What is the equation for molar enthalpy change?

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Study for the IGCSE Edexcel Chemistry Test. Access flashcards and multiple choice questions with hints and explanations. Get ready for your exam!

Multiple Choice

What is the equation for molar enthalpy change?

Explanation:
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.

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