2Thermodynamic Framework Energy - capacity to do work or produce heatLaw of conservation of energyPE (generally chemical potential energy) and KEHeat involves a transfer of energy - Very different from temperature (a measure of motions of particles) Heat is NOT a substanceWork is force acting over a distance
3State Functions Energy = work + heat Ball rolling down hill example - heat and work are different depending on pathway
4State Functions - con’t State function or state property only depends on current state not pathwayEnergy is state functionHeat and work are not state functionsAnother example, Elevation vs. Distance
5Chemical Energy System and surroundings Exothermic and endothermic Chemical PE <--> thermal energyRecall energy reaction diagramsOnly concerned with Energy of Reactants and Products (not pathway) Internal EnergyE = q + w q=heat, w=work1st Law of thermodynamicsEnergy of Universe is constant
6Universe Energy is Constant Exothermic orEndothermic?
7Chemical Energy - con’t Quantities have sign and magnitudeSystem’s perspective for sign, thus endo is flow into system so q is positive (gaining heat)E <0 exothermic, E>0 endothermicWork done on system is positive, w>0Work done by system is negative, w<0
8PV WorkCommon types of work are expansion by a gas and compression on a gasPV work,w = - PV if volume is expandingw = PV if volume is compressingV = Vfinal - Vinitial,
9PV Work - derivation Example that shows Pressure=force/area = F/A so F = P x AWork is force x dist = F x hso W = P x A x hvolume of cylinder = A x hThus W = PVThe sign is (-) for expanding gas, since work is done by system on surroundings
10Enthalpy, HH = E + PV, since E, P and V are state functions H is also a state functionAt constant pressure H = qp (qp is heat at const p) In general, for open laboratory chemical reactions pressure is constant, so the change in enthalpy is used interchangeably with the heat of a reaction.For a chemical reactionH = Hproducts - HreactantsExothermic means enthalpy, H < 0Endothermic means enthalpy, H > 0
11CalorimetryThe science of measuring heat. Substances absorb heat differently; heat capacity, C, measures thisC = heat absorbed / increase in tempSpecific heat capacity is per gram substanceA calorimeter measures heat change.q = C x m x T our text uses ‘s’ for CThe AP test will use C, actually Cp
12Calorimeter Simple styrofoam cup calorimeter is easily used for lab measurements and constant pressure measurements.
13Bomb CalorimeterA “Bomb” calorimeter schematic - it only looks like a bomb. It is used when constant volume measurements are needed.
14Hess’s LawSince enthalpy is a state function, heat of reactions can be calculated from a known set of simple chemical rxns combined together to get the final rxn.One step:N2(g) + 2O2(g) --> 2NO2(g) H = 68kJTwo distinct stepsN2(g) + O2(g) --> 2NO(g) H = 180kJ2NO(g) + O2(g) --> 2NO2(g) H = -112kJTotal these reactions
15Hess’s Law schematicThe overall reaction enthalpy is independent of pathway
16Hess’s Law rules Characteristics of H for a rxn If a reaction is reversed, the sign of H is also reversed.The magnitude of H is directly proportional to the quantities of reactants and products in the rxni.e. if the coefficients are multiplied by an integer, H is multiplied by same integer.
17Example 6.8 pg. 254We want to calculate the H for the synthesis of diborane, B2H6, from its elements.2B(s) + 3H2(g) --> B2H6 (g) H = ?Use the following data:2B(s) + 3/2O2(g) --> B2O3(s) H = kJB2H6(g) + 3O2(g) --> B2O3(s) +3H2O(g) H = kJH2(g) + 1/2O2(g) --> H2O(l) H = kJH2O(l) --> H2O(g) H = kJHints - 1) work backward from the required/desired reaction, 2) reverse any reaction as needed to align reactants and products, 3)multiply any reaction as necessary to get correct coefficients.Recall H is a state function (independent of pathway)
18Standard Enthalpies of Formation Hfo of a substance is the change in enthalpy that accompanies the formation of 1 mole of the substance from its elements in their standard states.Standard state: gas is 1 atm, liquid or solid is pure substance, solutions are 1M. Elements are state at 1atm and 25oC.
19CalculatingH from Hfo Since enthalpies are state functions independent of pathway, in chemical reactions, the reactants can be taken apart into their elements and the products can be constructed from their elements.For a chemical reactionH = npHfo (products) - nrHfo (reactants)Data found in Appendix pg A19 - A22