Enthalpy vs Entropy: Difference and Comparison

To understand thermodynamics, enthalpy and entropy are two foundational concepts that no one can miss. Knowing the difference between enthalpy and entropy helps us pass our science exam and provides a rational explanation for many processes we witness daily. Thermodynamics can explain everything from changing phases to the transfer of energy in a single state.

Key Takeaways

  1. Enthalpy represents the total energy of a system, whereas entropy measures the system’s degree of disorder or randomness.
  2. A positive change in enthalpy implies heat absorption, while a negative change signifies heat release; increasing entropy indicates increased disorder while decreasing entropy suggests order.
  3. Enthalpy and entropy both influence the spontaneity of reactions, with more negative enthalpy changes and more positive entropy changes making reactions more spontaneous.

Enthalpy vs Entropy

The difference between enthalpy and entropy is that enthalpy is the measurement of a system’s total energy, which is the sum of internal energy and the product of pressure and volume. On the other hand, entropy is the amount of thermal energy in a system that is not available for its conversion into work. 

Enthalpy vs Entropy

The Enthalpy of a thermodynamic system is defined as a state function that is calculated at constant pressure (large open atmosphere). The unit of enthalpy is the same as energy, i.e., J, in the SI unit, because it is the sum of a system’s internal energy and the product of pressure and change in volume. The total enthalpy of a system cannot be measured directly. So, we measure the change in the enthalpy of a system. 

In simple words, entropy is the measure of randomness or chaos in a system. It is an extensive property, meaning that the value of entropy changes according to the amount of matter in the system. If a system is highly ordered (less chaotic), it has low entropy and vice versa. The SI unit of entropy is J⋅K−1.

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Comparison Table

Parameters of ComparisonEnthalpyEntropy
DefinitionEnthalpy is the sum of internal energy and product of pressure and volume of a thermodynamic system. Entropy is the amount of thermal energy of a system that is not available for conversion into mechanical or useful work. 
MeasurementThe total enthalpy of a system cannot be measured directly hence we calculate the change in enthalpy. Measuring entropy of a system refers to the amount of disorder or chaos present in a thermodynamic system. 
UnitThe SI unit of enthalpy is the same as that of energy hence can be measured in J. The SI unit of entropy for unit mass is J⋅K−1⋅kg−1 and for entropy per unit amount of substance is J⋅K−1⋅mol−1.
SymbolEnthalpy is denoted by H.Entropy is denoted by S.
HistoryA scientist named Heike Kamerlingh Onnes coined the term “enthalpy.”A German physicist called Rudolf Clausius coined the term “entropy.”
Favoring ConditionsA thermodynamic system always favors minimum enthalpy. A thermodynamic system always prefers maximum entropy. 

What is Enthalpy?

 Enthalpy is a thermodynamic property that refers to the sum of the internal energy and product of the pressure and volume of a system. The Enthalpy of a system signifies its capacity to release heat, and thus it has the same unit as energy (joules, calories, etc.). Enthalpy is denoted by H. 

It is impossible to calculate a system’s total enthalpy as it is impossible to know the zero point. So, the change in enthalpy is calculated between one state and another when the pressure is constant. The formula of enthalpy is H = E + PV, where E is the system’s internal energy, P is the pressure, and V is the volume. 

There is a lot of significance of enthalpy in a thermodynamic system as it determines if a chemical reaction is endothermic or exothermic. It is also used to calculate the heat of the reaction, the minimum power requirement for a compressor, etc. 

What is Entropy?

 Entropy is an extensive property, and it is the measure of randomness or chaos in a thermodynamic system. The value of entropy changes with the change in the amount of matter in the system. Entropy is denoted by S, and the common units of entropy are joules per kelvin J⋅K−1 or J⋅K−1⋅kg−1 for entropy per unit mass. Since entropy measures randomness, a highly ordered system has low entropy.

There are several methods to calculate the entropy of a system. But two of the most common ways are calculating the entropy of a reversible process and an isothermal process. For calculating the entropy of a reversible process, the formula is S = kB ln W where kB is the Boltzmann’s constant and its value is equal to 1.38065 × 10-23 J/K, and W is the number of possible states. For calculating the entropy of an isothermal process, the formula is ΔS = ΔQ / T, where ΔQ refers to the change in heat and T is the absolute temperature of the system in Kelvin.

The melting of ice into the water, followed by its vaporization into steam, is an example of increasing chaos and decreasing entropy. When the ice cube gains energy, the heat energy loosens its structure to form liquid and thus increases the chaos in the system. A similar thing happens when liquid is changed to a vapour state. But, while focusing on the system, the entropy decreases while the entropy of the surroundings increases. 

Main Differences Between Enthalpy and Entropy

  1. Enthalpy is the sum of internal energy and the product of the pressure and volume of a thermodynamic system. On the other hand, entropy is the amount of thermal energy of a system that is not available for conversion into mechanical or useful work. 
  2. Measuring enthalpy means measuring a system’s change in enthalpy, whereas entropy refers to the amount of disorder or chaos in a system.
  3. The SI unit of enthalpy is the same as that of energy and hence can be measured in J, whereas the SI unit of entropy for unit mass is J⋅K−1⋅kg−1, and for entropy per unit amount of substance is J⋅K−1⋅mol−1.
  4. Enthalpy is denoted by H, whereas entropy is denoted by S.
  5. Heike Kamerlingh Onnes coined the term “enthalpy”, whereas Rudolf Clausius coined the term “entropy.”
  6. In a thermodynamic system, minimum enthalpy is preferred, whereas maximum entropy is preferred in the same system. 
References
  1. https://pubs.acs.org/doi/pdf/10.1021/j100362a018
  2. https://pubs.rsc.org/en/content/articlehtml/2014/md/c4md00057a
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