Convert Calories per Kelvin to Joules per Kelvin

The Calorie per Kelvin (symbol: cal/K) is a unit of entropy. A CGS entropy unit equal to exactly 4.184 J/K (using the thermochemical calorie definition per NIST SP 811). cal/K is a legacy unit that persists in older chemistry and biochemistry literature, with the equivalent molar form cal/(K·mol) (often abbreviated 'e.u.' for 'entropy unit' in older textbooks) appearing routinely in: classical physical-chemistry references pre-1980 (Lewis-Randall, Pitzer-Brewer thermodynamics texts), biochemistry literature on protein-folding entropy (typical ΔS_folding ranges from -50 to +50 cal/(K·mol) per residue), enzymology free-energy diagrams (Eyring-transition-state analysis), and pharmaceutical drug-binding thermodynamics (isothermal titration calorimetry / ITC papers in J. Mol. Biol., Biochemistry, Protein Sci. through the 1990s). The bond-dissociation entropy of H₂(g) → 2H(g) at 298 K is ΔS° = +24.4 cal/(K·mol). Modern publications increasingly use SI J/K, but cal/K appears in cross-references to older databases (NIST WebBook still dual-lists), in NMR-relaxation thermodynamic-cycle interpretation, and in physical-organic-chemistry literature where Arrhenius pre-exponential factors are quoted with entropy in cal-units.

It belongs to the metric measurement system.

Calories per Kelvin are commonly used in thermodynamics, refrigeration cycles, and chemical engineering.

The Joule per Kelvin (symbol: J/K) is a unit of entropy. The SI unit of entropy per ISO 80000-5 §5-19 — the thermodynamic state function that measures the spread of energy among accessible microstates (Boltzmann formulation S = k_B · ln Ω) or, equivalently, heat absorbed per unit temperature in reversible processes (Clausius formulation dS = δQ_rev/T). J/K is the universal unit for entropy across statistical mechanics (Gibbs, Helmholtz, Boltzmann formulations), classical thermodynamics, and the analogous information-theory definition (Shannon entropy in bits relates through the conversion 1 bit = k_B · ln 2 = 9.57 × 10⁻²⁴ J/K). The second law of thermodynamics (ΔS_universe ≥ 0 for spontaneous processes) and the third law (S → 0 as T → 0 for perfect crystals per Nernst) are both formalized in J/K. Reference values per NIST-JANAF Thermochemical Tables: standard molar entropy of liquid water at 298.15 K is 69.95 J/(mol·K); gaseous CO₂ at standard conditions is 213.79 J/(mol·K); diamond is 2.377 J/(mol·K) (very low — highly ordered crystal); graphite is 5.74 J/(mol·K). Boltzmann's constant k_B = 1.380649 × 10⁻²³ J/K (exact since 2019 SI revision).

It belongs to the metric measurement system.

Joules per Kelvin are commonly used in thermodynamics, refrigeration cycles, and chemical engineering.