Review of Terms. Additional Review. Energy, Enthalpy, & Thermochemistry

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Abraham Hines
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1 Energy, Enthalpy, & Thermochemistry 9.1 The Nature of Energy 9. Enthalpy 9. Thermodynamics of Ideal Gases 9.4 alorimetry 9.5 Hess s Law 9.6 Standard Enthalpies of Formation 9.7 Present Sources of Energy 9.8 New Energy Sources Week 1 HEM Sections L and M 1 Review of Terms = heat absorbed or emitted If heat is absorbed, then is positive If heat is emitted, then is negative w = work done by or on a system w = -P external E = + w H = enthalpy = We ll look at calculations involving these terms more closely later Week 1 HEM Sections L and M Additional Review Enthalpy of a Reaction Discussed during last class ΔH rxn = H products H reactants Enthalpy of a Phase hange H fusion H freezing H vaporization H condensation Week 1 HEM Sections L and M 1

2 Enthalpy of Phase hanges H O (s) H O (l) ΔH fusion = + 6 kj at 0 Week 1 HEM Sections L and M 4 Enthalpy of Phase hanges H O (l) H O (s) ΔH freezing = - 6 kj at 0 Week 1 HEM Sections L and M 5 Enthalpy of Phase hanges ΔH vaporization Week 1 HEM Sections L and M 6

Enthalpy of Phase hanges ΔH condensation Week 1 HEM 110 - Sections L and M 7 Enthalpy of Phase hanges What is the

ΔH freeze = - ΔH fusion ΔH vap = - ΔH condensation Note: constant T Week 1 HEM 110 - Sections L and M 8 Heat

3 Enthalpy of Phase hanges ΔH condensation Week 1 HEM Sections L and M 7 Enthalpy of Phase hanges What is the relationship between these enthalpy changes? ΔH freeze = - ΔH fusion ΔH vap = - ΔH condensation Note: constant T Week 1 HEM Sections L and M 8 Heat apacity Heat apacity,, is the amount of heat reuired to raise the temperature of a substance by one degree. = Δ T Heat in Joules = Nm Temp in K, most commonly Week 1 HEM Sections L and M 9

4 Facts About Heat apacity Heat capacity is always a + number = Δ T and ΔT must be both negative or positive If is negative, then heat is evolved or given off and the temperature decreases = T final - T initial is negative T final < T initial Week 1 HEM Sections L and M 10 Facts About Heat apacity Heat capacity is always a + number = Δ T and ΔT must be both negative or positive If is positive, then heat is absorbed and the temperature increases = T final - T initial is positive T final > T initial Week 1 HEM Sections L and M 11 Molar heat capacity is the amount of heat (in J) reuired to increase the temperature of one mole of material by one degree (usually in K). Molar Heat apacity Lowercase c c p = n p Here, assume constant pressure Units = Joules mol x K Week 1 HEM Sections L and M 1 4

5 Specific Heat apacity The word specific before the name of a physical uantity very often means divided by mass c s = p units are JK m -1-1 g Specific Heat apacity is the amount of heat reuired to raise the temperature of one gram of material by one degree (at constant P) Week 1 HEM Sections L and M 1 At onstant olume At onstant Pressure = R 5 P = + R = R Where do these relationships come from? Show on board Week 1 HEM Sections L and M 14 : onstant From hapter 5.6, (KE) avg represents the average, random, translational energy for 1 mole of a gas at a given T. (KE) ave = RT The energy reuired to change the energy of 1 mole of an ideal gas by ΔT is E reuired = R T Week 1 HEM Sections L and M 15 5

6 At onstant olume At onstant Pressure = R 5 P = + R = R Where do these relationships come from? Show on board Week 1 HEM Sections L and M 16 Thermodynamics of Ideal Gases From hapter 5.6, (KE) avg represents the average, random, translational energy for 1 mole of a gas at a given T. (KE) ave = RT The energy ( heat ) reuired to change the energy of 1 mole of an ideal gas by ΔT is Energy ("heat") reuired = R T D Week 1 HEM Sections L and M 17 Example Problems A gas is compressed from 40L to 15L at a constant pressure of 5 atm. In the course of this compression 10 kj of energy is released. What is the heat,, for this process? What is the work, w (in L x atm)? What is H in kj? What is E in kj? Work on this for Friday Show on board Week 1 HEM Sections L and M 18 6

Major Concepts Calorimetry (from last time)

Major Concepts Calorimetry (from last time) Heat capacity Molar heat capacity (per mole) Specific heat capacity (per mass) Standard state enthalpies: Hº Physical Changes Chemical Changes Hess's Law Balancing