1. What is Enthalpy Change (ΔH)?

Enthalpy change (ΔH) is the heat energy absorbed or released during a chemical reaction at constant pressure. It's a fundamental concept in thermochemistry that helps predict whether a reaction is endothermic (absorbs heat) or exothermic (releases heat).

2. How Does the Calculator Work?

The calculator uses the fundamental thermodynamic relationship:

Where:

• \( \Delta H \) — Enthalpy change (J)
• \( q \) — Heat energy transferred (J)

Explanation: At constant pressure, the enthalpy change of a system equals the heat transferred between the system and its surroundings.

3. Importance of ΔH Calculation

Details: Calculating enthalpy change is essential for understanding reaction thermodynamics, designing chemical processes, and predicting reaction spontaneity.

4. Using the Calculator

Tips: Simply enter the heat energy (q) in joules that was measured at constant pressure conditions. The calculator will output the enthalpy change (ΔH).

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between ΔH and q?
A: ΔH is a state function (path independent) while q is process-dependent. They are equal only at constant pressure.

Q2: How is heat energy typically measured?
A: Using calorimetry techniques, either through temperature changes (q = mcΔT) or direct heat flow measurements.

Q3: What does a negative ΔH value indicate?
A: A negative ΔH indicates an exothermic reaction (heat released to surroundings), while positive ΔH indicates endothermic.

Q4: Does this apply to phase changes?
A: Yes, the heat of fusion/vaporization at constant pressure equals the enthalpy change for the phase transition.

Q5: When is this relationship not valid?
A: When pressure isn't constant (use ΔU = q + w instead) or for open systems where mass transfer occurs.