Answer: Standard reaction enthalpy equals twice the standard enthalpy of formation of NH 3.
- A Standard reaction enthalpy equals twice the standard enthalpy of formation of NH<sub>3</sub>
- B Standard reaction enthalpy equals the standard enthalpy of formation of NH<sub>3</sub> divided by two
- C Standard reaction enthalpy is unrelated to the enthalpy of formation of NH<sub>3</sub>
- D Standard reaction enthalpy equals the standard enthalpy of formation of N<sub>2</sub> plus that of H<sub>2</sub>
Correct answer: A. Standard reaction enthalpy equals twice the standard enthalpy of formation of NH<sub>3</sub>
Explanation: Since 2 moles of NH<sub>3</sub> are formed from the elements in their standard states, the standard reaction enthalpy equals 2 times the standard enthalpy of formation of NH<sub>3</sub>.
Isothermal expansion follows a gentler curve (heat flows in to keep T constant) while adiabatic expansion drops in pressure more steeply (no heat exchange, so internal energy and temperature fall as the gas does work).
Concept context
Study energy changes in chemical reactions. Understand enthalpy, entropy, Gibbs free energy, and the laws of thermodynamics that decide whether a reaction will occur spontaneously or not.