Which of the following is a correctly written thermochemical equation?

• A. 2H_2(g) + O_2(g) ightarrow 2H_2O(l) \,\;\;\;\;\Delta H = -571.6\;\text{kJ}
• B. CH_4(g) + 2O_2(g) ightarrow CO_2(g) + 2H_2O(g) \,\;\;\;\;\Delta H = +890.3\;\text{kJ}
• C. C(s) + O_2(g) ightarrow CO_2(g)
• D. NaCl(s) ightarrow Na^+(aq) + Cl^-(aq) \,\;\;\;\;\Delta H = +411\;\text{kJ}

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 \$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)\$, verify that there are 4 hydrogen atoms and 2 oxygen atoms on both sides.
3. Confirm that the physical states are correctly indicated: (g) for gas, (l) for liquid, (s) for solid, and (aq) for aqueous solution. This is important because enthalpy changes depend on the states of substances.
4. Evaluate the sign of \$\Delta H\$ to see if it matches the reaction type. For example, formation of liquid water from hydrogen and oxygen is exothermic, so \$\Delta H\$ should be negative.
5. Review each given option using these criteria to identify which one is a properly balanced thermochemical equation with correct states and enthalpy sign.