Chapter 1 — IntroductionCHEM 101CHEM 101Fall 2013Fall 2013Dr. Michael Stollenz5. Energy and Chemical Reactions2•When a substance absorbs heat, its temperature will rise until it undergoes a phase change.•A phase change from solid to liquid requires heat of fusion.•A phase change from liquid to gas requires heat of vaporization.5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State3•The substance will continue to absorb energy during the phase change, however, its temperature remains constant: •Phase changes are isothermal processes. 5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State45. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State55. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State6•Changes of state can be either endothermic or exothermic. One example is the change between the liquid and gaseous state (vaporization, condensation):5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State+ energy+ energy+→22HO(l)HeatHO(g)→+22HO(g)HO(l)HeatChapter 1 — Introduction75. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State8•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State9Data information: ΔΔΔΔT, mass, heats of fusion/vaporization H2O, spec. heat cap.Warm ice from –50 ºC to 0 ºC.Step 1:Melt ice at 0 ºCStep 2:Warm water from 0 ºC to 100 ºC.Step 3:Evaporate water at 100 ºC.Step 4:q1Energy and Changes of StateEnergy and Changes of Stateq2q3q45. Energy and Chemical Reactions5. Energy and Chemical Reactions10Heat steam to 200 ºC.Step 5:qtotal= q1+ q2+q3+q4+ q55. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of Stateq4q5Step 6:11•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?5. Energy and Chemical Reactions5. Energy and Chemical Reactions►C1Energy and Changes of StateEnergy and Changes of State12•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 1: Warm ice from –50 ºC to 0 ºC:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of StateChapter 1 — Introduction13•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 2: Melt ice at 0 ºC:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State14•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 3: Warm water from 0 ºC to 100 ºC:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State15•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 4: Evaporate water at 100 ºC:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State16•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 5: Heat steam to 200 ºC:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State17•Example: What quantity of heat is required to convert 500 g of ice at –50 ºC to steam at 200 ºC ?Step 6: qtotal= q1+ q2+q3+q4+ q55. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnergy and Changes of StateEnergy and Changes of State185. Energy and Chemical Reactions5. Energy and Chemical ReactionsThe First Law of ThermodynamicsThe First Law of Thermodynamics•Example: Phase change by sublimation:CO2(s) → CO2(g) (at –78 ºC) ►C2Chapter 1 — Introduction19•We can only measure the change of enthalpy in a system, which is independent from the path between Hinitial(A) and Hfinal(B):ΔH = Hfinal– Hinitial•Therefore, enthalpy is a state function.AB5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnthalpyEnthalpy20•Since Enthalpies are state functions, we must specify the conditions at which they are measured.•H(T,P): Enthalpy is a function of temperature and pressure.5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnthalpyEnthalpy21•The standard state of an element or compound is the most stable form of the substance in the physical state that exists under standard conditions.•Standard state conditions are defined as:•100 kPa = 1.00 bar•usually 298.15 K or 25 ºC•“Hº” indicates that the Enthalpy is taken at standard state conditions.5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnthalpyEnthalpy22Reactants → Products∆H = Hfinal− HinitialEnthalpy of reaction = ∆rH = Hproducts− Hreactants5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnthalpyEnthalpyEnergyH (Products)H (Reactants) H (Products)H (Reactants)∆rH > 0∆rH < 0endothermicexothermic23CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) ΔrHo= –802 kJCO2(g) + 2 H2O(g) → CH4(g) + 2 O2(g) ΔrHo= +802 kJ•Energy can be considered as a product just like CO2or H2O.•From the chemical equation:5. Energy and Chemical Reactions5. Energy and Chemical ReactionsEnthalpyEnthalpy+4802 kJ of Energy Released1 mol CH (g) consumed2+802 kJ of Energy Released2 mol H O (g) produced24It is also common to write: ΔrHº = – 802 kJ/mol-rxnOne “mole of reaction” correlates to the exact amounts specified by the coefficients of the balanced equation.CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) ∆rHo= −802 kJEnthalpyEnthalpy5. Energy and Chemical Reactions5. Energy and Chemical ReactionsChapter 1 — Introduction25•Example: What is the enthalpy change when 128.5 g of methane, CH4(g) is combusted(ΔrHo= –802 kJ)?5. Energy and Chemical Reactions5. Energy and Chemical Reactions►C1EnthalpyEnthalpy26CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) ΔrHo= –802 kJexothermicCO2(g) + 2 H2O(g) → CH4(g) + 2 O2(g) ΔrHo= +802 kJendothermic5.