Internal energy change for reactions at constant volume vs constant pressure
The key point to keep in mind in this type of problem is that the energy of an ideal gas is only a function of temperature, not of volume or pressure$^\ast$. We also assume that the dependence of the energy of the condensed phases on p and V is negligible. Therefore the final pressure or volume is not going to affect $\Delta U$ for the reaction, provided n and T are constant. Since here the reaction refers to a conversion of a stoichiometric amount of reactants into products at the specified T, we therefore do not expect much of a change in $\Delta U$ with change in p or V. 

$^\ast$Note that the total differential for U can be written as 
$$\begin{align} dU &amp;= \left\{ T \left( \frac{\partial p}{\partial T} \right)_V -p\right\} dV + C_VdT \end{align}$$ Therefore at constant T,
$$\begin{align} dU = &amp;= \left\{ T \left( \frac{\partial p}{\partial T} \right)_V -p\right\} dV  \end{align}$$ 
The term in parentheses is zero for an ideal gas.