Chemistry 51 Unit 2: Thermodynamics

WORK DONE IN STRETCHING DNA Piezo translator v Force Optical fiber bends when pulled x lo = contour length = 15mm r = x/lo = 1 i.e contour length (actual end-to-end separation) Force (pN) 40 80 120 160 0.6 0.8 1.0 1.2 1.4 1.6 r Application of F extends to contour length Structural transition Zero F Coiled state r < lo Irreversible damage Cluzel et al. Science 271:792--794

THE CONCEPT OF REVERSABILTY P = 1 atm Pext = 1 atm pins P = 2 atm Pext = 1 atm (1) REMOVE PINS Irreversible Expansion Infinite number of steps Reversible Expansion P = 1 atm Pext = 1 atm P = 1.998 atm Pext = 1.998 atm Step 2 P = 1.999 atm Pext = 1.999 atm Step 1 (2) P = 2 atm Pext = 2 atm

IRREVERSIBLE EXPANSION Pext V1 V2 P V V1 V2 Pext w = -Pext(V2 – V1) P, V1 P, V2

How does the pressure of an ideal gas vary with volume? REVERSIBLE EXPANSION Pext = Pressure of gas. If the gas is ideal, then Pext = nRT/V How does the pressure of an ideal gas vary with volume? This is the reversible path. The pressure at each point along curve is equal to the external pressure. P V

IRREVERSIBLE EXPANSION Vi Vf Pi Pf A B The reversible path The shaded area is IRREVERSIBLE EXPANSION P V Vi Vf Pi Pf A B The shaded area is Pext = Pf Reversible expansion gives the maximum work

REVERSIBLE COMPRESSION Vf Vi Pf Pi A B The reversible path The shaded area is IRREVERSIBLE COMPRESSION V Vf Vi Pf Pi A B The shaded area is Pext = Pf Reversible compression gives the minimum work