Which of the following is required for a correctly written thermochemical equation?

• A. Only the reactants need to be balanced; products can be ignored.
• B. The physical states of all reactants and products must be specified.
• C. The enthalpy change should be omitted to avoid confusion.
• D. The equation must be written in reverse to indicate an endothermic process.

Answer: (B)

Verified step by step guidance

1. Understand that a thermochemical equation is a chemical equation that includes the enthalpy change (ΔH) associated with the reaction, providing information about heat absorbed or released.
2. Recognize that for a thermochemical equation to be correct, the chemical equation itself must be balanced, meaning the number of atoms of each element must be the same on both reactant and product sides.
3. Know that specifying the physical states (solid, liquid, gas, aqueous) of all reactants and products is essential because enthalpy changes depend on these states; for example, the enthalpy of vaporization or fusion varies with phase.
4. Remember that the enthalpy change (ΔH) should be included in the thermochemical equation to indicate whether the reaction is exothermic (releases heat) or endothermic (absorbs heat), so omitting it would make the equation incomplete.
5. Note that the direction of the reaction (forward or reverse) affects the sign of ΔH, but the equation itself does not have to be written in reverse to indicate an endothermic process; instead, the sign of ΔH changes accordingly.