What is the enthalpy change for the reaction $2C(\text{graphite}) + 3H_{2(g)} \rightarrow C_2H_{6(g)}$ called?

The standard heats of formation in $kcal \ mol^{-1}$ of $NO_{2(g)}$ and $N_2O_{4(g)}$ are $8.0$ and $2.0$ respectively. The heat of dimerization of $NO_2$ in $kcal$ for the reaction $2NO_{2(g)} \rightarrow N_2O_{4(g)}$ is:

Calculate the enthalpy change of the reaction for $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$, given the bond energies (in $\text{kJ mol}^{-1}$): $H-H = 436$, $Cl-Cl = 242$, $H-Cl = 431$.

Use the data from the table to estimate the enthalpy of formation of $CH_3CHO$.

Bond	Bond Enthalpy $(kJ \ mol^{-1})$	Enthalpy of formation $(kJ \ mol^{-1})$
$C-H$	$400$	$C(g): 700$
$C-C$	$350$	$H(g): 200$
$C=O$	$700$	$O(g): 250$

Consider the following data:
$(i)$ $2Al(s) + 6HCl(aq) \to Al_2Cl_6(aq) + 3H_2(g) + 1200 \text{ kJ/mol}$
(ii) $H_2(g) + Cl_2(g) \to 2HCl(g) + 164 \text{ kJ/mol}$
(iii) $HCl(g) + aq \to HCl(aq) + 83 \text{ kJ/mol}$
(iv) $Al_2Cl_6(s) + aq \to Al_2Cl_6(aq) + 663 \text{ kJ/mol}$
The enthalpy of formation of anhydrous solid $Al_2Cl_6$ is:

State and prove Hess's law of constant heat summation.