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The Nernst equation relates cell potential to concentration: E = E_standard - (RT/nF)ln(Q). At 25 C: E = E_standard - (0.0592/n)log(Q). Here n = electrons transferred in the balanced equation, Q = reaction quotient. Key applications: (1) Calculating E_cell at non-standard concentrations. (2) Concentration cells (E_standard = 0): E = (0.0592/n)log(C_high/C_low). (3) Finding K from E_standard: at equilibrium E = 0, so E_standard = (0.0592/n)log(K). (4) Thermodynamic link: delta_G = -nFE connects free energy to cell potential. delta_G_standard = -nFE_standard = -RTln(K) unifies electrochemistry, thermodynamics, and equilibrium. Temperature coefficient: delta_S = nF(dE/dT), delta_H = -nFE + nFT(dE/dT). Common traps: E_standard is intensive (doesn't change when reaction is scaled), but delta_G is extensive (scales with n). Always identify n from the balanced equation before applying Nernst.