermochemical equations, find out which one of the following algebraic relationships is correct?

Answer: (A)

Adding the formation of

CO

from graphite (

Δ H = y

) to its further oxidation to

CO X_{2}

(

Δ H = z

) reproduces the direct combustion of graphite to

CO X_{2}

(

Δ H = x

). By Hess's law the enthalpies combine the same way, so

x = y + z

JEE Main · 2019 · 12 Jan 2019 · Shift 2 · Single correct option