Mary J. Bojan Chem 110 Thermochemistry Energy • kinetic • potential First Law of Thermodynamics • ΔE = q + w Enthalpy ΔH Thermochemical Equations Thermochemistry: Study of energy changes in chemical processes 2H2 + O2 → 2H2O + energy 1 ΔH for chemical reactions Calorimetry, Heat Capacity Hess’s Law Heat of formation ΔHf Standard State ΔH°f Foods and FuelsFirst Law of Thermodynamics Law of conservation of energy Energy can be converted from one form to another… …but total energy remains constant. Mary J. Bojan 2 Chem 110Mary J. Bojan Chem 110 Energy Kinetic Energy energy of motion Mechanical moving mass (1/2mv2) Electrical moving charge Light photons Sound molecules moving uniformly Heat molecules moving randomly 3 Potential Energy stored energy Mechanical mass in a place where a force can act Chemical bonds Nuclear binding energyMary J. Bojan Chem 110 Energy H⎯H + H⎯H + O=O → + 3 bonds 4 bonds 4 Electronic Energy The potential energy associated with the electrons in atoms and molecules Chemical energy the Potential Energy associated with bondingMary J. Bojan Chem 110 Changes in Energy w = work= action of force through a distance (often P∆V) work done to the system (+) q = heat (thermal energy) = energy transferred due to a difference in temperature. heat added to the system (+) 5 E = internal energy: total energy (E) associated with a system; the sum of all sources of kinetic & potential energy. the capacity to do work or transfer heat. ΔE =Efinal - Einitial = q + wMary J. Bojan Chem 110 Changes in Energy 6 Energy lost by the SYSTEM … is gained by the SURROUNDINGS. The system: what you are interested in a atom, molecule, or chemical reaction energy System ⎯⎯→ Surroundings ΔE = − energy Surroundings ⎯⎯→ System ΔE = +Mary J. Bojan Chem 110 Energy changes Which one has more energy? Cool the water Heat the water 7Mary J. Bojan Chem 110 State Function a function whose value does not depend on the pathway used to get to the present state State Function: only depends on the current state (composition, T,P): does not depend on past history 8 ΔEMary J. Bojan Chem 110 State Functions State functions are written as uppercase letters (E, H, P, V, T, S…) q and w are not state functions but ∆E (= q + w) is a state function 9 Changes in state functions are path-independentMary J. Bojan Chem 110 Energy (ΔE) and Enthalpy (ΔH) When changes occur at constant pressure ΔE = q + w w = −PΔV = work done on system at constant P arises from expansion or contraction of the system: ΔV = Vfinal – Vinitial ΔE = qp + wexpansion 10 Energy transferred at constant V = ΔEEnthalpy Mary J. Bojan Chem 110 11 quantitative definition: ΔE = qp + wp = qp – PΔV P = constant so qp = ΔE + PΔV ≡ ΔH = Enthalpy qualitative definition: ΔH is heat transferred at constant P For many chemical processes, PΔV is small and Like ΔE, ΔH is a state function.Examples of enthalpy Mary J. Bojan Chem 110 12 physical changes weʼve seen before… " freezing & melting! add heat to ice ⇒" temperature does not change during melting"• " ! vaporizing & condensing! add heat to water ⇒" temperature does not change during vaporization "• "energy transfers accompany:"• " !• " "Mary J. Bojan Chem 110 Thermochemical Equations A balanced chemical equation that also includes the energy change H2(g) + 1/2 O2(g) → H2O(g) ΔH = −241.8kJ ΔH = enthalpy: heat given off or absorbed in the reaction 13Mary J. Bojan Chem 110 ENTHALPY OF REACTION If ΔH<0 (−) exothermic (heat released) If ΔH>0 (+) Endothermic (heat absorbed) 14 ΔH = H(products) – H(reactants)ENTHALPY 1. Enthalpy is an extensive property. 2. ΔH for a reaction is equal in magnitude and opposite in sign to ΔH for the reverse reaction. 3. ΔH for a reaction depends on the states of reactants and products (gas, liquid, solid).Mary J. Bojan Chem 110 How much heat is given off per mole of O2? 16 Is this reaction 1 exothermic 2 endothermic How much heat is given off per mole of H2?Mary J. Bojan Chem 110 How much heat will be needed to convert 9g of water into H2 + O2? 17 How much heat will be given off if liquid water is formed instead of gaseous water. ΔHvap of water = 44kJ/mol If I convert water to H2 + O2, the reaction will be ___________ 1 exothermic 2 endothermic How much heat will be given off if 10g of H2 is consumed?Mary J. Bojan Chem 110 CALORIMETRY Experimental measure of heat flow q = C m ΔT q = heat flow C = specific heat (heat capacity per gram) m = mass ΔT= Tfinal - Tinitial 18 For H2O: C = 4.184 J/g °C Molar heat capacity = 75.2 J/mole °C Note: H2O is usually part of the surroundings qsurr = Csurr m ΔTMeasuring heat of reaction Mary J. Bojan Chem 110 19 Mix 50ml of 1M NaOH + 50ml of 1M HCl What is ΔH°rxn ? = Ti = Tf =Mary J. Bojan Chem 110 Measuring heat of reaction 1. Write the balanced reaction. 2. ΔT = Tf – Ti = 3. Is qp (ΔH°rxn) positive or negative? 4. qp = C m ΔT V=100ml, assume d = 1g/ml Then: m = (100ml)(1g/ml) = 100g qp =(4.184J/goC)(100g) ΔT 20Mary J. Bojan Chem 110 Hess’ Law ΔH for a sum of steps is the same as ΔH for the overall process. 21 True because ΔH is a state function. A → B ΔH1 B → C ΔH2 A+B → B+C ΔH1 + ΔH2 = ΔHrxnMary J. Bojan Chem 110 Hess’ Law Example 22 Given the following information A 2 H2(g) + F2(g) → 2 HF(g) ΔHA = -537kJ B 2 H2(g) + O2(g) → 2 H2O(g) ΔHB = -572kJ Determine ΔH for reaction C C 2 F2(g) + 2 H2O(g) → 4 HF(g) + O2(g) ΔHC = ??? SOLUTION: find combination of reactions such that n A + m B = C Then: n ΔHA + m ΔHB = ΔHCMary J. Bojan Chem 110 Hess’ Law Example 23 Given the following information A 2 H2(g) + F2(g) → 2 HF(g) ΔHA = −537kJ B 2 H2(g) + O2(g) → 2 H2O(g) ΔHB = −572kJ Determine ΔH for reaction C C 2 F2(g) + 2 H2O(g) → 4 HF(g) + O2(g) ΔHC = ??? SOLUTION: 2 A − 1 B = C 2xA 4 H2(g) + 2 F2(g) → 4 HF(g) ΔH = 2x(−537kJ) −1x B 2 H2O(g) → 2 H2(g) + O2(g) ΔH = − (−572kJ) 2 F2(g) + 2 H2O(g) → 4 HF(g) + O2(g) Then: 2 (−537kJ) − 1 (−572kJ) = −502kJMary J. Bojan Chem 110 You try! Given the following information: ΔH 2SO2(g) + O2(g) →
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