ch06---energetics.ppt

New Way Chemistry for Hong Kong A-Level Book 11Chapter 6Energetics6.1 What is Energetics?6.2 Ideal Enthalpy Changes Related to Breaking and Formation of Bonds6.3 Standard Enthalpy ChangesNew Way Chemistry for Hong Kong A-Level Book 126.4 Experimental Determination of Enthalpy Changes by Calorimetry6.5 Hesss Law6.6 Calculations involving Enthalpy Changes of ReactionsNew Way Chemistry for Hong Kong A-Level Book 13What is Energetics?Energetics is the study of energy changes associated with chemical reactions.Thermochemistry is the study of heat changes associated with chemical reactions.Some termsEnthalpy(H)=heat content in a substanceEnthalpy change(H)=heat content of products-heat content of reactants=Hp-Hr6.1 What is Energetics?(SB p.140)New Way Chemistry for Hong Kong A-Level Book 14Internal Energy and Enthalpye.g.Zn(s)+2HCl(aq)ZnCl2(aq)+H2(g)Heat change atconstant pressure=Change in internal energy+Work done on the surroundings(Heat change at constant volume)Enthalpy change6.1 What is Energetics?(SB p.140)New Way Chemistry for Hong Kong A-Level Book 15Exothermic and Endothermic ReactionsAn exothermic reaction is a reaction that releases heat energy to the surroundings.(H=-ve)6.1 What is Energetics?(SB p.142)New Way Chemistry for Hong Kong A-Level Book 16Exothermic and Endothermic ReactionsAn endothermic reaction is a reaction that absorbs heat energy from the surroundings.(H=+ve)6.1 What is Energetics?(SB p.141)New Way Chemistry for Hong Kong A-Level Book 17Enthalpy Changes Related to Breaking and Forming of Bondse.g.CH4+2O2 CO2+2H2O 6.2 Enthalpy Changes Related to Breaking and Formation of Bonds?(SB p.143)In an exothermic reaction,the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products(products contain stronger bonds).New Way Chemistry for Hong Kong A-Level Book 18Enthalpy Changes Related to Breaking and Forming of BondsIn an endothermic reaction,the energy required in breaking the bonds in the reactants is more than the energy released in forming the bonds in the products(reactants contain stronger bonds).6.2 Enthalpy Changes Related to Breaking and Formation of Bonds?(SB p.143)New Way Chemistry for Hong Kong A-Level Book 19Bond EnthalpiesTo be discussed in later chapters.6.2 Enthalpy Changes Related to Breaking and Formation of Bonds?(SB p.143)BondMean Bond Enthalpy(kJ mol-1)H HC CCCC CN NN NN N436348612837163409944New Way Chemistry for Hong Kong A-Level Book 110Standard Enthalpy ChangesCH4(g)+2O2(g)CO2(g)+2H2O(g)H=-802 kJ mol-1 CH4(g)+2O2(g)CO2(g)+2H2O(l)H=-890 kJ mol-1As enthalpy changes depend on temperature and pressure,chemists find it convenient to report enthalpy changes based on an internationally agreed set standard conditions:1.elements or compounds in their normal physical states;2.a pressure of 1 atm(101325 Nm-2);and3.a temperature of 250C(298 K)6.3 Standard Enthalpy Changes(SB p.144)Enthalpy change under standard conditions denoted by symbol:HNew Way Chemistry for Hong Kong A-Level Book 111Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of Neutralizatione.g.The standard enthalpy change of neutralization between HNO3 and NaOH is-57.3 kJ mol-1e.g.The standard enthalpy change of neutralization between HCl and NaOH is-57.1 kJ mol-16.3 Standard Enthalpy Changes(SB p.144)The standard enthalpy change of neutralization(Hneut)is the enthalpy change when one mole of water is formed from the neutralization of an acid by an alkali under standard conditions.e.g.H+(aq)+OH-(aq)H2O(l)Hneut=-57.3 kJ mol-1New Way Chemistry for Hong Kong A-Level Book 112Standard Enthalpy Changes of neutralizationH+(aq)+OH-(aq)H2O(l)6.3 Standard Enthalpy Changes(SB p.145)-57.1-57.2-52.2-68.6NaOHKOHNH3NaOHHClHClHClHF Hneu AlkaliAcidNew Way Chemistry for Hong Kong A-Level Book 113Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of SolutionNote that enthalpy changes of solution associate with physical changes.6.3 Standard Enthalpy Changes(SB p.146)The standard enthalpy change of solution(Hsoln)is the enthalpy change when one mole of a solute is dissolved to form an infinitely dilute solution under standard conditions.e.g.NaCl(s)+water Na+(aq)+Cl-(aq)Hsoln=+3.9 kJ mol-1e.g.LiCl(s)+water Li+(aq)+Cl-(aq)Hsoln=-37.2 kJ mol-1New Way Chemistry for Hong Kong A-Level Book 1146.3 Standard Enthalpy Changes(SB p.146)Standard Enthalpy Changes of solution-44.7+3.9-57.8+20.0NaOHNaClKOHKBrHsoln(kJ mol-1)SaltNew Way Chemistry for Hong Kong A-Level Book 115Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of