# What is the Standard Enthalpy of Water?

• Stephanus
In summary, the standard enthalpy change of formation for water is -241.818 kJ/mol for gas, -285.8 kJ/mol for liquid, and 43.682 kJ/mol for water vapor. The gas has a negative value, indicating that more energy is released by the gas phase than the liquid phase, and the liquid has a positive value, indicating that more energy is released by the liquid phase than the gas phase. The temperature for the reaction is not important; the gas and liquid reactions occur at any temperature between 25 and 1000 C.
Stephanus
Dear PF Forum,
As per wiki: https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)
Standard enthalpy of water:
Gas: -241.818 kJ/mol
Liquid: -285.8 kj/mol
What does it means?
That to produce H2O per mol at 1000C, it releases 241.818 kJ.
That to produce H2O per mol in liquid, it releases 285.5 kJ.
Because in liquid water is not 1000C, (say 250C), so actualy the difference 43.682kJ is to increase the temperature to 1000C?.
Is that what it means?
And what temperature is the gas?
What temperatur is the liquid?
Thank you very much.

Hi Stephanus, please consider the image below

at 1000C values of enthalpy corresponds to an isotherm(temperature fixated or constant at that point, see image). You have correponsding enthalpies for gas and liquid consequently, considering the 2 phase(liquid-gas) region.
Read something on Classical Thermodynamics, you will learn a lot on this.

Stephanus
Thanks Ronie Bayron,
Next month I want to introduce the benefit of organic waste processing.
Beside fertilizer, I'd like to know how much energy can we gain from 1 Kg organic waste, considering any organic waste has elements composition in common
Such as: 65% Oxygen (by mass), 10% carbon, 6.3% Hydrogen.
So I can have some idea, roughly (very roughly), how many joules can we gain from 1 kg organic waste.

Stephanus said:
Thanks Ronie Bayron,
Next month I want to introduce the benefit of organic waste processing.
Beside fertilizer, I'd like to know how much energy can we gain from 1 Kg organic waste, considering any organic waste has elements composition in common
Such as: 65% Oxygen (by mass), 10% carbon, 6.3% Hydrogen.
So I can have some idea, roughly (very roughly), how many joules can we gain from 1 kg organic waste.
Yup, nice to know, should you need a professional help, just pm me. I am open to a freelance job somewhere (just not to advertise the site)

I appreciate it. But I do it for my own home and for my church, so I can teach them the benefit of organic waste processing.
By the way I live in Indonesia.

Wow, that's a good thing to know. I am glad to help in anyways I can.

Stephanus
Stephanus said:
Dear PF Forum,
As per wiki: https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)
Standard enthalpy of water:
Gas: -241.818 kJ/mol
Liquid: -285.8 kj/mol
What does it means?
That to produce H2O per mol at 1000C, it releases 241.818 kJ.
That to produce H2O per mol in liquid, it releases 285.5 kJ.
Because in liquid water is not 1000C, (say 250C), so actualy the difference 43.682kJ is to increase the temperature to 1000C?.
Is that what it means?
And what temperature is the gas?
What temperatur is the liquid?
Thank you very much.
These values don't have anything to do with 100 C. The value of -285.8 kJ/mol represents the change in enthalpy in going from a state of 1 mole of oxygen and 0.5 moles of H2 to a state of 1 mole of liquid water, both states at 25 C and 1 bar pressure.

Stephanus
But, why this number between gas in liquid differ?
Oh, perhaps you mean if the end produc is liquid then it releases 285kj/mol. If it's gas then it will release 241kj/mol. Even it's water vapor at 250C?

Stephanus said:
But, why this number between gas in liquid differ?
Oh, perhaps you mean if the end produc is liquid then it releases 285kj/mol. If it's gas then it will release 241kj/mol. Even it's water vapor at 250C?
Chestermiller is right on this. Considering the negative sign as convention of direction of heat out of the gas to the environment. The enthalpy of the gas is way lesser than that of liquid since, some of the energy is still contained in the water vapor to maintain it in a gas phase. If you try to recover that amount of energy, the gas could give up more, about 44kJ/mol energy though this will consequently turned the H2O gas into liquid phase.

