Which of these species has a standard enthalpy of formation equal to zero?

The $H-H$ bond energy is $430 \ kJ \ mol^{-1}$ and $Cl-Cl$ bond energy is $240 \ kJ \ mol^{-1}$. $\Delta H$ for the formation of $HCl$ is $-90 \ kJ \ mol^{-1}$. The $H-Cl$ bond energy is about:

Compounds with high heat of formation are less stable because

Calculate the enthalpy change for the reaction
$H_2 + F_2 \longrightarrow 2HF$
given that
Bond energy of $H-H$ bond $= 434 \ kJ/mol$
Bond energy of $F-F$ bond $= 158 \ kJ/mol$
Bond energy of $H-F$ bond $= 565 \ kJ/mol$
Result in $kJ$.

Molar enthalpy of combustion of $C_2H_{2(g)}$,$C_{(s)}$,and $H_{2(g)}$ are $-1300$,$-394$,and $-286 \ kJ/mol$ respectively. The standard enthalpy of formation of $C_2H_{2(g)}$ is $:$

The heat change for the reaction $H_2 + \frac{1}{2} O_2 \to H_2 O$ is called