The bond enthalpies of $H_2$,$X_2$,and $HX$ are in the ratio of $2 : 1 : 2$. If the standard enthalpy of formation of $HX$ is $-50 \ kJ \ mol^{-1}$,the bond enthalpy of $H_2$ is .... $kJ \ mol^{-1}$.

Enthalpy of combustion of carbon to $CO_{2}$ is $-393.5 \, kJ \, mol^{-1}$. Calculate the heat released upon formation of $35.2 \, g$ of $CO_{2}$ from carbon and dioxygen gas.

The calorific value of hydrogen gas is $-143 \ kJ \ g^{-1}$. The standard enthalpy of formation of $H_2O$ will be $—$

Which substance has a standard molar enthalpy of formation equal to zero at $298 \ K$?

Given that:
$C_{(s)} + O_{2(g)} \to CO_{2(g)}, \Delta H = -394 \ kJ$
$2H_{2(g)} + O_{2(g)} \to 2H_2O_{(l)}, \Delta H = -568 \ kJ$
$CH_{4(g)} + 2O_{2(g)} \to CO_{2(g)} + 2H_2O_{(l)}, \Delta H = -892 \ kJ$
Calculate the heat of formation of $CH_{4(g)}$ in $kJ$.

If the combustion of $1 \ g$ of graphite produces $20.7 \ kJ$ of heat,what will be the molar enthalpy change? Give the significance of the sign also.