The key difference between enthalpy and entropy is that enthalpy is the heat transfer taking place in a constant pressure whereas entropy gives an idea of the randomness of a system.
For the study purposes in chemistry, we divide the universe into two as a system and surrounding. At any time, the part we are going to study is the system, and the rest is surrounding. Enthalpy and entropy are two terms describing the reactions taking place in a system and the surrounding. Both enthalpy and entropy are thermodynamic state functions.
CONTENTS
1. Overview and Key Difference
2. What is Enthalpy
3. What is Entropy
4. Side by Side Comparison – Enthalpy vs Entropy in Tabular Form
5. Summary
What is Enthalpy?
When a reaction takes place, it may absorb or evolve heat, and if we carry out the reaction at constant pressure, we call it the enthalpy of the reaction. However, we cannot measure the enthalpy of molecules. Therefore, we need to measure the change in enthalpy during a reaction. We can obtain the enthalpy change (∆H) for a reaction in a given temperature and pressure by subtracting the enthalpy of reactants from the enthalpy of products. If this value is negative, then the reaction is exothermic. If the value is positive, then the reaction is endothermic.
Figure 01: Relationship Between Enthalpy Change and Phase Change
The change in enthalpy between any pair of reactants and products is independent of the path between them. Moreover, enthalpy change depends on the phase of the reactants. For instance, when the oxygen and hydrogen gases react to produce water vapour, the enthalpy change is -483.7 kJ. However, when the same reactants react to produce liquid water, the enthalpy change is -571.5 kJ.
2H2 (g) +O2 (g) → 2H2O (g); ∆H= -483.7 kJ
2H2 (g) +O2 (g) → 2H2O (l); ∆H= -571.7 kJ
What is Entropy?
Some things happen spontaneously, others do not. For example, heat will flow from a hot body to a cooler one, but we cannot observe the opposite even though it does not violate the conservation of energy rule. When a change occurs, the total energy remains constant but is parcelled out differently. We can determine the direction of change by the distribution of energy. A change is spontaneous if it leads to greater randomness and chaos in the universe as a whole. We can measure the degree of chaos, randomness, or dispersal of energy by a state function; we name it as the entropy.
Figure 02: A Diagram showing the Change in Entropy with Heat Transfer
The second law of thermodynamics is related to entropy, and it says, “the entropy of the universe increases in a spontaneous process.” Entropy and the amount of heat generated relate each other by the extent to which the system used energy. In fact, the amount of entropy change or extra disorder caused by a given amount of heat q depends on the temperature. If it is already very hot, a bit of extra heat doesn’t create much more disorder, but if the temperature is very low, the same amount of heat will cause a dramatic increase in disorder. Therefore, we can write it as follows: (where ds is changed in entropy, dq is changed in heat and T is temperature.
ds=dq/T
What is the Difference Between Enthalpy and Entropy?
Enthalpy and entropy are two related terms in thermodynamics. The key difference between enthalpy and entropy is that enthalpy is the heat transfer takes place in a constant pressure whereas entropy gives an idea of the randomness of a system. Moreover, enthalpy relates to the first law of thermodynamics while entropy relates to the second law of thermodynamics. Another important difference between enthalpy and entropy is that we can use enthalpy to measure the change in energy of the system after reaction whereas we can use entropy to measure the degree of disorder of the system after the reaction.
Summary – Enthalpy vs Entropy
Enthalpy and entropy are thermodynamic terms that we often use with chemical reactions. The key difference between enthalpy and entropy is that enthalpy is the heat transfer takes place in a constant pressure whereas entropy gives an idea of the randomness of a system.
Reference:
1. Libretexts. “Enthalpy.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018. Available here
2. Drake, Gordon W.F. “Entropy.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 7 June 2018. Available here
Image Courtesy:
1.”Phase change – en”By F l a n k e r, penubag – Own work, (Public Domain) via Commons Wikimedia
2.”Entropy Hot to Cold”By Ibrahim Dincer and Yunus A. Cengel – Entropy 2001, 3(3), 116-149; doi:10.3390/e3030116 http://www.mdpi.com/1099-4300/3/3/116, (CC BY 3.0) via Commons Wikimedia