Standard Entropy of Reaction Calculator

The Measurement of Chaos

In thermodynamics, Entropy ($S$) is the scientific measure of chaos, disorder, and the number of possible microscopic arrangements a system can have. The universe fundamentally prefers high entropy.

The Standard Entropy of Reaction ($\Delta S^\circ_{rxn}$) tells us whether a chemical reaction is becoming more chaotic (favorable) or more ordered (unfavorable).

How to Predict Entropy Changes

You can often predict the sign of $\Delta S$ just by looking at the states of matter in a chemical equation:

• Solid to Liquid/Gas: Entropy massively increases ($+$).
• Fewer Moles to More Moles: Entropy increases ($+$) because there are more independent particles flying around.
• Gas dissolving into a liquid: Entropy decreases ($-$) because the wild gas molecules are now trapped in the liquid.

The Summation Equation

Just like Enthalpy, Entropy is a state function. We calculate the total change by summing up the absolute entropies of the products and subtracting the absolute entropies of the reactants.

Note: Unlike Enthalpy of Formation (where pure elements are zero), the Absolute Entropy of pure elements is never zero unless the substance is a perfect crystal at Absolute Zero (0 Kelvin).