CHEM 101 1st Edition Lecture 14 Outline of Last Lecture I. SpectrophotometryII. ThermodynamicsIII. Specific Heat CapacityOutline of Current Lecture I. Energy and Changes of StateII. The First Law of ThermodynamicsCurrent Lecture - Energy and Changes of Stateo When a substance absorbs heat, its temperature will rise until it undergoes a phase change…o The substance will continue to absorb energy during the phase change, however, its temperature remains constant:o Phase changes are isothermal processes.o Changes of state can be either endothermic or exothermic. One example is the change between the liquid and gaseous state (vaporization,condensation): H2O(l) + Heat H2O(g)o Ex: what quantity of heat is required to convert 500 g of ice at -50 °C to steam at 200 °C?Step 1: Warm ice from -50 °C to 0 °C q1 = mH2O x CH2O(s) x ΔT = 500 g x 2.06 J x 50 K g x K = 5.15 x 10^(4) JStep 2: Melt ice at 0 °C q2 = mH2O x Heat of Fusion (Ice) = 500 g x 333 J g =1.67 x 10^(5) JStep 3: Warm water from 0 °C to 100 °C q3 = mH2O x CH2O(s) x ΔT = 500 g x 4.184 J x 100 K g x K = 2.09 x 10^(5) JStep 4: Evaporate water at 100 °C q4 = mH2O x Heat of Evaporation (Water) = 500 g x 2.256 Jg = 1.13 x 10^(6) JStep 5: Heat steam to 200 °C q5 = mH2O x CH2O(g) x ΔT = 500 g x 1.86 J x 100 K g x K = 9.30 x 10^(4) JStep 6: qtotal = q1 + q2 + q3 + q4 + q5 = 5.15 x 10^(4) J + 1.67 x 10^(5) J +2.09 x 10^(5) J + 1.13 x 10^(6) J + 9.30 x 10^(4) J = 1.65 x 10^(6) J - The Fist Law of Thermodynamicso Ex: Phase change by sublimation:CO2(s) CO2(g) (at -78 °C)Δu = q x wWhere u is internal energyq is generally energy transferred as heat to or from the systemand w is energy transformed as work to or from the systemEndothermic q > 0+ u increasesExothermic q < 0- u decreasesWork done on the system w > 0+ u increasesWork done by the system w < 0- u decreaseswork = -P x ΔVΔV = 0 (c0nstant volume) thenw= 0ΔU = qV + wV then:ΔU = qVConditions of Constant pressure:ΔU = qp + wp = qp – p x ΔV qp = ΔU + p x ΔV known as enthalpy (H)Enthalpy = ΔU
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