3H2 (g) + CO (g) --> CH4 (g) + H2O (g) was given to figure out
2H2 + CO2 --> CH4 @ /\Hf 319.1kJ the one you calculated
Definitely not the same, right?
And this is aside from the fact that the O2 doesn't cancel out (1O2 consumed in the first, 2 produced in the second)
Also the book says no diatomic gases (or elements) count towards heat formation and are considered zero. Is there any reason for this? and does this mean anything that is 'double' is negated down to 0?
it's the definition since those molecules are in their basic state. And for basic states: the heat of formation for any element in its basic state is arbitrarily set to 0 kJ / mole.