# If you are given ΔH for this reaction which occurs at constant pressure, what additional information you need to determine ΔE?

Reaction: $\ce{2Al + 3Cl2 → 2AlCl3}$

Answer: We must have either temperature, or the values of P and ΔV in order to calculate ΔE from ΔΗ.

Since ΔH is given, we can use the formula ΔE=ΔH-PΔV to calculate change in internal energy, and only pressure and ΔV are required to calculate ΔΕ. So my question is, why do we need temperature in this question?

1) your question assumes that the volumes of the solids ($\ce{Al}$ and $\ce{AlCl3}$) are negligible compared to the volume of the $\ce{Cl2}$ gas. This means that $\Delta V \approx V(\ce{Cl2})$.
2) your question assumes that chlorine gas behaves ideally. This means that $V(\ce{Cl2}) = \frac{nRT}{p}$.
In the end, it means you only need two of the three variables ($p$, $T$, $V$) in order to obtain all the information you want.
I would however be very careful about using molar quantities, in particular $V_m$ (since $p$ and $T$ are intensive properties), if the question is asking you to find $\Delta H$ and $\Delta E$ in terms of $\text{kJ mol}^{-1}$.