Are chemical treated as ideal gases when trying to figure out heat of formation using calorimetry?
Internal energy of any gas is given by
$$U=U_\text{trans} + U_\text{rotational} + U_\text{vibrational} + U_\text{intermolecular} + U_\text{electronic} + U_\text{relativistic} + U_\text{bonds}$$
Last three aren't affected by ordinary heating. And $U_\text{intermolecular}$ for ideal gas is zero. That's why for ideal gas $U$ is only function of temperature. But during a chemical reaction (even if it involves an ideal gas)
$U_\text{bonds}$ comes into play. The formation and destruction of bonds results in a decrease of potential energy.
Therefore, for a chemical reaction, even if the temperature remains constant the internal energy changes.