Calculadora de Variação de Entropia
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
ΔS = ΔH / T
About this calculator
The Entropy Change Calculator determines the change in entropy (ΔS) of a thermodynamic system undergoing a reversible transformation at constant temperature. Entropy is a measure of disorder or energy dispersion in a system. In reversible processes, the entropy change is calculated by the ratio of heat exchanged reversibly (Q_rev) to absolute temperature (T), or, at constant pressure, by the ratio of enthalpy change (ΔH) to temperature. This tool is useful for students and professionals in chemistry and physics who need to quickly calculate entropy changes in chemical reactions or phase transitions.
The operation is simple: the user enters the enthalpy change (ΔH) in joules or kilojoules and the temperature (T) in kelvin. The calculator applies the formula ΔS = ΔH / T, assuming a reversible and isothermal process. The result is given in joules per kelvin (J/K) or kilojoules per kelvin (kJ/K), depending on the input unit. It is important to remember that the formula is only valid for reversible processes at constant temperature. For irreversible processes, the entropy change of the universe (system + surroundings) is always positive, but the calculation for the isolated system may be more complex.
When to use this calculator? It is ideal for calculating entropy changes in phase transitions, such as melting, vaporization, or sublimation, where the process occurs at constant temperature and pressure. For example, when melting ice at 0°C, the enthalpy change is the latent heat of fusion, and the temperature is 273.15 K. The calculator can also be used for chemical reactions under standard conditions, provided the reaction is reversible. However, for spontaneous and irreversible reactions, the obtained values may not represent the actual entropy change of the system.
Cautions: the main limitation is that the formula assumes a reversible process and constant temperature. In practice, many processes are irreversible, and the calculated entropy change underestimates the entropy generated. Also, ensure you use temperature in kelvin (K = °C + 273.15) and that ΔH corresponds to the heat exchanged reversibly. For chemical reactions, ΔH is the standard enthalpy change, but the standard entropy of reaction is usually calculated from standard molar entropies, not by this simple formula. The calculator is an educational tool for ideal cases.
Frequently asked questions
What is entropy and why is it important?
Entropy is a measure of disorder or the amount of energy dispersed in a system. It is important because it helps predict the spontaneity of processes, according to the second law of thermodynamics.
Can I use this calculator for irreversible processes?
Not directly. The formula ΔS = ΔH/T is only valid for reversible and isothermal processes. For irreversible processes, the entropy change of the system can be calculated by other methods, but the total entropy of the universe always increases.
What unit should I use for temperature?
Temperature must be entered in kelvin (K). Celsius is not accepted because the formula requires the absolute scale. Convert: K = °C + 273.15.
What does a positive or negative ΔS mean?
Positive ΔS indicates an increase in disorder (e.g., melting ice). Negative ΔS indicates a decrease in disorder (e.g., freezing water). Spontaneity also depends on temperature and enthalpy change.
Is this calculator suitable for common chemical reactions?
It is only suitable for reversible reactions at constant temperature. Most chemical reactions are irreversible, so the calculated value is approximate. For precise calculations, use standard molar entropies.