entropy
Summary
Entropy (S) measures how many microscopic arrangements are available to a system, often described as disorder or energy dispersal. In a spontaneous process the entropy of the universe increases, but the system's own entropy can fall if the surroundings gain more.
Detail
Meaning: Entropy is a state function that reflects the number of microscopic arrangements (microstates) available to a system. More microstates means higher entropy. It usually rises when a solid melts or a liquid boils, a solute dissolves, temperature rises, or a reaction produces more moles of gas. The second law says the entropy of the universe increases in a spontaneous process.
Equation: ΔS = qrev/T for an isothermal process, where ΔS is the entropy change (J/K), qrev is heat absorbed along a reversible path (J), and T is absolute temperature (K). Spontaneity: ΔG = ΔH − TΔS, where ΔG and ΔH are in J.
On the MCAT: You predict the sign of ΔS from phases and moles of gas, then combine it with ΔH. Negative ΔH with positive ΔS is spontaneous at all temperatures. Positive ΔH with negative ΔS is spontaneous at no temperature. The mixed cases depend on T.
Common confusion: A system's entropy can fall (freezing, condensation) without violating the second law, because the heat released raises the surroundings' entropy by more. Also, ΔS is usually given in J/(mol·K) but ΔH in kJ/mol, so convert units before using ΔG = ΔH − TΔS.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.