Ncert Exercises & Solutions

On the basis of thermochemical equations (i), (ii), and (iii), find out which of the algebraic relationships given in options (a) to (d) is correct:

(i)	C(graphite) + O₂(g) → CO₂(g) ; ∆_rH = x kJmol^-1
(ii)	C(graphite) + \(\frac{1}{2}\)O₂(g) → CO(g) ; ∆_rH = y kJ mol^-1
(iii)	CO(g) + \(\frac{1}{2}\)O₂(g) → CO₂(g); ∆_rH = z kJ mol^-1

1. z=x+y

2. x=y-z

3. x=y+z

4. y=2z-x

Hint: Hess's Law of Constant Heat Summation

The algebraic relationships of the given reaction is

equation (a) - equation (b) - equation (c)

C (graphite) + O_{2} (g) \to {CO}_{2} (g); ∆_{r} H = x kJ {mol}^{- 1}

C (graphite) + \frac{1}{2} O_{2} (g) \to CO (g); ∆_{r} H = y kJ {mol}^{- 1}

CO (g) + \frac{1}{2} O_{2} (g) \to {CO}_{2} (g); ∆_{r} H = z kJ {mol}^{- 1}

Hence, x-y=z or x=y+z