Enthalpy and entropy are two very important concepts in thermodynamics, which describe two different aspects of the state of a system, and there is a certain relationship between them.
1.Enthalpy (h).
Enthalpy is a thermodynamic state function defined as the product of the internal energy (u) of the system plus the pressure (p) and volume (v) of the system (i.e., h = u + pv). In a constant pressure process, the enthalpy change (δh) of the system is equal to the heat (q) absorbed or released by the system.
2.Entropy(s).
Entropy is a measure of the disorder or randomness of a system. In thermodynamics, the change in entropy (δs) describes the change in the number of microstates of a system as it transitions from one state to another. An increase in entropy is often associated with an increase in the disorder of a system.
Relationship between enthalpy and entropy**:
In the context of the second law of thermodynamics, the relationship between enthalpy and entropy can be linked by Gibbs free energy (G). Gibbs free energy is defined as g = h - ts, where t is the temperature. The change in Gibbs free energy (δg) can be used to determine whether a process proceeds spontaneously or not. If δg < 0, the process is spontaneous; If δg > 0, then the process is non-spontaneous; If δg = 0, the system is in equilibrium.
Under the condition of constant temperature and pressure, the spontaneous process of the system can be judged by the enthalpy change (δH) and entropy change (δS). According to the Gibbs-Helmholtz equation, δg = δh - tδs. This means that even if the enthalpy change (δh) is negative (exothermic process), if the entropy change (δs) is large enough and the temperature (t) is high enough, the system may increase the entropy (δs > 0) by absorbing heat (δh > 0), so that the Gibbs free energy (δg) becomes negative and the process proceeds spontaneously.
In practice, changes in enthalpy and entropy usually need to be determined from experimental data or calculated from thermodynamic tables and graphs.
In summary, enthalpy and entropy are two key parameters that describe the energy and disorder of a system, and together they determine the behavior of the system during thermodynamic processes. With Gibbs free energy, we can combine these two parameters to analyze the spontaneity of chemical reactions.