FormationStandard enthalpy change of formation of NaCl is-411 kJ mol-1.6.3 Standard Enthalpy Changes(SB p.147)The standard enthalpy change of formation(Hf)is the enthalpy change of the reaction when one mole of the compound in its standard state is formed from its constituent elements under standard conditions.e.g.2Na(s)+Cl2(g)2NaCl(s)H=-822 kJ mol-1 Na(s)+Cl2(g)NaCl(s)Hf=-411 kJ mol-1 1 moleNew Way Chemistry for Hong Kong A-Level Book 116Standard Enthalpy Changes of ReactionsThe enthalpy change of formation of an element is always zero.N2(g)N2(g)6.3 Standard Enthalpy Changes(SB p.147)What is Hf N2(g)?Hf N2(g)=0New Way Chemistry for Hong Kong A-Level Book 117Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of Combustione.g.C3H8(g)+5O2(g)3CO2(g)+4H2O(l)H1=-2220 kJ 2C3H8(g)+10O2(g)6CO2(g)+8H2O(l)H2=?H2=-4440 kJ It is more convenient to report enthalpy changes per mole of the main reactant reacted/product formed.6.3 Standard Enthalpy Changes(SB p.149)New Way Chemistry for Hong Kong A-Level Book 118Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of CombustionThe standard enthalpy change of combustion of propane is-2220 kJ mol-16.3 Standard Enthalpy Changes(SB p.149)The standard enthalpy change of combustion(Hc)of a substance is the enthalpy change when one mole of the substance burns completely under standard conditions.e.g.C3H8(g)+5O2(g)3CO2(g)+4H2O(l)Hc=-2220 kJ mol-1 1 moleNew Way Chemistry for Hong Kong A-Level Book 119Standard Enthalpy Changes of Reactions6.3 Standard Enthalpy Changes(SB p.150)-285.8-395.4-393.5-283.0-890.4H2(g)C(diamond)C(graphite)CO(g)CH4(g)Hc(kJ mol-1)SubstanceNew Way Chemistry for Hong Kong A-Level Book 120Experimental Determination of Enthalpy Changes by CalorimetryCalorimeter=a container used for measuring the temperature change of solutionDetermination of Enthalpy Change of Neutralization6.4 Experimental Determination of Enthalpy Changes by Calorimetry(p.151)New Way Chemistry for Hong Kong A-Level Book 1216.4 Experimental Determination of Enthalpy Changes by Calorimetry(p.151)Heat evolved=(m1c1+m2c2)TWhere m1 is the mass of the solution,m2 is the mass of calorimeter,c1 is the specific heat capacity of the solution,c2 is the specific heat capacity of calorimeter,And T is the temperature change of the reaction.New Way Chemistry for Hong Kong A-Level Book 122Determination of Enthalpy Change of Combustion6.4 Experimental Determination of Enthalpy Changes by Calorimetry(p.153)New Way Chemistry for Hong Kong A-Level Book 123Heat evolved=(m1c1+m2c2)TWhere m1 is the mass of water in the calorimeter,m2 is the mass of the calorimeter,c1 is the specific heat capacity of the water,c2 is the specific heat capacity of calorimeter,And T is the temperature change of the reaction.6.4 Experimental Determination of Enthalpy Changes by Calorimetry(p.153)New Way Chemistry for Hong Kong A-Level Book 124Hesss LawA+BC+DRoute 1H1EH2H3Route 2 H1=H2 +H3Hesss Law states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place.Why?6.5 Hesss Law(p.157)New Way Chemistry for Hong Kong A-Level Book 125Importance of Hesss Lawthe reactions cannot be performed in the laboratorythe reaction rates are too slowthe reactions may involve the formation of side productsThe enthalpy change of some chemical reactions cannot be determined directly because:But the enthalpy change of such reactions can be determined indirectly by applying Hesss Law.6.5 Hesss Law(p.158)New Way Chemistry for Hong Kong A-Level Book 126Enthalpy Change of Formation of CO(g)H2+O2(g)CO2(g)H1+O2(g)=-393.5-(-283.0)=-110.5 kJ mol-16.5 Hesss Law(p.158)Given:Hf CO2(g)=-393.5 kJ mol-1;Hc CO(g)=-283.0 kJ mol-1C(graphite)+O2(g)CO(g)Hf CO(g)Hf CO(g)+H2 =H1Hf CO(g)=H1-H2New Way Chemistry for Hong Kong A-Level Book 127Enthalpy Change of Formation of CO(g)6.5 Hesss Law(p.158)Hf CO(g)+H2 =H1New Way Chemistry for Hong Kong A-Level Book 128Enthalpy Change of Hydration of MgSO4(s)6.5 Hesss Law(p.160)aqMgSO4(s)+7H2O(l)MgSO47H2O(s)Mg2+(aq)+SO42-(aq)+7H2O(l)H H2 aqH1 H =enthalpy of hydration of MgSO4(s)H1=molar enthalpy change of solution of anhydrous magnesium sulphate(VI)H2=molar enthalpy change of solution of magnesium sulphate(VI)-7-water H =H1-H2New Way Chemistry for Hong Kong A-Level Book 129Calculations involving standard enthalpy changes of reactionsreactantsproductsHreaction6.6 Calculations involving Enthalphy Changes of Reactions(p.161)elements-Hf reactants Hf productsHreaction=Hf products-Hf reactantsNew Way Chemistry for Hong Kong A-Level Book 130