# Gibbs free energy problem

• Kqwert
In summary, the equilibrium vapor pressure of water at 100°C is 1 bar. This means that at equilibrium, the partial pressure of water would be 1 atm. However, the reaction is reversible and continuously occurring, with vapor molecules condensing into the liquid phase and liquid molecules evaporating into the gas phase. When P(H2O) is less than 1 atm, the rate of evaporation exceeds the rate of condensation, and when P(H2O) is greater than 1 atm, the opposite occurs. Only when P(H2O) = 1 atm will there be no net exchange of material between the liquid and gaseous phases. Therefore, it is incorrect to use an "=" sign between the reactants

#### Kqwert

Please post this type of questions in HW section using the template.
Calculate deltaG for the reaction:

H2O(l) = H2O(g). 100 degrees celsius, water is clean. P(H2O) = 0.1 bar.

Given that it is an equilibrium, I'd think that deltaG would be zero. But the answer is in fact negative. How is that possible?

What is the equilibrium vapor pressure of water at 100°C?

1 bar? So it's really not an equilibrium..?

At equilibrium, the partial pressure of water would be 1 atm. Because P(H2O) is less than that, the forward reaction is favored.

So it's wrong to call it an equilibrium?

The reaction is at equilibrium only at a specific P(H2O).

Thank you!
isn't it wrong to write the equation with a "=" sign between the reactants and products then?

H2O(g) ##\rightleftharpoons## H2O(l) is the correct notation.

The reaction is reversible, with vapor molecules continuously condensing into the liquid phase and liquid molecules continuously evaporating into the gas phase. When P(H2O) is less than 1 atm, the rate of evaporation exceeds the rate of condensation. When P(H2O) is greater than 1 atm, the rate of condensation exceeds the rate of evaporation. Only when P(H2O) = 1 atm will evaporation and condensation occur at the same rate, so there will be no net exchange of material between the liquid and gaseous phases.

Kqwert said:
Thank you!
isn't it wrong to write the equation with a "=" sign between the reactants and products then?
I think it would have been better if they had used an arrow. ---->