The second law The Concepts Chemistry 331
Chemistry 331 The Second Law The Concepts
The Laws of Thermodynamics First law You cant get something for nothing Second law You can t even break even Chemistry 331
Chemistry 331 The Laws of Thermodynamics • First Law You can’t get something for nothing. • Second Law You can’t even break even
Heat engines a heat engine converts heat into work Th=temperature ofheat source Engine c=temperature of heat sink gh= heat supplied 」qn qe heat released W=work produced Chemistry 331
Chemistry 331 Heat Engines • A heat engine converts heat into work. • Th = temperature of heat source • Tc = temperature of heat sink • qh = heat supplied • qc = heat released • w = work produced Th Tc Engine qh qc w
efficiency of heat engines An engine operates in You can iget a cycle:. AU=O something for and w=gh-lqcl nothing. efficiency is given b 8=w /qr q cl / gh q>o implies a< 1 You can t even breakeven Chemistry 331
Chemistry 331 Efficiency of Heat Engines • An engine operates in a cycle; \ DU = 0 and |w| = qh - |qc | • efficiency is given by e = |w| / qh \ e = 1 - |qc | / qh • |qc |>0 implies e < 1 You can’t get something for nothing. You can’t even break even
The Second law(Kelvin) No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work In other words, w<gh and the efficiency of a heat engine is less than 100% Chemistry 331
Chemistry 331 The Second Law (Kelvin) • No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work. • In other words, |w| < qh and the efficiency of a heat engine is less than 100%
Propositions For a reversible cycle, absorbing q, atT and g2 at T2, the ratio q: q2 depends only on Tand t2 2. This ratio may be written: q q2=t,/ T2 3. Th he entropy, S, defined by ds=(dq/t) rev S a function of state 4. In an isolated system, As is zero for a reversible process, and positive for a spontaneous one Chemistry 331
Chemistry 331 Propositions 1. For a reversible cycle, absorbing q1 at T1 and q2 at T2 , the ratio q1 :q2 depends only on T1 and T2 . 2. This ratio may be written: q1 /q2 = T1 /T2 3. The entropy, S, defined by dS=(dq/T)rev is a function of state. 4. In an isolated system, DS is zero for a reversible process, and positive for a spontaneous one
The Entropy function The entropy s is defined by reversible state B y ds= dg/t' ds-dqre T dS≠ dqirrey/ irreversible 4S≠d/T state A Chemistry 331
Chemistry 331 • The entropy S is defined by dS = dqrev/T • dS dqirrev/T The Entropy Function reversible: dS = dq/T irreversible: dS dq/T state A state B
The Second Law(clausius) The entropy s is a function of state ds >dq / T(clausius inequality o ds= dq /t for a reversible change o ds> dg/t for an irreversible change Corollary For an isolated system ds=0 for a reversible change ds>o for an irreversible change Chemistry 331
Chemistry 331 The Second Law (Clausius) • The entropy S is a function of state. • dS dq / T (Clausius inequality) • dS = dq / T for a reversible change • dS > dq / T for an irreversible change • Corollary: For an isolated system • dS = 0 for a reversible change • dS > 0 for an irreversible change
Carnot cycle isothermal: 100 K adiabatic isothermal: 200 K diabatic V/L Chemistry 331
Chemistry 331 Carnot Cycle 0 1 2 3 0 10 20 30 40 50 V / L p / atm isothermal: 100 K adiabatic isothermal: 200 K adiabatic
Carnot efficiency △S=△S +△S≥0 engine but△S =0 engine △S surr (qh/T)+(q/T)≥0 8 E≤1-(T/T Chemistry 331
Chemistry 331 Carnot Efficiency DS = DSengine + DSsurr 0 but DSengine = 0 \ DSsurr = - (qh / Th ) + (qc / Tc ) 0 \ qc / qh Tc / Th e = |w| / qh = 1 - |qc | / qh \ e 1 - (Tc / Th )