Question

The melting of a substance at its melting point OR the vaporization of a substance at its boiling point are reversible isothermal processes.

Calculate the Entropy change (in J/K) that occurs when 25.0 grams ethanol (C2H5OH(g)) condenses at 78.2oC (ethanol's normal boiling point).

ΔHvaporization = 38.6 kJ/mole

Include a negative sign in front of your answer if the Entropy decreases!!

Answer 1

From the second law of thermodynamics we have

\[\Delta S = \frac{q}{T}\]
Since we're doing a state change, the temperature and pressure remain constant during this process, so that takes care of \(T\) and all enthalpy is, is the heat transfer at constant pressure. So we have that too, awesome.

\[\Delta_f S = \frac{\Delta_f H}{T}\]

We're not entirely out of the woods yet, there are two more things we have to look at, is the sign positive or negative? Condensing is going from gas to liquid and evaporating is going from liquid to gas. So knowing that entropy increases as disorder increases and that gasses are much more disordered than liquids you should be able to figure out if the change in entropy is increasing or decreasing depending on your end states.

And one final thing, this will just get you the molar entropy change. Check your units on enthalpy! So you must go to the periodic table and figure out how many moles of ethanol you have since you were given it in grams.

That should do it!