Entropy Calculator
Calculate the standard entropy change ΔS° of any chemical reaction from product and reactant entropies using ΔS° = ΣS°(products) − ΣS°(reactants). Free chemistry entropy calculator with charts and breakdowns.
About This Calculator
The Entropy Calculator (Standard Reaction Entropy Change) computes the change in entropy ΔS° for any chemical reaction using the formula ΔS° = ΣS°(products) − ΣS°(reactants). Chemistry students at high school and university levels, laboratory researchers, and chemical engineers use this tool to determine whether reactions increase or decrease the disorder of a system, which is essential for predicting reaction spontaneity alongside enthalpy and temperature.
Standard molar entropy (S°) is a property of each chemical substance measured in joules per kelvin per mole (J/mol·K) at standard conditions of 298.15 K (25°C) and 1 bar pressure. To use the calculator, multiply the S° of each product by its stoichiometric coefficient and sum the results; do the same for reactants. The difference between these sums gives the standard reaction entropy change. A positive ΔS° indicates products are more disordered than reactants, while a negative ΔS° indicates the opposite.
Common standard molar entropies include: H₂(g) 131.0 J/mol·K, O₂(g) 205.0 J/mol·K, H₂O(g) 188.7 J/mol·K, H₂O(l) 69.9 J/mol·K, CO₂(g) 213.7 J/mol·K, CH₄(g) 186.3 J/mol·K, NaCl(s) 72.1 J/mol·K, and C(diamond) 2.4 J/mol·K. Gases generally have higher entropy than liquids, which have higher entropy than solids. More complex molecules with more atoms also tend to have higher standard entropies.
Regional Notes
Global (SI units): Standard molar entropies are universally expressed in J/(mol·K) using the SI system. Temperature is measured in Kelvin (K) for all thermodynamic calculations worldwide. The standard state pressure of 1 bar (100 kPa) is used in IUPAC conventions.
United States: Follows the same SI conventions for entropy. Standard entropies are typically taught in AP Chemistry and university general chemistry courses. Some older textbooks may reference the calorie-based unit cal/(mol·K) where 1 cal/(mol·K) = 4.184 J/(mol·K).
United Kingdom & Europe: Entropy and Gibbs free energy are core topics in A-level Chemistry and IB Chemistry curricula. Standard entropies at 298 K from the NIST-JANAF thermochemical tables are widely used. European textbooks follow IUPAC conventions with J/(mol·K) units.
Frequently Asked Questions
What is entropy in chemistry?
Entropy (S) is a thermodynamic quantity that measures the degree of disorder or randomness in a system. The second law of thermodynamics states that the total entropy of an isolated system always increases over time. In chemistry, entropy determines the spontaneity of reactions when combined with enthalpy and temperature through Gibbs free energy.
How do you calculate standard reaction entropy change?
Calculate standard reaction entropy change ΔS° using the formula ΔS° = ΣS°(products) − ΣS°(reactants), where S° values are standard molar entropies measured at 298.15 K and 1 bar pressure. Multiply each substance's S° by its stoichiometric coefficient, sum products and reactants separately, then subtract reactants from products. A positive ΔS° means products are more disordered than reactants.
What are typical standard molar entropy values?
Standard molar entropies S° vary by phase and complexity: gases like H₂ (131 J/mol·K) and O₂ (205 J/mol·K) have high entropy due to molecular motion; liquids like H₂O (70 J/mol·K) have moderate entropy; solids like diamond (2.4 J/mol·K) have very low entropy because of their ordered crystal structure. More complex molecules generally have higher entropy.
What does a positive ΔS° indicate?
A positive ΔS° indicates that the products of a reaction have higher entropy (more disorder) than the reactants. This means the reaction increases the overall disorder of the system. Reactions that produce gas from solids or liquids, or that increase the number of gas molecules, typically have positive ΔS° values. Positive ΔS° favors spontaneous reactions, especially at high temperatures.
What is the relationship between entropy and Gibbs free energy?
Gibbs free energy (ΔG) relates entropy and enthalpy through ΔG = ΔH − TΔS. A reaction is spontaneous when ΔG < 0. Even reactions with negative ΔS° can be spontaneous if they are exothermic enough (large negative ΔH). Conversely, endothermic reactions with positive ΔS° become spontaneous above a certain temperature where TΔS exceeds ΔH.
How does temperature affect entropy?
Increasing temperature increases the entropy of a system because molecules gain kinetic energy and have more available microstates. The entropy change from heating is ΔS = ∫(dq_rev/T). For phase transitions, the entropy change is ΔS = ΔH_transition/T_transition, such as ΔS_vap = ΔH_vap/T_bp for vaporization. This is why boiling water (high T) has much higher entropy than ice (low T).
Can entropy ever decrease in a system?
Entropy can decrease in a specific system (like freezing water to ice), but the total entropy of the system plus its surroundings always increases for spontaneous processes, as stated by the second law of thermodynamics. When water freezes, the entropy of the water decreases, but the entropy of the surroundings increases by a larger amount as heat is released.
What is the third law of thermodynamics?
The third law of thermodynamics states that the entropy of a perfect crystalline substance is zero at absolute zero temperature (0 K or −273.15°C). This provides an absolute reference point for entropy values. As temperature increases from 0 K, entropy increases as the atoms and molecules gain thermal energy and begin to vibrate, rotate, and translate.