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Stephanus
Stephanus said:
But, why this number between gas in liquid differ?
Oh, perhaps you mean if the end produc is liquid then it releases 285kj/mol. If it's gas then it will release 241kj/mol. Even it's water vapor at 250C?
I'm having trouble understanding what you are saying. I think you're saying that the difference between heat released by the reaction of H2 and O2 to form liquid water at 25 C and the heat released by the reaction of H2 and O2 to form water vapor at 25 C is equal to the heat of vaporization of water at 25 C. That's -241.8-(-285.8). If this is what you are saying, then it is correct.

Stephanus
Chestermiller said:
I'm having trouble understanding what you are saying. I think you're saying that the difference between heat released by the reaction of H2 and O2 to form liquid water at 25 C and the heat released by the reaction of H2 and O2 to form water vapor at 25 C is equal to the heat of vaporization of water at 25 C. That's -241.8-(-285.8). If this is what you are saying, then it is correct.
Hi Chet, yes, I affirmed you are right with that and add up, the explanation of the concept High Heat Value (-285.8) and Low Heat Value (-241.8).

"The[/PLAIN] quantity known as lower heating value (LHV) (net calorific value (NCV) or lower calorific value (LCV)) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. This treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not released as heat.
LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value (HHV) (a.k.a. gross calorific value or gross CV) which assumes that all of the water in a combustion process is in a liquid state after a combustion process."
- From Wikipedia, I think that would do a better explanation.

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Stephanus
This stuff is in every chemical thermodynamics book.

Stephanus

Chestermiller said:
I'm having trouble understanding what you are saying. I think you're saying that the difference between heat released by the reaction of H2 and O2 to form liquid water at 25 C and the heat released by the reaction of H2 and O2 to form water vapor at 25 C is equal to the heat of vaporization of water at 25 C. That's -241.8-(-285.8). If this is what you are saying, then it is correct.
Yes that is. Is it because even if the temperature doesn't change, there is heat/energy needed to change liquid water to water vapor?

Stephanus said:
Yes that is. Is it because even if the temperature doesn't change, there is heat/energy needed to change liquid water to water vapor?
Sure.

Stephanus
Chestermiller said:
Sure.
Thanks a lot Chestermiller. I've known this since high school. But I do need confirmation for may "suspicion". So the different of heat enthalpy
Water:
Gas -> -241
Liquid -> -281
The difference is to convert liquid to gas and vice versa

Chestermiller

## What is the Standard Enthalpy of Water?

The Standard Enthalpy of Water, also known as the Heat of Formation, is the amount of energy released or absorbed when one mole of water is formed from its elements at standard conditions (25°C and 1 atm pressure).

## Why is the Standard Enthalpy of Water important?

The Standard Enthalpy of Water is important because it is used as a baseline for comparing the enthalpy changes of other reactions. It is also a key component in thermodynamic calculations and in understanding the stability of different chemical compounds.

## What is the difference between the Standard Enthalpy of Water and the Enthalpy of Formation?

The Standard Enthalpy of Water is the enthalpy change for the formation of water at standard conditions, while the Enthalpy of Formation is the enthalpy change for the formation of a compound from its elements in their standard states. In other words, the Enthalpy of Formation of water is the same as the Standard Enthalpy of Water, but the Enthalpy of Formation can be calculated for any compound, not just water.

## How is the Standard Enthalpy of Water measured?

The Standard Enthalpy of Water is measured using a calorimeter, which is a device that measures the heat released or absorbed during a chemical reaction. The reactants are placed in the calorimeter and the change in temperature is recorded, which can then be used to calculate the enthalpy change.

## What factors can affect the Standard Enthalpy of Water?

The Standard Enthalpy of Water can be affected by changes in temperature, pressure, and the presence of other substances. It may also vary slightly depending on the source of the water, as different sources may have slightly different compositions.

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