Entropy Calculator
The entropy calculator computes the entropy change for chemical reactions and physical processes. Entropy (S) measures the degree of disorder or the number of available microstates in a system. The second law of thermodynamics requires that any spontaneous process increases the total entropy of the universe. This calculator uses standard molar entropies to compute delta S for reactions via: delta S_rxn = sum of product entropies minus sum of reactant entropies. It also handles entropy changes for phase transitions (melting and boiling), heat transfer processes, and isothermal expansion of ideal gases. Understanding entropy is essential in thermodynamics, physical chemistry, chemical engineering, and biology.
Entropy formulas
Reaction: delta S = sum(n*S_products) - sum(n*S_reactants)
Phase transition: delta S = delta H / T
Heat transfer: delta S = q / T
Isothermal expansion: delta S = n*R*ln(V2/V1)
Standard molar entropies (J/mol*K at 298 K)
- H2(g): 130.7; O2(g): 205.0; N2(g): 191.6
- H2O(g): 188.7; H2O(l): 69.9
- CO2(g): 213.7; CH4(g): 186.3
- Fe(s): 27.3; Cu(s): 33.2
Entropy: frequently asked questions
What is entropy?
Entropy (S) is a measure of the number of microscopic configurations (microstates) available to a thermodynamic system. More disorder = more microstates = higher entropy. The second law of thermodynamics states that the total entropy of the universe always increases for any spontaneous process. Units are J/(mol*K) or J/K.
How is entropy change calculated for a reaction?
Using standard molar entropies (S degrees) from tables: delta S_rxn = sum(n_i * S_degrees(products)) - sum(n_j * S_degrees(reactants)). Unlike enthalpies of formation, standard molar entropies of elements are NOT zero at standard conditions - they have positive values.
What is the entropy change for a phase transition?
For melting: delta S = delta H_fusion / T_melting. For boiling: delta S = delta H_vaporization / T_boiling. These are positive (entropy increases as solid becomes liquid, liquid becomes gas). Example: water boiling at 100 degC has delta S = 40,700 J/mol / 373 K = 109 J/(mol*K).
What factors increase or decrease entropy?
Entropy increases when: gases are produced, number of moles increases, temperature increases, solid or liquid dissolves, molecules become more complex. Entropy decreases when: gas is compressed, particles aggregate, temperature decreases. Reactions producing gas almost always have positive delta S.
What is the third law of thermodynamics?
The third law states that the entropy of a perfect crystal at absolute zero (0 K) is zero. This gives an absolute reference point for entropy, unlike enthalpy. All standard molar entropies tabulated in chemistry data are positive values measured relative to 0 J/(mol*K) at 0 K.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.