2
$\begingroup$

enter image description here

Been trying to understand this question. Here is my reasoning:

1) True, because T is positive (105 + 273)K while ΔS should obviously be negative because the final entropy is lower than the initial entropy. We are after all going from water's gaseous state to its liquid state. There are fewer degrees of freedom in water's liquid state.

2) If there is an equilibrium between water vapor and water liquid, then by definition relative humidity is 100%. The air is saturated with water vapor to the point where it can't even hold the water vapor and water vapor exists in a dynamic equilibrium with liquid water. From Wikipedia:

Saturated air is really just air which contains water vapor in equilibrium with a liquid water source.

3) False, because ΔG, not ΔG°. ΔG for a system at equilibrium is always 0. ΔG° - not necessarily 0. So we have 0 = ΔG = ΔH - TΔS. This implies ΔH = TΔS.

4) True. For liquid water to boil, vapor pressure must equal atmospheric pressure. Similarly for water to condense, atmospheric pressure must equal vapor pressure. Also, vapor pressure is solely dependent on temperature of the pure liquid. The below statement is from UC Davis's ChemWiki. I would think that adding solute would change the vapor pressure (and yes it does, and it's discussed further down the page of the site).

Vapor pressures are dependent only on temperature and nothing else.

Taking into consideration that water boils at 100 degrees Celsius, we may conclude that at 105 degrees Celsius, the vapor pressure of liquid water is greater than 1 atm. And therefore, for this equilibrium to exist, both the vapor pressure of liquid water and the atmospheric pressure of gaseous water must be equal and thus greater than 1 atm.

5) This is true. This would make ΔG = 0 since ΔG = G(products) - G(reactants).

$\endgroup$
  • $\begingroup$ ok, so what is the question? $\endgroup$ – Satwik Pasani May 20 '14 at 4:58
  • $\begingroup$ Can you verify my logic? $\endgroup$ – Dissenter May 20 '14 at 5:30
  • $\begingroup$ I think it is principally accurate. I will still give a detailed review in short time. $\endgroup$ – Satwik Pasani May 20 '14 at 6:36
  • $\begingroup$ I appreciate it. $\endgroup$ – Dissenter May 20 '14 at 7:25
3
$\begingroup$

All the arguments are perfect except the fourth.

The statement is true, but the premise that the vapour pressure of water must equal the atmospheric pressure is incorrect. A phase equilibrium between the gaseous and the liquid phase can be established at a particular temperature without the liquid boiling. Boiling is a special case of this equilibrium when the atmospheric pressure is entirely constituted by water vapour in the vicinity of the liquid surface and hence the vapour pressure can no longer rise at that pressure. In normal cases, as is possible in this case also, vapour pressure of water is just one component of the atmospheric pressure and the liquid need not be boiling.

A more accurate sentence would be $P_{atm}\geq P_{vap}$. This would still mean that both vapour pressure and the atmospheric are higher than 1 atm but not necessarily equal.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.