What's the difference between entropy and enthalpy in terms of system's energy?

Entropy is a measure of disorder in a system, while enthalpy is a measure of the system's total energy.

Entropy, symbolised as 'S', is a fundamental concept in thermodynamics which quantifies the degree of randomness or disorder in a system. It's often described as the measure of the system's thermal energy per unit temperature that is unavailable for doing useful work. In simpler terms, it's the measure of the number of specific ways in which a system may be arranged, often taken to be a measure of 'disorder'. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.

On the other hand, enthalpy, denoted as 'H', is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. Enthalpy is a state function, which means it only depends on the final and initial state of the system, not on the path taken to change states. It's often used to calculate the heat exchange in chemical reactions, especially constant pressure reactions.

In the context of chemical reactions, the change in enthalpy (ΔH) and the change in entropy (ΔS) are crucial for determining whether a reaction is spontaneous or not. This is determined by the Gibbs free energy equation, ΔG = ΔH - TΔS, where T is the absolute temperature and ΔG is the change in Gibbs free energy. If ΔG is negative, the reaction is spontaneous; if it's positive, the reaction is non-spontaneous.

In summary, while both entropy and enthalpy are fundamental concepts in thermodynamics, they measure different aspects of a system. Entropy measures the disorder or randomness, while enthalpy measures the total energy of the system.