Which of the following is a correctly written thermochemical equation?

• A. 2 H_2(g) + O_2(g) ightarrow 2 H_2O(l) \,\;\;\;\;\Delta H = -571.6\;\text{kJ}
• B. CH_4(g) + 2 O_2(g) ightarrow CO_2(g) + 2 H_2O(g) \,\;\;\;\;\Delta H = +890.4\;\text{kJ}
• C. NaCl(s) ightarrow Na^+(aq) + Cl^-(aq) \,\;\;\;\;\Delta H = +411\;\text{kJ}
• D. C(s) + O_2(g) ightarrow CO_2(g)

Answer: (A)

Verified step by step guidance

1. Understand that a thermochemical equation must include the balanced chemical equation along with the correct physical states of reactants and products, and the enthalpy change (\$\Delta H\$) with the correct sign indicating whether the reaction is exothermic or endothermic.
2. Check if the chemical equation is balanced in terms of atoms on both sides. For example, in the equation \$2\ H_2(g) + O_2(g) \rightarrow 2\ H_2O(l)\$, count hydrogen and oxygen atoms to confirm balance.
3. Verify the physical states of all species are correctly indicated, such as (g) for gas, (l) for liquid, (s) for solid, and (aq) for aqueous solution, since these affect the enthalpy values.
4. Examine the sign of \$\Delta H\$ to see if it matches the nature of the reaction: negative for exothermic (heat released) and positive for endothermic (heat absorbed). For example, combustion reactions are typically exothermic and should have a negative \$\Delta H\$.
5. Confirm that the enthalpy change corresponds to the balanced equation as written, meaning the \$\Delta H\$ value is for the exact stoichiometric coefficients shown in the equation.