Enthalpy Change (ΔH) Calculator

Enthalpy Change (ΔH) Calculator

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What is Enthalpy Change (ΔH)?

The Enthalpy Change (ΔH) of a reaction is a measure of the total heat energy absorbed or released during a chemical reaction at constant pressure. It tells you whether a reaction is endothermic (absorbs heat from the surroundings, ΔH is positive) or exothermic (releases heat into the surroundings, ΔH is negative).

Hess's Law & Enthalpy of Formation

This calculator uses Hess's Law, which states that the total enthalpy change for a reaction is the same no matter which path it takes. This allows us to calculate the reaction enthalpy (ΔH°rxn) using the standard enthalpies of formation (ΔH°f) of its components.

The ΔH°f is the heat change when one mole of a compound is formed from its elements in their standard states. The formula used is:

ΔH°rxn = Σ(m·ΔH°f)products - Σ(n·ΔH°f)reactants

Frequently Asked Questions

What is the 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 constituent elements in their most stable form at standard conditions (25°C and 1 atm). By definition, the ΔH°f for a pure element in its standard state (like O₂(g), C(graphite), H₂(g)) is zero. You must look up the ΔH°f values for your compounds in a textbook or online reference.

Exothermic vs. Endothermic: What's the difference?

• Exothermic (ΔH is negative): The reaction releases heat into the surroundings. It feels hot. Combustion (burning) is a classic example.
• Endothermic (ΔH is positive): The reaction absorbs heat from the surroundings. It feels cold. An instant cold pack is a good example.

What's the difference between enthalpy (ΔH) and entropy (ΔS)?

They are both key parts of thermodynamics, but they measure different things.
• Enthalpy (ΔH) measures the change in heat energy (is it hot or cold?).
• Entropy (ΔS) measures the change in disorder or randomness.
Both are used together in the Gibbs Free Energy (ΔG) equation to determine if a reaction will be spontaneous.