Enthalpy Calculator
What is Enthalpy?
Enthalpy ($H$) is a measurement of the total energy in a thermodynamic system. It is one of the most important concepts in chemistry and engineering, used to understand everything from steam engines to chemical reactions.
It combines two things:
- Internal Energy ($U$): The energy required to create the system (moving molecules, heat, vibrations).
- Work ($PV$): The energy required to "make room" for the system by pushing against the environment's pressure and volume.
The Equation
- H: Enthalpy (Joules).
- U: Internal Energy (Joules).
- P: Pressure (Pascals).
- V: Volume (Cubic Meters).
Why the "$PV$" Term Matters
Why isn't Internal Energy ($U$) enough? Imagine you want to create a balloon full of gas out of nothing.
First, you need the energy to create the gas itself (Internal Energy). But that gas takes up space. To exist, it has to push the atmosphere out of the way. Pushing that air back requires work.
That "pushing" energy is $P \times V$. Enthalpy accounts for both the creation energy AND the pushing energy.
State Functions
Enthalpy is a "State Function." This means the value depends only on the current state of the system (Temperature, Pressure, Volume), not on how it got there.
It doesn't matter if you heated the gas slowly or quickly; if the final T, P, and V are the same, the Enthalpy is the same.
Exothermic vs. Endothermic
In chemistry, we usually measure the Change in Enthalpy ($\Delta H$) during a reaction, rather than the total enthalpy.
| Change ($\Delta H$) | Type | Feeling | Example |
|---|---|---|---|
| Negative (-) | Exothermic | Releases Heat (Hot) | Fire, Hand Warmers |
| Positive (+) | Endothermic | Absorbs Heat (Cold) | Instant Ice Pack, Melting |
In physics, the standard unit is Joules (J). In chemistry, you will often see kJ/mol (Kilojoules per mole). Always convert units before plugging them into the formula. Remember: $1 \text{ Pa} \times 1 \text{ m}^3 = 1 \text{ Joule}$.