7Discussions next class: Application of the first law Group 1: Understanding about the atmosphere and climate phenomenon(1) Marine climate/ Continental climate(2) Altitude/temperatureGroup 2: Is it possible water being used as fuel?Group 3: Food and energy reserves.
82.9 The thermochemistry U Q D = S ( - The energy changes in chemical reactionsthe heat produced or required by chemical reactionsUQrVD=Sreactant）product）(-MeasurablepredictableAt the same temperature ( reactants, products)
9molar enthalpy of reaction Standard molar enthalpy of reactionThe enthalpy change per mole for conversion of reactants in their standard states into products in their standard states, at a specified temperature and pressure PθH2(g,p) + I2(g,p)=2HI(g,p)△rHm(298.15K) = -51.8kJ·molFor example:
10Calculation of standard enthalpy of reactions (1) By standard molar enthalpy of formationThe enthalpy change when one mole of the compound is formed at 100 kPa pressure and given temperature from the elements in their stable states at that pressure and temperature.(298.15K)△rHm(298.15K)
11Understanding standard molar enthalpy of formation KJ.mol-1-300-200-100100200300F(g)+H(g)F Cl H2H(g)+218Cl(g)+H(g)HF(g)HCl(g)-564-431-92-269Standard molar enthalpy of formation of the stable forms of the elements is zero;Any form of elements other than the most stable will not be zero; such as C (diamond), C (g), H (g), and S (monoclinic).
12The ∆fH depend on state of substances KJ.mol-1-250-200-150-100-50-300H2+O2H2O2H2O(g)H2O(l)-188-242-285
13Example: Calculate the standard enthalpy of following reaction at 25℃ by using standard molar enthalpy of formationC2H5OH(g) C4H6(g) H2O(g) H2(g)
14(2) By standard molar enthalpy of combustion Definition: The enthalpy change when a mole of substance is completely burnt in oxygen at a given temperature and standard pressure.All these complete products have an enthalpy of combustion of zero.
15Standard molar enthalpy of combustion 400-110CO300200-394KJ.mol-1-284100CO2
20The experimental determination of standard enthalpy of combustion A sample of biphenyl (C6H5)2 weighing g was ignited in a bomb calorimeter initially at 25°C, producing a temperature rise of 1.91 K. In a separate calibration experiment, a sample of benzoic acid C6H5COOH weighing g was ignited under identical conditions and produced a temperature rise of 1.94 K. For benzoic acid, the heat of combustion at constant pressure is known to be 3226 kJ mol–1 (that is, ΔU° = –3226 kJ mol–1.) Use this information to determine the standard enthalpy of combustion of biphenyl.ΔH = ΔU + Δ(PV) = Qv + ΔngRT
21Hess’s law For example：（1） （2） （1）-（2）=（3） （3） The enthalpy change for any sequence of reactions that sum to the same overall reaction is identical.(Based on Enthalpy being a state function)For example：（1）（2）（1）-（2）=（3）（3）