Question

Two kg of liquid lead initially at 500°C are poured into a form. It then cools at constant pressure down to room temperature of 20°C as heat is transferred to the room. The melting point of lead is 327°C and the enthalpy change between the phases, hif , is 24.6 kJ/kg. The specific heats are  Calculate the net entropy change for this process. 

Answer 1

C.V. Lead, constant pressure process

mPb(u₂ - u₁)Pb = ₁Q₂ - P(V₂ - V₁)

We need to find changes in enthalpy (u + Pv) for each phase separately and then add the enthalpy change for the phase change.

Consider the process in several steps:

Cooling liquid to the melting temperature

Solidification of the liquid to solid

Cooling of the solid to the final temperature