gibbs free energy when vapor condenses

[datdat]

when water vapor condenses,
bonds are formed and energy released so, dH <0
entropy decreases(gas-->liquid) so TdS <0

so I think the condesation will be spontaneous only when dH>TdS right? not dH<TdS as in kaplan book p.869.

what do you guys think?

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[UCB05]

when water vapor condenses,
bonds are formed and energy released so, dH <0
entropy decreases(gas-->liquid) so TdS <0

so I think the condesation will be spontaneous only when dH>TdS right? not dH<TdS as in kaplan book p.869.

what do you guys think?

(deltaH-TdeltaS < 0) deltaH and TdeltaS are both negative, which then means deltaH has to be more negative than TdeltaS. So Kaplan is right to say dH<TdS

 

[joonkimdds]

delta G= delta H - Tdelta S

we know that TdeltaS is negative.
? - - = ? + = spontaneous = -
thus ? must be big negative number.

 

[bennijai]

Heat is given off thus H is negative and so is dS since it is going from gas to liquid. Under these conditions it is spontaneous only if the temperature is low.

So going by Gibb's free energy formula: dG = H - TdS, H has to be a larger negative value otherwise a high temperature will cause dS to be a high value that will cause dG to be positive, which leads to a non spontaneous reaction. But a low temperature will lead to a low TdS and thus the dG will be negative. Maybe some numbers will make more sense.

Here we have dG = -10(H) - [2(T)][-3(dS)] = -4 = spontaneous. Notice the low temperature will allow the TdS value to remain low.
Then we have dG = -10(H) - [5(T)][-3(dS)] = +5 = non spontaneous. Notice the higher temperature causes the TdS value to become larger.

So remember when H is negative along with a negative dS it depends on temperature to affect spontaneous reactions.