The value of ΔfH° for NH₃ is - 45.9 KJ mol⁻¹. Calculate enthalpy change for the reaction:

• 2 NH₃(g) → N₂(g) + 3 H₂(g)

To calculate the enthalpy change for the reaction 2 NH₃(g) → N₂(g) + 3 H₂(g), we can use the standard enthalpy of formation (ΔfH°) values. The enthalpy change for a reaction can be determined using the formula:

Enthalpy Change Calculation

The formula is:

• ΔH° = Σ(ΔfH° of products) - Σ(ΔfH° of reactants)

Step 1: Identify the ΔfH° values

For this reaction:

• ΔfH° for NH₃(g) = -45.9 kJ/mol
• ΔfH° for N₂(g) = 0 kJ/mol (elements in their standard state)
• ΔfH° for H₂(g) = 0 kJ/mol (elements in their standard state)

Step 2: Calculate the total ΔfH° for reactants and products

Now, we can calculate:

• Reactants: 2 NH₃(g) → 2 × (-45.9 kJ/mol) = -91.8 kJ
• Products: N₂(g) + 3 H₂(g) → 0 + 3 × 0 = 0 kJ

Step 3: Apply the formula

Now, substituting these values into the formula:

• ΔH° = (0 kJ) - (-91.8 kJ) = +91.8 kJ

Final Result

The enthalpy change for the reaction 2 NH₃(g) → N₂(g) + 3 H₂(g) is +91.8 kJ.