JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Chemical Thermodynamics

The standard enthalpies of formation of

C O_{2} (g), H_{2} O (l)

and glucose

(s)

at 25 °C are

- 400 k J / m o l, - 300 k J / m o l

and

- 1300 k J / m o l

, respectively. The standard enthalpy of combustion per gram of glucose at 25 °C is

(A) $+ 2900 k J$
(B) $- 2900 k J$
(C) $- 16.11 k J$ ✓ Correct
(D) $+ 16.11 k J$

Answer: (C)

Solution

The combustion reaction is:

C_{6} H_{12} O_{6} (s) + 6 O_{2} (g) \to 6 C O_{2} (g) + 6 H_{2} O (l)

Using Hess's law:

Δ H_{co mb} = 6Δ H_{f} (C O_{2}) + 6Δ H_{f} (H_{2} O) - Δ H_{f} (g l u cose)

= 6 (- 400) + 6 (- 300) - (- 1300)

= - 2400 - 1800 + 1300 = - 2900 kJ/mol

Molar mass of glucose = 180 g/mol

Per gram:

\frac{- 2900}{180} = - 16.11 kJ/g

JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Single correct option

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