© University of South Carolina Board of Trustees Overview Energy (  E = w + q ) ●work ( w ) ●heat ( q ) Enthalpy (  H =  E + P  V) Entropy ( S ) Gibb Free Energy ( G )  G =  H - T  S K eq = exp(-  G / RT )

© University of South Carolina Board of Trustees Calorimetry (Heat Measurement) Constant PressureConstant Volume

© University of South Carolina Board of Trustees Energy vs Enthalpy H = E + PV Constant Volume  E = q Constant Pressure  H = q  H rxn =  E rxn + (  n gas ) RT new!

© University of South Carolina Board of Trustees Calorimetry (Heat Measurement) Constant Pressure  H = q = C  T Constant Volume  E = q = C  T

© University of South Carolina Board of Trustees Calorimetry Example 1.00 g of octane (C 8 H 18, MW = g/mol) is burned completely in a bomb calorimeter. The original temperature is 25 °C and the temperature change is °C. The heat capacity of the calorimeter is 6.22 kJ/°C. (a) What is the molar reaction energy? (b) What is the molar reaction enthalpy?

© University of South Carolina Board of Trustees Overview Energy (  E = w + q ) ●work ( w ) ●heat ( q ) Enthalpy (  H =  E + P  V) Entropy ( S ) Gibb Free Energy ( G )  G =  H - T  S K eq = exp(-  G / RT )

© University of South Carolina Board of Trustees Chapt. 17 Thermodynamics Sec. 3 and 4 2 nd Law: Concepts

© University of South Carolina Board of Trustees Entropy ( S ) Entropy is a quantitative measure of disorder. Units = J/K S gas > S liq > S solid S gas > S sol’n S sol’n > S solid S sol’n > S liq S high T > S low T

© University of South Carolina Board of Trustees 2 nd Law of Thermodynamics spontaneously Even though reactions conserve energy, they may or may not happen spontaneously. > ●In an isolated system, the entropy must increase in a spontaneous reaction  S > 0 the Gibbs Free Energy must decrease in a spontaneous reaction < ●In a system at constant pressure and temperature, the Gibbs Free Energy must decrease in a spontaneous reaction  G < 0

© University of South Carolina Board of Trustees Gibbs Free Energy ( G ) G = H - T S  G =  H - T  S (const. T)  G =  E - P  V - T  S (const. T and P) “tax” to keep temperature constant “tax” to keep pressure constant “gross” energy “free” energy

© University of South Carolina Board of Trustees Gibbs Free Energy ( G ) G = H - T S  G =  H - T  S  G =  H - T  S (const. T) Entropy increase (  S > 0) favors reaction. Heat release (  H < 0) favors reaction.  G < 0 needed for a spontaneous reaction. Entropy effect increases at high temperature.