Solve for ΔH for the following problem

Question

MgO (s) + 2 HCl (aq) -> MgCl2 (aq) + H2O (l) ΔH= ?

Mg (s) + 2 HCl (aq) -> MgCl2 (aq) + H2 (g) ΔH=?

H2 (g) + 1/2 O2 (g) -> H2O (l) ΔH= -285.8 kJ

Answer 1

The first one you look up the heats of formation of MgCl₂, H⁺(aq), and MgO ( Δ H₃ is Heat of formation of liquid water). Both HCl and MgCl₂ are present as ions (Cl^- is a spectator)

ΔH₁ = H_f,H2O+H_f,Mg2+(aq) - 2H_f,H+(aq) - H_f,MgO

ΔH₂ is also the sum of the moles of products times the molar heats of formation of the products minus the sum of the moles of reactants times their heats of formation.

Remember that H_f of H⁺, H₂, and Mg₂ are all 0 by definition.