6 ThermochemistryThermochemisty is the study of the relationship between heat and chemical reactions.1. Kinetic energy is energy possessed by matter because it is in motionThermal energy-- random motion of the particles in any sample above 0 KHeat -- causes a change in the thermal energy of a sample. Flows from hot to cold
8 Potential Energy2. Potential energy is energy possessed by matter because of its position or condition.A brick on top of a building has potential energy that is converted to kinetic energy when it is dropped on your headChemical energy is energy possessed by atoms as a result of forces which hold the atoms together (Boxes!)
9 Where is the Energy? Definitions we will use: System: Reaction (bonds) Surrounding: solvent, reactionvessel, air, etc.An everyday example: burning woodInitially, much energy stored as potential in C-H bonds, little kinetic energy in the airFinally, lower potential energy in the C=O bonds, higher kinetic energy in the air
10 Total Energy Total Energy = kinetic + potential Law of Conservation of Energy - The total energy of universe is constantInternal Energy - E - the sum of all the kinetic and potential energies of all the atoms and molecules in a sample.
11 Change in Energy of System Change in internal energy of system = heat + workConvention: point of view of system
12 Change in Internal Energy DE = q + wWork = Force x distanceWhat happens to your internal energy when you push a boulder?What happens to your internal energy when you push a boulder on a rough surface?
13 Chemical Work ׀W׀ = ׀F x Dh׀ P = F/A ׀W׀ = ׀P x A x h׀ ׀W׀ = ׀PV׀ Sign Convention:W = -PDV
14 Test Your Understanding For the following three reactions:Are they performed under constant pressure or not?What is the sign of work in each case?
15 State Function State Function Path Dependent Internal Energy Pressure VolumePath DependentWorkheatProperty depends only on present state
16 Enthalpy Most reactions are done in open containers, so P is constant Need a term for constant pressure where only work is PVAt constant pressure, qpΔE = qp – PΔVqp = ΔE + PVΔH = ΔE + PΔV (definition)ΔH = qp
17 Enthalpy IF pressure is constant and only work is PV Change in enthalpy is equal to flow of energy in form of heatMeasurable by TemperatureChange in enthalpy is “heat of reaction”If no net change in moles of gas, enthalpy ~ energy
18 Enthalpy Signs on ΔH Endothermic ΔH = + Exothermic ΔH = - + heat is taken in by system- heat is given off by systemEndothermic ΔH = +Exothermic ΔH = -
19 Exothermic2 Al (s) + Fe2O3 (s) Al2O3 (s) Fe (s) + energyWhich bonds have more potential energy? Which bonds are stronger?
20 Endothermic Ba(OH)2. 8H2O (s) + 2 NH4SCN (s) + energy Ba(SCN)2 (aq) NH3 (g) H2O (l)
21 Reaction Enthalpy It is useful to know how much energy is released CH4 (g) O2 (g) CO2 (g) H2O (l) + ENERGYIt is useful to know how much energy is releasedCH4 (g) O2 (g) CO2 (g) H2O (l) ΔH = kJThis is a stoichiometric amount890 kJ released = 1 mol CH4 = 2 mol O2 = 1 mol CO2 = 2 mol H2O
24 Standard Reaction Enthalpy Standard State - a compound in its pure state at 1 atm pressure, all solutions are 1 MTemperature can vary but usually KStandard Reaction Enthalpy (ΔHro)- reaction enthalpy when all products and reactants are in the standard stateCH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l) ΔHo = kJ
26 Application: Enthalpies of Combustion Standard Enthalpy of Combustion (ΔHco) -- change in enthalpy for the combustion of one mole substance at standard conditionsCombustion is combination with O2 to give CO2 and waterSpecific Enthalpy -- the enthalpy of combustion per gramenthalpy density -- enthalpy of combustion per liter
30 How Do We Determine Standard Reaction Enthalpies? Tabular Data: Hess’s LawCombining appropriate reactionsHeat of FormationBond enthalpiesExperimentalCalorimetry: constant pressureCalorimetry: constant volume
32 How Do We Determine Standard Reaction Enthalpies? Tabular Data: Hess’s LawCombining appropriate reactionsHeat of FormationBond enthalpiesExperimentalCalorimetry: constant pressureCalorimetry: constant volume
34 Using Hess’s Law Find ΔHo for C (s) + ½ O2 (g) CO (g) C (s) + O2 (g) CO2 (g) ΔHo = kJ2 CO (g) + O2 (g) 2 CO2 (g) ΔHo = kJ
35 How Do We Determine Standard Reaction Enthalpies? Tabular Data: Hess’s LawCombining appropriate reactionsHeat of FormationBond enthalpiesExperimentalCalorimetry: constant pressureCalorimetry: constant volume
36 Standard Enthalpies of Formation The standard enthalpy of formation is the enthalpy change when one mole of a substance in its standard state is formed from the elements in their standard states. HofWrite an equation for the standard heat of formation of carbon dioxideC(s) O2 (g) CO2 (g) Hof (CO2)Write an equation for DHof of CH3OHWrite an equation for DHof of N2 (g) and explain why its value is zero.
