At $1 \ bar$ and $298 \ K$,the standard molar enthalpy of formation of which substance is zero?

For the reaction $H_{2} + I_{2} \rightarrow 2 HI$,provide the potential energy profile diagram and prove that it is an exothermic reaction.

For the reaction $H_{2(g)} + C_{2}H_{4(g)} \rightarrow C_{2}H_{6(g)}$,the enthalpy change is ....... $Kcal \, mol^{-1}$. Given bond energies: $H-H = 103$,$C-H = 99$,$C-C = 80$,and $C=C = 145 \, Kcal \, mol^{-1}$.

The standard enthalpies of formation of $Al_{2}O_{3}$ and $CaO$ are $-1675 \ kJ \ mol^{-1}$ and $-635 \ kJ \ mol^{-1}$ respectively.
For the reaction $3 CaO + 2 Al \rightarrow 3 Ca + Al_{2}O_{3}$,the standard reaction enthalpy $\Delta_{r}H^{\circ}$ is .......... $kJ$.
(Round off to the Nearest Integer).

The formation of ammonia is shown by the reaction $N_{2(g)} + 3H_{2(g)} \to 2NH_{3(g)}, \Delta_r H^o = -91.8 \, kJ \, mol^{-1}$. What will be the enthalpy of reaction for the decomposition of $NH_3$ according to the reaction $2NH_{3(g)} \to N_{2(g)} + 3H_{2(g)}; \Delta_r H^o = ?$ ..... $kJ \, mol^{-1}$

When equal volumes of $1 \ M \ HCl$ and $1 \ M \ H_2SO_4$ are separately neutralized by an excess volume of $1 \ M \ NaOH$ solution,$x \ J$ and $y \ J$ of heat are liberated,respectively. The value of $y / x$ is . . . .