# Calculate the Internal Energy at 298k for the Formation of One Mole of Ammonia, If the Enthalpy Change at Constant Pressure is – 42.0 Kj Mol-1. - Physics

Sum

Calculate the internal energy at 298K for the formation of one mole of ammonia, if the enthalpy change at constant pressure is – 42.0 kJ mol-1.

(Given: R = 8.314 J K-1 mol-1)

#### Solution

Formation of 1 mole of ammonia

1/2 N_(2(g)) + 3/2 H_(2(g)) -> NH_(3(g))

Δn = (no.of moles of gaseous product) - (no.of moles of gaseous reactant)

= 1 - (1/2 + 3/2)

= -1

ΔH = ΔU + PΔV

PΔV = -ΔnRT

=-(-1)xx8.314xx298

= 2477 J

= 2.477 KJ

ΔH = -42.0 KJ

∴ -42.0 KJ = ΔU + 2.477

ΔU = -42 - 2.477

= -44.47 KJ

Concept: Chemical Thermodynamics and Energetic - First Law of Thermodynamics
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