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Phase changes

Posted: Sun Jan 27, 2019 12:55 pm
by Jenna Salas 2H
I know phase changes from liquid to vapor, solid to liquid, and solid to vapor are always endothermic. Would the opposite phases changes (vapor to liquid, and liquid to solid) always be exothermic?

Re: Phase changes

Posted: Sun Jan 27, 2019 1:00 pm
by Andie Jian 1D
I think you are correct. Because enthalpy is additive, if you reverse the reaction so that the product becomes the reactant, then the enthalpy should just be the negative of the forward reaction enthalpy.

Re: Phase changes

Posted: Sun Jan 27, 2019 1:04 pm
by Milena Aragon 2B
Yes, the reverse reaction would be exothermic. Remember this so that If you're ever given the enthalpy for the reaction from liquid to vapor for example, you would know to change the sign from + to - and that would be the enthalpy for the reverse reaction.

Re: Phase changes

Posted: Sun Jan 27, 2019 2:23 pm
by KatrinaPho_2I
Yes, to turn ice to water (melting), heat must be added to the system (endothermic) and the molecules move around quicker and melt. To turn water into ice (freezing), the water loses heat energy (exothermic) and the molecules slow down in movement and freeze into a solid.

Re: Phase changes

Posted: Sun Jan 27, 2019 2:53 pm
by MadelineHlobik
If a reaction gives off heat, it has a negative delta h right? And this process is exothermic so it will release heat and then the surroundings will warm?

Re: Phase changes

Posted: Sun Jan 27, 2019 3:01 pm
by Emilia z
Yes that sounds right

Re: Phase changes

Posted: Sun Jan 27, 2019 10:34 pm
by Jchellis 1I
Yes, I believe if it gives off heat it would be negative delta H.

Re: Phase changes

Posted: Sun Jan 27, 2019 11:18 pm
by Tony Ong 3K
I believe that negative delta H is exothermic so it gives off heat and positive delta H is endothermic, so it requires heat. But would that be the same for negative delta G and positive?

Re: Phase changes

Posted: Sun Jan 27, 2019 11:52 pm
by 404982241
you are correct. phase changes like gas to liquid are exothermic and therefore release energy.

Re: Phase changes

Posted: Mon Jan 28, 2019 12:36 am
by 505211599
You are correct. Phase changes from vapor to gas will be exothermic and the other way around is endothermic.

Re: Phase changes

Posted: Tue Jan 29, 2019 9:40 am
by lindsey_ammann_4E
Jenna Salas 2H wrote:I know phase changes from liquid to vapor, solid to liquid, and solid to vapor are always endothermic. Would the opposite phases changes (vapor to liquid, and liquid to solid) always be exothermic?


Yes, you are correct. In the first changes you listed, heat acts like a reactant. This means the reaction is endothermic with a positive deltaH.
In the second list of phase changes, the reverse reactions, heat acts like a product. This means the reaction is exothermic with a negative deltaH.

Re: Phase changes

Posted: Tue Jan 29, 2019 10:29 am
by JT Wechsler 2B
The reverse reactions would be exothermic because you would be losing heat to go from liquid to solid.

Re: Phase changes

Posted: Tue Jan 29, 2019 8:14 pm
by Chem_Mod
s --> l --> g
Endothermic, meaning DeltaH is positive
Increase in entropy, meaning DeltaS is positive
DeltaG = DeltaH - T*DeltaS
At low temperatures, DeltaG is positive and this is unfavorable
At high temperatures, DeltaG is negative and this is favorable

s <-- l <-- g (backwards)
Exothermic, meaning DeltaH is negative
Decrease in entropy, meaning DeltaS is negative
DeltaG = DeltaH - T*DeltaS
At low temperatures, DeltaG is negative and this is favorable
At high temperatures, Delta G is positive and this is unfavorable

Gibbs free energy (G) is the amount of energy that is available to do work. Conditions that make DeltaG the most favorable are when DeltaH is - and DeltaS is +.

Re: Phase changes

Posted: Wed Jan 30, 2019 1:30 pm
by Megan_Ervin_1F
Why is it that the phase change for water from a liquid to a gas than from a solid to a liquid has more energy?

Re: Phase changes

Posted: Wed Jan 30, 2019 4:11 pm
by Miya Lopez 1I
Megan_Ervin_1F wrote:Why is it that the phase change for water from a liquid to a gas than from a solid to a liquid has more energy?


I think when water changes from a liquid to a gas there is more energy because the particles need to be moving faster as a gas and the all bonds in the liquid need to be broken for it to change to a gas.