These mathematical definitions are needed to understand any concept that involves entropy. And therefore the effect of entropy on the surroundings must be taken into account.įor a spontaneous process, ΔS must be > q/T, and at equilibrium, ΔS surroundings = -q/T = -ΔH/T. So this law relates spontaneity to entropy. We use entropy as part of the second law of thermodynamics which states that the total entropy of a system and its surroundings always increases for a spontaneous process (in general the universe is always increasing its total entropy). If ΔS is negative, the process results in more order (less entropy). In terms of reactions, if ΔS is positive, the process results in more disorder (more entropy). Recall from physics the term Δ as in ΔS which is the difference between the initial and final states (ΔS = S f – S i). It is a measure of the disorder of a system (though this definition does not fully describe what is happening it is a good approximation as an introductory concept). Mathematically the thermodynamic term used to describe entropy is S, as seen in formulas like Gibb’s Free energy where G = H – TS. More broadly we define a spontaneous process as one that occurs in the direction a chemical reaction is written (from left to right, from reactants to products). Like enthalpy, entropy is a thermodynamic term that is also a state function.
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