Entropy Calculator: ΔS for Reactions & Phase Changes
Calculate the change in entropy for a reaction or phase change from the standard molar entropies of products and reactants.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
deltaS = Q_rev / T; deltaS_rxn = sum(S_products) - sum(S_reactants)
For a reversible process, entropy change equals heat transferred divided by absolute temperature. For chemical reactions, entropy change is the difference between total standard molar entropies of products and reactants, weighted by stoichiometric coefficients.
Worked Examples
Example 1: Entropy Change of Ice Melting
Problem:Calculate the entropy change when 1 mole of ice melts at 273.15 K. The enthalpy of fusion is 6,010 J/mol.
Solution:deltaS = Q / T\ndeltaS = 6010 J / 273.15 K\ndeltaS = 22.00 J/(mol*K)\n\nThis is positive because melting increases molecular disorder — liquid water has more microstates than solid ice.
Result:deltaS = 22.00 J/(mol*K) — entropy increases during melting
Example 2: Reaction Entropy for Combustion of Carbon
Problem:Find deltaS for C(s) + O2(g) -> CO2(g). Standard entropies: C(s) = 5.7, O2(g) = 205.2, CO2(g) = 213.8 J/(mol*K).
Solution:deltaS_rxn = S(products) - S(reactants)\ndeltaS_rxn = [213.8] - [5.7 + 205.2]\ndeltaS_rxn = 213.8 - 210.9\ndeltaS_rxn = 2.9 J/(mol*K)
Result:deltaS = 2.9 J/(mol*K) — slight entropy increase (1 mol gas produces 1 mol gas)
Frequently Asked Questions
What is entropy in chemistry?
Entropy (S) is a thermodynamic quantity that measures the degree of randomness or disorder in a system. In statistical mechanics, entropy is defined as S = k_B * ln(W), where k_B is Boltzmann constant and W is the number of microstates available to the system. A higher entropy means more possible arrangements of particles and energy. Entropy always increases for the universe as a whole (Second Law of Thermodynamics), though individual systems can decrease in entropy if a greater increase occurs elsewhere. In chemistry, entropy changes during reactions help determine whether a process is spontaneous by contributing to the Gibbs free energy equation deltaG = deltaH - T*deltaS.
How do you predict the sign of entropy change for a reaction?
Several qualitative rules help predict whether entropy increases or decreases in a chemical reaction. Entropy generally increases when: solids dissolve into solution, liquids vaporize to gases, the number of gas molecules increases (e.g., 1 mol gas producing 2 mol gas), temperature increases, or complex molecules decompose into simpler ones. Entropy generally decreases when: gases condense or are absorbed, molecules combine to form larger molecules, or crystallization occurs from solution. For example, the reaction 2H2O(l) producing 2H2(g) + O2(g) has a large positive entropy change because liquid water becomes three moles of gas, dramatically increasing the number of possible microstates.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy