CHAPTER 10: THERMOCHEMISTRYThe Zeroth LawExothermic and Endothermic ReactionsHeatRelated quantititesTypical Summer in San FranciscoEquilibration temperaturesExample, cont’dCalorimetersEnthalpyEnthalpy of ReactionThermodynamics of Phase ChangesHess’s LawHess’s Law, cont’dStandard-state EnthalpiesStandard enthalpies of compoundsStandard molar enthalpies of formation, exampleStandard molar enthalpies of formation, cont’dHeats of formation exampleBond EnthalpiesBond enthalpy exampleWork in ThermodynamicsPressure-Volume WorkFirst Law of ThermodynamicsCHAPTER 10: THERMOCHEMISTRY• Definition of heat• Calorimetry and relationship between heat and temperature changes• Enthalpy• Hess’s Law• Bond enthalpies• First Law of ThermodynamicsCHEM 1310 A/B Fall 2006The Zeroth Law• “Zeroth Law” of Thermodynamics: Different objects at different temperatures which are in contact will eventually reach the same temperature• Thermal insulators like styrofoam cups try to slow down this process• Pan cools down & water heats up because the pan transfers energy in the form of heat to the waterCHEM 1310 A/B Fall 2006Exothermic and Endothermic Reactions• Exothermic reactions (example, right) give off heat. • Endothermic reactions require heat. • How exactly do we define heat?CHEM 1310 A/B Fall 2006Heat• Heat is a randomized form of energy which is transferred between materials, accompanied by a temperature change. Denoted by q.• (Recall kinetic theory of gases gave us a connection between temperature and kinetic energy of gas molecules.)• How much heat is transferred for a given temperature change ∆T = Tf–Ti? Depends on the heat capacity of a substanceheat capacity = Cp= q / ∆T (in J/K)for constant pressure and assuming Cpis constant with respect to T.CHEM 1310 A/B Fall 2006Related quantitites• Molar heat capacity: cp= Cp/ n (in J K-1mol-1). Heat capacity per mole.• Specific heat capacity: cs= Cp/ m (J K-1kg-1or J K-1g-1). Heat capacity per unit mass (kg for SI units, but frequently given per g)• A high heat capacity means it takes a lot of heat to change the temperature of a substance; a low heat capacity means a little heat will change the temperature a lot.CHEM 1310 A/B Fall 2006Typical Summer in San Francisco• Solar radiation hits the whole region equally, but inland area gets hotter. Why?• Heat capacity of water to the west is much higher than that of rock/land to the east•cs(H2O) = 4.18 J K-1g-1, cs(SiO2) = 0.74 J K-1g-1 .•SiO2quartz in granite, sandstone•.CHEM 1310 A/B Fall 2006Equilibration temperatures• A 4kg iron skillet at 100oC is immersed in 5L of H2O at 25oC. If we wait until the skillet and water reach the same temperature T2, what is T2? The specific heat capacities are cs(Fe(s)) = 0.449 J K-1g-1, cs(H2O(l)) = 4.18 J K-1g-1.CHEM 1310 A/B Fall 2006Example, cont’dCHEM 1310 A/B Fall 2006Calorimeters• Device to measure heat absorbed or given off by a chemical process• Thermometer records ∆T• Insulated container to minimize heat flow to/from outside• In example to right, measure heat given off by dissolving a solute in H2O by recording ∆T of water and knowing Cpof water. In practice, calorimeter also absorbs some heat; can use “calorimeter constant” C, q=C∆TCHEM 1310 A/B Fall 2006Enthalpy• Enthalpy is a quantity which is a thermodynamic property of a system. At constant pressure, heat absorbed by a system is equal to the change in enthalpy,qp= ∆H• Enthalpy is a “state function” that does not depend on path; similar to altitude, for example.Mountain climbers taking paths 1 or 2 still experience same change in altitude, ∆h. Likewise, can reach a given enthalpy Hfvia several different chemical processesCHEM 1310 A/B Fall 2006Enthalpy of Reaction• For constant pressure, the heat absorbed is the change in enthalpy for the reaction∆H > 0 endothermic (heat absorbed)∆H < 0 exothermic (heat given off)•N2H4(l) + 2Cl2(g) → 4HCl(g) + N2(g)∆H = -420 kJ • How much does the enthalpy change when 25.4g of hydrazine (N2H4) reacts with excess chlorine?CHEM 1310 A/B Fall 2006Thermodynamics of Phase Changes• A change of phase (e.g., solid to liquid) typically either consumes or gives off heatsolid → liquid endothermicliquid → solid exothermicat const P, heat absorbed is “enthalpy of fusion,” ∆HfusionReverse process is ∆Hfreezing∆Hfreezing= -∆Hfusion• Liquid vs gasliquid → gas endothermicgas → liquid exothermic∆Hcondensation= -∆Hvaporization• Enthalpies of fusion ~ 1-30 kJ/mol (6.007 kJ/mol for H2O)• Enthalpies of vaporization ~1-200 kJ/mol (40.7 kJ/mol for H2O)CHEM 1310 A/B Fall 2006Hess’s Law• Suppose we wish to know the enthalpy of reaction for(1) Cs(s) + O2(g) → CsO2(s) ∆H1= ?• Suppose also this reaction is much easier to perform with liquid cesium… how could we get ∆H1?• Recall H is a state function … we can take any path. (2) Cs(s) → Cs(l) ∆H2(3) Cs(l) + O2(g) → CsO2(s) ∆H3-------------------------------------------------------------(2)+(3) Cs(s) + O2(g) → CsO2(s) ∆H2 + ∆H3But (2)+(3) is just (1)! So ∆H1= ∆H2 + ∆H3. Can measure ∆H for reactions (2) and (3) if that’s easier than measuring it for reaction (1).CHEM 1310 A/B Fall 2006Hess’s Law, cont’d• Two ways to get from A to B --- straight there, or via a detour through C. ∆H is the same either way!• Hess’s Law: ∆H is the same whether a reaction occurs “directly” or as several steps. When we add/subtract equations, we add/subtract their ∆H values.CHEM 1310 A/B Fall 2006Standard-state EnthalpiesIn physics and chemistry, we need to define the “zero” of energy. For enthalpy, the zero is defined for the “standard state” of an element.• For solids and liquids, the standard state is at 1atm and a given temperature• For gases, the standard state is at a pressure of 1atm, the specified temp, and assuming ideal-gas behavior• For solvated species, the standard state is 1 molar, 1 atm, specified T, and ideal solution•Ho= 0 for each element (in its most stable form) at 298.15 K. Superscript omeans standard state.CHEM 1310 A/B Fall 2006Standard enthalpies of compounds• Now we can define a zero level for standard states of elements. What about compounds?• The standard molar enthalpy of formation, denoted ∆Hof, is the enthalpy change for the reaction producing 1 mol of the