The thermite reaction involves aluminum and iron(III) oxide 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l) This reaction is highly exothermic and the liquid iron formed is used This reaction is highly exothermic and the liquid iron formed is used to weld metals. Calculate the heat released in kilojoules per

Question

The thermite reaction involves aluminum and iron [latex] (III) [/latex]oxide

[latex]2Al(s) + Fe_{2}O_{3}(s) → Al_{2}O_{3}(s) + 2Fe(l)[/latex]

This reaction is highly exothermic and the liquid iron formed is used to weld metals. Calculate the heat released in kilojoules per gram of Al reacted with [latex]Fe_{2}O_{3}[/latex] . The ΔHf° for Fe(l) is 12.40 kJ/mol.

Strategy The enthalpy of a reaction is the difference between the sum of the enthalpies of the products and the sum of the enthalpies of the reactants. The enthalpy of each species (reactant or product) is given by its stoichiometric coefficient times the standard enthalpy of formation of the species.

Accepted Answer

Using the given [latex]\Delta H^{°}_{f} [/latex] value for Fe(l) and other [latex]\Delta H^{°}_{f}[/latex] values in Appendix 2 and Equation (6.18), we write

[latex]\Delta H^{°}_{f}=[\Delta H^{°}_{f}(Al_{2}O_{3})+2\Delta H^{°}_{f}(Fe)]-[2\Delta H^{°}_{f}(Al)+\Delta H^{°}_{f}(Fe_{2}O_{3})][/latex]

=(-1669.8 kJ/mol)+2(12.40 kJ/mol)]-[2(0)+(822.2 kJ/mol)]

=-822.8 kJ/mol

This is the amount of heat released for 2 moles of Al reacted. We use the ratio

[latex]\frac{-822.8 kJ}{2 mol Al}[/latex]

to convert to kJ/g Al. The molar mass of Al is 26.98 g, so

heat released per gram of Al=[latex]\frac{-822.8 kJ}{2 mol Al} imes \frac{1 mol Al}{26.98 g Al} [/latex]

=-15.25 kJ/g

Check Is the negative sign consistent with the exothermic nature of the reaction? As a quick check, we see that 2 moles of Al weigh about 54 g and give off about 823 kJ of heat when reacted with [latex]Fe_{2}O_{3}[/latex] . Therefore, the heat given off per gram of Al reacted is approximately −830 kJ/54 g or −15.4 kJ/g.

Question 6.10: The thermite reaction involves aluminum and iron(III) oxide ...

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