1. What is Standard Enthalpy of Formation?

The standard enthalpy of formation (ΔH_f) is the change in enthalpy when one mole of a substance is formed from its elements in their standard states. For water, this value is -285.8 kJ/mol at standard conditions (25°C, 1 atm).

2. How Does the Calculator Work?

The calculator uses the standard enthalpy of formation equation:

Where:

• \( \Delta H \) — Total enthalpy change (kJ)
• \( n \) — Number of moles of water
• \( \Delta H_f \) — Standard enthalpy of formation (-285.8 kJ/mol for water)

Explanation: The equation calculates the total enthalpy change based on the amount of water formed.

3. Importance of ΔH_f Calculation

Details: Standard enthalpy of formation values are essential for calculating enthalpy changes in chemical reactions using Hess's Law and for thermodynamic analysis.

4. Using the Calculator

Tips: Enter the amount of water in moles. The value must be positive. The calculator will compute the total enthalpy change for that amount of water.

5. Frequently Asked Questions (FAQ)

Q1: Why is ΔH_f negative for water?
A: The negative sign indicates that heat is released when water is formed from its elements (exothermic process).

Q2: Does this value apply to all phases of water?
A: No, this value is for liquid water at 25°C. The value differs for water vapor or ice.

Q3: How was this value determined experimentally?
A: Through calorimetry experiments measuring heat released during water formation from hydrogen and oxygen.

Q4: Can this calculator be used for other substances?
A: No, this is specific to water. Each substance has its own unique ΔH_f value.

Q5: How does temperature affect ΔH_f?
A: ΔH_f values are temperature-dependent, but standard values are typically reported at 25°C.