Question

define the standard heat of formation of MgCO3.

Answer 1

Mg(s) + C(graphite) + 3/2O₂(g) MgCO₃(s)
This reaction that cannot be carried out directly. So the enthalpy change for the reaction must be calculated via indirect means.

we got several reaction with each H to find the H of MgCO₃ formation, these are:

Mg (s) + 2HCl (aq) MgCl₂ (aq) + H₂ (g)   H = - 641.8 kJ/mol (1)

MgCO₃ (s) + 2HCl (aq) MgCl₂ (aq) + H₂O (l) + CO₂ (g) H = -90kJ /mol (2)

C(s) + O₂ (g) CO₂ (g) H = -393kJ /mol (3)

H₂ (g) + 1/2 O₂ (g) H₂O (l) H = -285kJ /mol (4)

Reversing equation 2,

MgCl₂ (aq) + H₂O (l) + CO₂ (g)      MgCO₃ (s) + 2HCl (aq)   H = 90kJ /mol (5)

Adding equation (1),(3),(4) & (5) to get the required equation :

Mg(s) + C(s) + 3/2O₂(g) MgCO₃(s)

H =  (- 641.8 kJ/mol) + (-393 kJ /mol) + ( -285kJ /mol ) + (90kJ /mol )

H = - 1229.8 kJ/mol