What is the equation for molar enthalpy change?

What is the equation for molar enthalpy change?

• A. ΔH = -Q / 1000 n
• B. ΔH = Q / 1000 n
• C. ΔH = -Q / n
• D. ΔH = -Q

Answer: (A)

Molar enthalpy change is the heat change per mole of substance involved in the reaction. At constant pressure, the heat exchanged with the surroundings equals the enthalpy change (q_p = ΔH). To express this per mole, convert the total heat to kilojoules and divide by the number of moles: ΔH (in kJ/mol) = - Q (in J) / (1000 × n). The minus sign reflects that when the reaction releases heat, the system’s enthalpy decreases, so ΔH is negative. If heat is absorbed, Q would be negative in this convention, yielding a positive ΔH per mole. Example: if 5,000 J of heat is released by 2 moles, ΔH = -5000 / (1000 × 2) = -2.5 kJ/mol.

Molar enthalpy change is the heat change per mole of substance involved in the reaction. At constant pressure, the heat exchanged with the surroundings equals the enthalpy change (q_p = ΔH). To express this per mole, convert the total heat to kilojoules and divide by the number of moles: ΔH (in kJ/mol) = - Q (in J) / (1000 × n). The minus sign reflects that when the reaction releases heat, the system’s enthalpy decreases, so ΔH is negative. If heat is absorbed, Q would be negative in this convention, yielding a positive ΔH per mole.

Example: if 5,000 J of heat is released by 2 moles, ΔH = -5000 / (1000 × 2) = -2.5 kJ/mol.