Entropy Change Calculator
Entropy is a thermodynamic quantity that measures the degree of disorder or the dispersal of energy within a system. The second law of thermodynamics states that the total entropy of an isolated system never decreases over time: natural processes tend toward greater disorder. This calculator covers two common ways to quantify entropy change. The first is the classical heat-transfer formula: deltaS equals Q divided by T, where Q is the heat added to the system in joules and T is the absolute temperature in kelvin at which the transfer occurs. This formula applies to reversible processes at constant temperature, such as melting or boiling at the equilibrium point. A positive deltaS means heat entered the system and disorder increased; a negative deltaS means heat left. The second mode applies to the isothermal expansion of an ideal gas, where the volume changes from V1 to V2 at constant temperature. The formula is deltaS equals n times R times the natural logarithm of V2 divided by V1, where n is the number of moles and R is the universal gas constant 8.314 J per mol per K. Expanding the gas (V2 greater than V1) increases entropy because the molecules have more space to occupy. Enter your values and choose the mode that fits your problem. Temperature must be in kelvin. This calculator is suited to students studying thermodynamics, chemistry and engineering, and is for educational use.
Temperature must be in kelvin (K). For Celsius, add 273.15. Results apply to reversible processes at constant temperature only.
How it works
Two modes are available. Heat transfer mode applies deltaS = Q / T. Enter heat Q in joules (positive if heat flows into the system, negative if it flows out) and the temperature T in kelvin. Ideal gas expansion mode applies deltaS = n x R x ln(V2/V1), where n is moles of gas, R = 8.314 J/(mol K), and V1 and V2 are initial and final volumes (any consistent unit, since the ratio is dimensionless). If V2 is greater than V1 the result is positive (entropy increases on expansion); if V2 is less than V1 the result is negative (entropy decreases on compression).
Worked example
A system absorbs 1,000 J of heat at a constant temperature of 300 K. Entropy change: deltaS = 1000 divided by 300 = 3.33 J/K. The positive result confirms that entropy increased, consistent with heat flowing into the system. These match the default values pre-filled above.
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