Standard Enthalpy of Formation Calculator for Gas

Standard Enthalpy of Formation for Gas:

\[ \Delta H_{f,gas} = \Delta H_{f,liquid} + \Delta H_{vap} \]

ΔH_f,gas:

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1. What is Standard Enthalpy of Formation for Gas?

The standard enthalpy of formation for gas (ΔH_f,gas) is the change in enthalpy when one mole of a substance in the gas phase is formed from its pure elements under standard conditions. This calculator determines it by adding the standard enthalpy of formation of the liquid phase (ΔH_f,liquid) and the enthalpy of vaporization (ΔH_vap).

2. How Does the Calculator Work?

The calculator uses the equation:

\[ \Delta H_{f,gas} = \Delta H_{f,liquid} + \Delta H_{vap} \]

Where:

\( \Delta H_{f,gas} \) — Standard enthalpy of formation for gas (kJ/mol)
\( \Delta H_{f,liquid} \) — Standard enthalpy of formation for liquid (kJ/mol)
\( \Delta H_{vap} \) — Enthalpy of vaporization (kJ/mol)

Explanation: The equation accounts for the energy required to form the substance in liquid phase plus the energy needed to vaporize it to gas phase.

3. Importance of ΔH_f,gas Calculation

Details: The standard enthalpy of formation for gas is crucial for thermodynamic calculations, including reaction enthalpies, Gibbs free energy calculations, and predicting reaction spontaneity under standard conditions.

4. Using the Calculator

Tips: Enter both ΔH_f,liquid and ΔH_vap values in kJ/mol. The values can be positive or negative depending on whether energy is absorbed or released.

5. Frequently Asked Questions (FAQ)

Q1: What are standard conditions for this calculation?
A: Standard conditions are 1 atm pressure and usually 25°C (298 K) for thermodynamic calculations.

Q2: Can this be used for solids converting to gas?
A: For solids, you would need to include the enthalpy of fusion (ΔH_fus) as well: ΔH_f,gas = ΔH_f,solid + ΔH_fus + ΔH_vap.

Q3: Where can I find ΔH_f,liquid and ΔH_vap values?
A: These values are typically found in thermodynamic tables or chemical databases like NIST Chemistry WebBook.

Q4: Why are some ΔH_f values negative?
A: Negative values indicate the formation process releases energy (exothermic), while positive values indicate energy is absorbed (endothermic).

Q5: How does temperature affect this calculation?
A: The equation assumes standard conditions. For other temperatures, heat capacity corrections would be needed.