Sublimation printing of water?
The following question relates to the following task:
Calculate the pressure required for lyophilization (freeze-drying of biological samples, i.e., sublimation of water) at T = -20 C. Note: Start by reducing the pressure from the triple point.
I've only provided the phase diagram of water with the corresponding values, for example, for the triple point. I can actually read the pressure easily, albeit only somewhat accurately, for this phase equilibrium point at -20°C for solid (polymorphic phase I)/gas. However, a calculation is required. For a Clausius-Clapeyron equation, however, I would need the sublimation enthalpy, which I haven't provided…What is the purpose of this exercise?
Thanks for your help!
(1 votes)
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The enthalpy of sublimation can always be derived from the thermodynamic data if standard enthalpy values for the solid and gaseous states are known.
So you need the table for that. You'd be given it in the exam, too. Just calculate the difference.
Thanks for your answer. I understand. But I don't think I'm allowed to work with these values for the standard enthalpy of formation. I only provided the phase diagram…so, technically, I should only read the equilibrium point at the required temperature…but that wouldn't have been calculated…
If necessary, ask: I think the table is allowed because I don't see any other way you could get these values.
Yes, perfect! Everything worked out well. Thanks for your input.
Korrekt, das sollten funktionieren
Sure, definitely. 👍The problem is also from a series of exercises, and it's not entirely clear which tabulated values, for example, you're allowed to use. 😅 I'm assuming, then, that the required values will also be given in an exam. However, I see that the evaporation and fusion enthalpies of water are (were) known from previous subtasks. Then I could take the sublimation enthalpy as the sum of the other two enthalpies, right?