1. What is Enthalpy of Reaction?

The enthalpy of reaction (ΔH_rxn) is the change in enthalpy that occurs during a chemical reaction. It represents the heat absorbed or released at constant pressure. A negative ΔH indicates an exothermic reaction (releases heat), while a positive ΔH indicates an endothermic reaction (absorbs heat).

2. How Does the Calculator Work?

The calculator uses the enthalpy of reaction equation:

Where:

• \( \Delta H_{products} \) — Sum of standard enthalpies of formation of products (kJ/mol)
• \( \Delta H_{reactants} \) — Sum of standard enthalpies of formation of reactants (kJ/mol)

Explanation: The equation calculates the difference between the total enthalpy of the products and the total enthalpy of the reactants.

3. Importance of ΔH_rxn Calculation

Details: Calculating enthalpy of reaction is essential for understanding reaction thermodynamics, predicting whether reactions will occur spontaneously, and designing chemical processes in industry.

4. Using the Calculator

Tips: Enter the sum of standard enthalpies of formation for products and reactants in kJ/mol. The calculator will compute the enthalpy change for the reaction.

5. Frequently Asked Questions (FAQ)

Q1: What are standard enthalpies of formation?
A: The enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 1 atm pressure and 298 K.

Q2: How does ΔH relate to reaction spontaneity?
A: While ΔH is important, spontaneity depends on Gibbs free energy (ΔG = ΔH - TΔS). However, exothermic reactions (negative ΔH) often are spontaneous.

Q3: What are typical ΔH values for reactions?
A: Values vary widely: -286 kJ/mol for H₂ combustion, +178 kJ/mol for CaCO₃ decomposition, etc.

Q4: Can this be used for phase changes?
A: Yes, but you must use the appropriate enthalpy values (e.g., ΔH_fusion or ΔH_vaporization).

Q5: How does temperature affect ΔH?
A: ΔH varies slightly with temperature, but for small temperature ranges, it's often considered constant.