Entalphy of Formation: Why Use Water at Std Cond?

In summary, the enthalpy of formation of water at standard conditions is used in calculations because it takes into account the latent heat of condensation when gaseous water condenses to liquid. The condensation enthalpy is dependent on local saturation conditions, so it must be separately calculated to determine the additional energy released. The enthalpy of formation for both liquid and gaseous water is included in tables, but the hypothetical state value is often used in calculations involving water vapor.
  • #1
Est120
51
3
why do we use the entalphy of formation of water at standard conditions to calculate the entalphy of a reaction even if water is not gas at 1bar and 298K
 
Chemistry news on Phys.org
  • #2
A lot of water is in gaseous form at room temperature and pressure...
When gaseous water condenses to liquid, thermal energy is released. This is the latent heat of condensation. It is taken into account separately because formation enthalpy and condensation enthalpy are different things.
condensation enthalpy depends on the local saturation conditions, so if you want to know how much additional energy is released due to condensation, you have to compute how much water condenses. The difference in heat release between combustion with and without water condensation is the difference between the Higher Heating Value and the Lower Heating Value.
 
  • #3
Est120 said:
why do we use the entalphy of formation of water at standard conditions to calculate the entalphy of a reaction even if water is not gas at 1bar and 298K

To be honest, I have no idea what you are asking about. Doesn't enthalpy of formation of water at STP assume water to be in a liquid form?
 
  • #4
In the tables, there are two values for the heat of formation of water. One is for liquid water at 25 C and the standard pressure of 1 bar, and the other for the hypothetical state of water vapor at 25 C and 1 bar. The hypothetical state value is useful in many calculations where water vapor is part of a gaseous product mixture.
 

1. What is the enthalpy of formation?

The enthalpy of formation is the amount of energy released or absorbed when a substance is formed from its constituent elements in their standard states at standard conditions. It is a measure of the stability of a substance.

2. Why is water used as the standard state for enthalpy of formation?

Water is used as the standard state for enthalpy of formation because it is the most abundant and stable compound on Earth. It also has a well-defined and easily reproducible standard state at standard conditions (25°C and 1 atm).

3. What are standard conditions?

Standard conditions refer to a set of specific and well-defined conditions used for comparing and measuring physical and chemical properties of substances. These conditions include a temperature of 25°C and a pressure of 1 atm.

4. How is the enthalpy of formation of a substance determined?

The enthalpy of formation of a substance is determined by measuring the heat released or absorbed during a chemical reaction in which the substance is formed from its constituent elements in their standard states at standard conditions. This is typically done using calorimetry.

5. Why is the enthalpy of formation important?

The enthalpy of formation is important because it provides valuable information about the stability and energy content of a substance. It can also be used to calculate other thermodynamic properties, such as the enthalpy of reaction, which is useful in predicting the feasibility of a chemical reaction.

Similar threads

Replies
1
Views
2K
  • Biology and Chemistry Homework Help
Replies
2
Views
2K
  • Chemistry
2
Replies
39
Views
3K
Replies
2
Views
2K
  • Chemistry
Replies
7
Views
2K
Replies
2
Views
2K
  • Thermodynamics
Replies
7
Views
790
  • Chemistry
Replies
1
Views
2K
  • Biology and Chemistry Homework Help
Replies
9
Views
5K
Back
Top