For the reaction $R \rightarrow P$,the following potential energy diagram is given. What will be the enthalpy change $(\Delta H)$ for the given reaction (in $kJ$)?

For which of the following reactions is the $\Delta H^o$ of the reaction equal to the $\Delta H_f^o$ of the product?

The heat evolved per mole of $H^+$ ions during the neutralization of a strong acid and a strong base is ......

The atomisation enthalpy of $CH_4$ is $1660 \ kJ \ mol^{-1}$. The $C-H$ bond enthalpy of each successive step in $CH_4$ $\rightarrow CH_3$ $\rightarrow CH_2$ $\rightarrow CH$ are $+15, +30$ and $+45 \ kJ \ mol^{-1}$ higher than the mean bond enthalpy of $C-H$ bonds,respectively. The bond enthalpy of the last $C-H$ unit is

In which of the following reactions does the heat change represent the heat of formation of water?

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