37 Standard Enthalpies of Formation DHof can be compiled in table form
38 Application of Heat of Formation Hess’s LawWithout the limitations of combining limited number of reactionsCommon starting point: elements
40 How Do We Determine Standard Reaction Enthalpies? Tabular Data: Hess’s LawCombining appropriate reactionsHeat of FormationBond enthalpiesExperimentalCalorimetry: constant pressureCalorimetry: constant volume
41 Using Bond Enthalpies Most versatile Least exact Must be able to draw Lewis Dot structures
42 Bond Enthalpies Bond Dissociation Energies Hess’s Law Positive valuesHess’s LawIn principle, “free atoms” formedDHrxn = SBDE(broken) – SBDE(formed)
43 Bond EnthalpiesCalculate the heat of reaction for the combustion of formamide (CH3NO).EquationLewis DotCalculate
44 How Do We Determine Standard Reaction Enthalpies? Tabular Data: Hess’s LawCombining appropriate reactionsHeat of FormationBond enthalpiesExperimentalCalorimetry: constant pressureCalorimetry: constant volume
45 Heat CapacityHeat Capacity (C) - the heat required to raise the temperature of an object by 1 KC = q/ΔTextensive property
46 Specific Heat Capacity Specific heat capacity (Cs) - the heat required to raise 1 g of a substance by 1 KSpecific heatCs = C/mintensive propertyq = m Cs ΔT
48 Each Substance “Stores” Heat Differently Increased263 oC25 oC1 g copper100 JIncreased24 oC25 oC1 g waterPutting the same amount of heat into two different substances will raise their temperature differently based on the specific heat of each substanceq=mCsDT
49 CalorimetryCalorimeter - an insulated container fitted with a thermometerOpen to atmosphere, so P is constantqp = m Cs ΔTqp = ΔH
50 Calorimetry ProblemIn a coffee cup calorimeter, 50.0 mL of M silver nitrate and 50.0 mL of M HCl are mixed. The following reaction occurs: Ag+ (aq) + Cl- (aq) AgCl (s). If the two solutions are initially at oC, and if the final temperature is oC, calculate the change in enthalpy for the reaction. (What assumptions need to be made?)
51 Bomb Calorimeter Volume is constant q = ΔE qsystem= -qsurroundings qrxn= -(qbomb+ qwater)qwater = mwaterCsΔTqbomb = mbombCsΔT or CDT
52 Thermodynamics of Ideal Gas Heat capacity of monoatomic gasFrom KMT, KE = 3/2RTKE = translational energyHeat required to raise temp 1 degree is 3/2RAt constant volume, no work is doneMolar heat capacity Cv = 3/2R = J/mol KIs this constant for all gases?
53 Consider polyatomic gases For polyatomic molecules, _____ energy has to be put into the same amount of gas to raise it by 1 degreeWhere does this energy go? (Not translational!)Trend??
54 Monoatomic Gas at Constant Pressure Energy input does two things: increase translational energy (T) and expand gas (w)w = PDV = nRDTMolar heat capacityCp = 3/2R + R = 5/2R
55 Polyatomic at Constant Pressure Explain physical basis of this equation:Cp =Cv + RThis is observed!When would you see a deviation from Cp-Cv = R?