Chapter 9Energy, Enthalpy and ThermochemistryThe study of energy and its interconversions is called thermodynamics.Kinetic Energy: energy due to the motion of the object (1/2 mv2)Potential Energy: energy due to position or compositionHeat: the transfer of energy between two objects due to a temperature differenceWork: a force acting over a distance

Figure 9.1 (a): Initial position of balls

Figure 9.1 (b): Final position of balls

Energy of Matters

State FunctionA property of the system depends only on its present state. A state function does not depend in any way on the systems past.Energy is a state function, but work and heat are not state function

Combustion of methane

Nitrogen/oxygen

First Law of ThermodynamicsThe energy of the universe is constant

In closed systemE= U=q+w (=+)q: the heat added to the system during the processw: the work done on the system during the processq>0 heat flows into the system from the surroundingsq0 work is done on the system by the surroundingsw

P-V Work

P-V Work

Enthalpy ()The heat qp absorbed in a constant-pressure process equals the systems enthalpy change.

For a chemical reaction H=Hproducts-HreactantsIf HreactantsHproducts (exothermic)

Consider a constant-volume processdw=-PdV=0 ()U=q+w=qvU=qv

Heat Capacity ()

Thermodynamics of Ideal Gases

Heat Capacity of Heating an Ideal Monatomic GasUnder constant volume, the energy flowing into the gas is used to increase the translational energy of the gas molecules.

Heat Capacity of Ideal Monatomic Gases

GasCv(J/K mol)Cp(J/K mol)He12.4720.8

Ne12.4720.8 Ar12.4720.8

Heat Capacity of Diatomic Gases

Heat Capacity of Polyatomic Gases

Heat Capacity of Heating a Polyatomic GasPolyatomic gases have observed values for Cv that are significantly greater than 3/2 R.This larger value for Cv results because polyatomic molecules absorb energy to excite rotational and vibrational motions in addition to translational motions.

Cv and Cp of molecules

Cv and Cp of Monatomic Gas

Cv and Cp of H2O at 373K

2 mol of monatomic ideal gasCalculate q, w, U and H for both pathway

TA=122K, TC=366K, TB=183K, TD=61K Cv=3/2R, Cp=5/2RPath 1(AC)w1=-PV=-2atm(30-10)L101.3J/Latm=-4.05103Jq1=qp=nCpT=25/2(R)(366-122)=1.01104J=H1U1=nCvT= 23/2(R)(366-122)=6.08103JPath 2(CB)q2=qv=nCvT=23/2(R)(183-366)=-4.56103J=U2H2=nCpT= 25/2(R)(183-366)=-7.6103JV=0 w2 =-PV =0

Path 3(AD)q3=qv=nCvT=23/2(R)(61-122)=-1.52103J=U3H3=nCpT= 25/2(R)(61-122)=-2.53103JV=0 w2 =-PV =0Path 4(DB)w1=-PV=-1atm(30-10)L101.3J/Latm=-2.03103Jq4=qp=nCpT=25/2(R)(183-61)=5.08104J=H4U4=nCvT= 23/2(R)(183-61)=3.05103J

SummaryPath 1qpath1=q1+q2=5.5 103Jwpath1=w1+w2= -4.05103JHpath1= H1 +H2= 2.55103JUpath1= U1 +U2= 1.52103JPath 2qpath2=q3+q4=3.56103Jwpath2=w3+w4= -2.03103JHpath2= H3 +H4= 2.55103JUpath2= U3 +U4= 1.52103J

CalorimetrySpecific heat capacity with unit JK-1g-1Molar heat capacity with unit JK-1mol-1

A constant-pressure calorimetry is used in determining the change in enthalpy equals the heat.H=qp=nCpTCoffee Cup Calorimeter

V=0U=qv+0=qvU=qv=nCv T

Bomb Calorimeter

2SO2(g)+O2(g)2SO3(g) H=-198 KJ2 mol. 1 mol. 2 mol.Calculate H and U

P is constant, H=qp=-198 KJ (energy flow out of system)U= qp +ww=-PV and V=n(RT/P)T and P are constant, n=nfinal-ninitial=-1 molSo w=-PV=-Pn (RT/P) =- nRT=-(-1)(8.314)(298)=2.48 kJU= qp +w=-198 kJ+2.48 kJ=-196 kJ

Hesss LawIf a reaction is carried out in a series of steps, H for the reaction will be equal to the sum of the enthalpy changes for the individual stepsThe overall enthalpy change for the process is independent of the number of steps or the particular nature of the path by which the reaction is carried out.

Consider the combustion reaction of methane to form CO2 and liquid H2OCH4(g) + 2O2(g) CO2(g) + 2H2O(l) H1 =-890KJ/mol This reaction can be thought of as occurring in two steps:CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H2 = -802 kJ/mol2H2O(g)2H2O(l) H3 =-88KJ/molH1 = H2 + H3

Standard Enthalpies of FormationThe change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states.The superscript zero indicates that the corresponding process has been carried out under standard conditions.

Petroleum and Natural Gas Coal

Present Sources of EnergyNew Energy SourcesCoal ConversionHydrogen as a fuel

CO2 capture