Question

The enthalpy change is -400 kJ for reaction: 2 Al(s)+ 3/2 O2(g) ? Al2O3(s). What is the ?H for the following reaction? 2 Al2O3(s) ? 4Al(s) + O2(g) is

Answer 1

Given the enthalpy change for the given reaction is -400 kJ

                                 2 Al(s)+ 3/2 O₂(g) Al₂O₃(s)   ; =-400 kJ     ------------- (1)

   The enthalpy change for the reaction    Al₂O₃(s) 2 Al(s)+ 3/2 O₂(g) : ₁ = ?            -------------- (2)

The above reaction is obtained by reversing the reaction so the enthalpy change for the reaction is the -ve value of

So ₁ = -

                 = - (-400 kJ)

                 = 400 kJ

2Al₂O₃(s) 4 Al(s)+ 3 O₂(g) : ₂=?

The enthalpy change for the above reaction is obtained by multiplying Eqn(2) , so ₂ = 2x₁

                                                                                                                                            = 2x400 kJ

                                                                                                                                             = 800 kJ

NOTE:

There is a correction in the given equation 2 Al2O3(s) ? 4Al(s) + O2(g) as 2 Al2O3(s) ? 4Al(s) + 3O2(g)