Tag: Thermodynamics

Physical Chemistry · Equilibria The Phase Rule and Reading Phase Diagrams One short equation that tells you how many variables you are free to change, and a diagram that shows the consequence. BSc & MSc · Physical Chemistry · Concept The short answer: The phase rule states F = C − P + 2, where F is the degrees of freedom, C the number of components and P the number of phases in equilibrium. Applied to a one-component diagram it explains why an area has two degrees of freedom, a line one, and the triple point none. The rule F…

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Physical Chemistry · Thermodynamics Gibbs Free Energy: Why It Is the Criterion for Spontaneity Entropy decides spontaneity, but only for the universe. Gibbs energy repackages that so the system alone is enough. BSc & MSc · Physical Chemistry · Concept The short answer: The second law says the entropy of the universe must increase. Rewriting that condition for a system at constant temperature and pressure gives ΔG < 0. So Gibbs energy is not a new law — it is the second law expressed in variables you can actually measure inside the system. The problem Gibbs energy solves The second…

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Physical Chemistry · Thermodynamics Maxwell Relations: Where They Come From and How to Use Them Maxwell relations look like four formulas to memorise. They are actually one idea applied four times, and deriving them beats remembering them. BSc & MSc · Physical Chemistry · Concept The short answer: Each thermodynamic potential is an exact differential, so its mixed second derivatives are equal in either order. Applying that to U, H, A and G gives the four Maxwell relations. Their practical value is converting quantities that cannot be measured directly, such as an entropy derivative, into ones that can. The one…

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