In a thermite reaction Fe2O3 + 2Al -> Al2O3 + 2Fe, the standard enthalpies of formation are Delta_fH deg(Al2O3)=-1676 and Delta_fH deg(Fe2O3)=-824 kJ/mol. The heat released when 5.4 g of Al (Al = 27) is completely consumed, assuming excess Fe2O3, is

JEE Advanced levelSingle correct optionDifficulty 3 of 3mt107

In a thermite reaction $Fe X_{2} O X_{3} + 2 Al Al X_{2} O X_{3} + 2 Fe$ , the standard enthalpies of formation are $Δ_{f} H^{\circ} (Al X_{2} O X_{3}) = - 1676$ and $Δ_{f} H^{\circ} (Fe X_{2} O X_{3}) = - 824 kJ/mol$ . The heat released when $5.4 g$ of Al (Al = 27) is completely consumed, assuming excess $Fe X_{2} O X_{3}$ , is

Δ H^{\circ} = - 1676 - (- 824) = - 852 kJ

per mole of reaction, which uses 2 mol Al. Moles of Al

= 5.4/27 = 0.2

, so extent of reaction

= 0.1

. Heat released

= 0.1 \times 852 = 85.2 kJ