Question

Assume 2 kg of hydrogen (Item B 28) are initially at 40 atm, 100 K. (a) If the hydrogen changes pressure to 1 atm without a change of temperature, find the changes of enthalpy and entropy. (b) If the hydrogen changes temperature to 35 K with no change of entropy, find the change of pressure and enthalpy.

Answer 1

$m = 2\textsf{ kg}\\ P_1 = 40\textsf{ atm}\\ T_1 = 100\textsf{ K}\\ P_2= 1\textsf{ atm}\\ T_2 = 100\textsf{ K}\\ ∆P = 39\textsf{ atm}$

Change of enthalpy $(ΔH) = ΔE + Δ(PV)= 0 + mRT\ln\dfrac{P_1}{P_2} = 2 × 8.314 × 100 × \ln\dfrac{40}1=6133.9\ J$

Change of entropy = $\dfrac{∆Q}T = 0$

Change of enthalpy = ΔH = ΔE + Δ(PV)$.$