1. If the coefficient in an equation are double, ∆H must be double
Ca(s)+ C(graphite)+3/2 O2(g) ->CaCO3(s) ∆H=-1206.9 kJ/mol

2. If reverse reaction and the opposite sign of ∆H
4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O (1) ∆H =- 904 kJ/mol

3. If reverse reaction and the coefficient in an equation is triple, ∆H must be triple with opposite sign
C6H12O6(s) -> C2H5O(1) + 2CO2 (g) ∆H =-67.9 kJ/mol​