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Gibbs Free Energy vs. Enthalpy

Hi,

What’s the difference between Gibbs free energy and enthalpy?

Thanks.
Gibbs energy is a combination of enthalpy and entropy - it's defined as βˆ†G = βˆ†H - Tβˆ†S

Some reactions are endothermic (positive βˆ†rH) yet still occur spontaneously - this is due to entropy so Gibbs energy accounts for this - reactions are spontaneous if they have negative βˆ†G.
Reply 2
Does enthalpy apply only to heat? (Sorry. I haven’t learnt this in chemistry yet; I’m learning this for a foundation in biology.)
I guess, yeah. At constant pressure change in enthalpy is the same as the change in heat energy. So a reaction that has negative reaction enthalpy will release heat energy to the environment.
Reply 4
I see. Yet my biology textbook says that β€œGibbs free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell.”

If the temperature is constant, then heat energy would not be produced? Perhaps other forms of energy that can be used to drive other reactions?

Also, I don’t understand why only Gibbs free energy can be used to do work. Enthalpy is total energy, correct? What happens to that part of energy that apparently can’t do work?
Original post by velocitous
I see. Yet my biology textbook says that β€œGibbs free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell.”

If the temperature is constant, then heat energy would not be produced? Perhaps other forms of energy that can be used to drive other reactions?

Also, I don’t understand why only Gibbs free energy can be used to do work. Enthalpy is total energy, correct? What happens to that part of energy that apparently can’t do work?


Enthalpy is defined as the heat absorbed at constant pressure. Internal energy is the total energy, at constant volume enthalpy=internal energy (H=U)
I personally found it quite a confusing topic without looking at it in a more rigorous way.

Enthalpy is defined as H = U + pV, where U is the internal (i.e. total) energy. It definitely isn't total energy, although in many circumstances βˆ†H = βˆ†U and enthalpy is easier to measure directly (it's so useful in chemistry because βˆ†H = βˆ†U at constant pressure, and experiments done in an open container are at constant pressure).

It requires some manipulation of their definitions to show that for systems at constant temperature and pressure, Gibbs energy is the amount of energy that can do work. If you're going to do more chemistry later, it'll come up! I'd wait until then to look at the whole topic properly.
Reply 7
Original post by KombatWombat
I personally found it quite a confusing topic without looking at it in a more rigorous way.

Enthalpy is defined as H = U + pV, where U is the internal (i.e. total) energy. It definitely isn't total energy, although in many circumstances βˆ†H = βˆ†U and enthalpy is easier to measure directly (it's so useful in chemistry because βˆ†H = βˆ†U at constant pressure, and experiments done in an open container are at constant pressure).

It requires some manipulation of their definitions to show that for systems at constant temperature and pressure, Gibbs energy is the amount of energy that can do work. If you're going to do more chemistry later, it'll come up! I'd wait until then to look at the whole topic properly.


So Gibbs free energy is the part of ENTHALPY that can be used for work, right? What happens to that other part that cannot be used for work?
Original post by KombatWombat
I personally found it quite a confusing topic without looking at it in a more rigorous way.

Enthalpy is defined as H = U + pV, where U is the internal (i.e. total) energy. It definitely isn't total energy, although in many circumstances βˆ†H = βˆ†U and enthalpy is easier to measure directly (it's so useful in chemistry because βˆ†H = βˆ†U at constant pressure, and experiments done in an open container are at constant pressure).

It requires some manipulation of their definitions to show that for systems at constant temperature and pressure, Gibbs energy is the amount of energy that can do work. If you're going to do more chemistry later, it'll come up! I'd wait until then to look at the whole topic properly.


I'm not sure that's strictly true, dH=(dQ)PdH=(dQ)_P and dU=(dQ)VdU=(dQ)_V
Original post by velocitous
So Gibbs free energy is the part of ENTHALPY that can be used for work, right? What happens to that other part that cannot be used for work?


No, it's not a part of enthalpy. It's a different way of measuring energy that is defined as enthalpy minus temperature times entropy. Gibbs energy is the maximum amount of work that can be done.

There's nothing particularly intrinsic about enthalpy or Gibbs energy, it's just that you can prove that they have certain properties in certain conditions from their definitions (H = U + pV, G = H -TS). They're both different measures of energy.
(edited 9 years ago)
Original post by langlitz
I'm not sure that's strictly true, dH=(dQ)PdH=(dQ)_P and dU=(dQ)VdU=(dQ)_V


Yeah, you're right. We're doing stat mech at the moment and I'm getting my E/Q/Us confused!

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