Question 13

Given

N2(g) + 3H2(g) → 2NH3(g) ; ΔrH0 = –92.4 kJ mol–1

What is the standard enthalpy of formation of NH3 gas?

Answer

Standard enthalpy of formation of a compound is the change in enthalpy that takes place during the formation of 1 mole of a substance in its standard form from its constituent elements in their standard state.

Re-writing the given equation for 1 mole of NH3(g),

½ N2(g) + 3/2 H2(g) → NH3(g)

\therefore Standard enthalpy of formation of NH3(g)

= ½ ΔrHθ

= ½ (-92.4 kJ mol-1)

= - 46.2 kJ mol-1

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