Question

The value of DHfe for Al2O3 (s) is __________ kJ.

Answer 1

The ΔH_f of a substance is actually the heat of formation released or absorbed during the reaction. Generally it is calculated for the entire reaction.

Now if we consider the formation of Al2O3 (s), then the formation reaction is-

2 Al(s) + 3 O2(g) ------------------> 2 Al2O3(s)

Now ΔH_f for this reaction will be-

ΔH_f = net heat of formation of products - net heat of formation of reactants

= Σn(ΔH_f)Products − Σn(ΔH_f)Reactants

= H_f (2 Al2O3(s)) - [H_f (2 Al(s)) + H_f (3 O2(g)) ]

= 2 * H_f (Al2O3(s)) - 2* H_f (Al(s)) - 3* H_f (O2(g))

Now putting the values form a reference table-

ΔH_f = 2 * H_f (Al2O3(s)) - 2* H_f (Al(s)) - 3* H_f (O2(g))

= 2 mols * (−1675.5) - 2* 0 - 3* 0 because heat of formation of naturally available elements = 0

= - 3,351 kJ

So value of ΔH_f for Al2O3 (s) is _- 3,351 kJ