Question

For the reaction
N₂ (g) + 3H₂(g) ⇋ 2NH₃(g), ∆H = ?
(a) ∆E + 2RT (b) ∆E –2RT
(c) ∆E – RT (d) None of these

Answer 1

N₂ (g) + 3H₂(g) ⇋ 2NH₃(g),

∆H is calculated as below :

• First law of thermodynamics states

that,

• ∆H = ∆E + ∆(PV)

• For an ideal gas, PV = nRT ;

∆H = ∆E + ∆(nRT)

• If n = constant i.e closed system

then,

• ∆H = ∆E + nR∆T

• If T = constant then,

• ∆H = ∆E + RT∆ng

Where ∆ng = Total no. of moles of

gaseous products - Total no. of

moles of gaseous reactants.

• Therefore, ∆ng = 2 - 4 = -2

• Hence, ∆H = ∆E -2RT