Which statement is true about the standard enthalpy change of a reaction, ${\Delta }^{\circ }{H}_{\mathrm{rxn}}$, and standard enthalpies of formation, ${\Delta }^{\circ }{H}_f$?

• A. ${\Delta }^{\circ }{H}_{\mathrm{rxn}}$ is independent of the physical states of reactants and products.
• B. ${\Delta }^{\circ }{H}_{\mathrm{rxn}}$ can be calculated as the sum of ${\Delta }^{\circ }{H}_f$ for products minus the sum of ${\Delta }^{\circ }{H}_f$ for reactants, each multiplied by their stoichiometric coefficients.
• C. For any element in its standard state, ${\Delta }^{\circ }{H}_f$ is always less than $0$.
• D. ${\Delta }^{\circ }{H}_{\mathrm{rxn}}$ is always positive because bond breaking requires energy.

Answer: (B)