#2CO + O_2 = 2CO_2#
#DeltaH"of reaction" = DeltaH_f^@ of "product(s)" - DeltaH_f^@ of "reactants"#
The product in this reaction is CO
#DeltaH_f^@# of CO is -253.7KJ/mol
The reactants in this reaction are #O_2# and #CO_2#
But the #DeltaH_f^@# of #O_2# is not there
The #DeltaH_f^@# of all elements is 0kJ/mol
So the #DeltaH_f^@# of #O_2# is
0kJ/mol
The combined #DeltaH_f^@#of products should be multiplied by their coefficients .
-253.7KJ/mol 2 + 0kJ/mol1 = -507.4kJ/mol
#DeltaH_f^@# of products
#CO_2# = -314.3kJ/mol
And remember also multiply by the coefficient
-314.3kJ/mol* 2 = -628.6
Plug the values in the equation
#DeltaH"of reaction" = DeltaH_f^@ of "product(s)" - DeltaH_f^@ of "reactants"#
#"-628.6kJ/mol - (-507.4kJ/mol)"#
#DeltaH"of reaction"= "-121.2kJ/mol"#
