CHEMICAL THERMODYNAMICS 1 chem 16 4 slides per page

if a particular reaction is carried out in a series

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Unformatted text preview: ess’s Law – Balance the reaction first! • Hess’s Law of Heat Summation – Arrange reactions [1] and [2] so that they also have FeO and O2 as reactants and Fe2O3 as a product. • Each reaction can be doubled, tripled, or multiplied by half, etc. • The ∆Ho values are also doubled, tripled, halved, etc. • If a reaction is reversed the sign of the ∆Ho is changed. - if a particular reaction is carried out in a series of steps, the ∆H for the reaction is equal to the SUM of enthalpy changes of each step. – Hess’s Law is true because ∆H is a state function. Given the following enthalpies of formation: [1] 2 Fe(s) + O2(g) → 2 FeO(g) [2] 4 Fe(s) + 3 O 2(g) → 2 Fe2O3(s) Determine the ∆Hrxn for : ∆H o = −544 kJ ∆H o = −1648 kJ FeO (s) + O 2(g) → Fe 2 O 3 (s) Hess’s Law • Example: Given the following equations and ∆Ho values For [1]: reverse, 2[-1] 2 ( 2 FeO (s) → 2 Fe (g) + O 2(g) ) multiply by 2 2[-1] 4FeO (s) → 4Fe (g) + 2O 2(g) For [2]: retain ∆H o = 2(+544 kJ/mole) ∆H o = +1088 kJ/mole) [2] 4Fe (s) + 3O 2(g) → 2Fe 2 O 3(s) ∆H o = −1648 kJ 4FeO (s) + O 2(g) → 2Fe 2 O 3 (s) ∆H rxn = -568 kJ/mole Hess’s Law calculate ∆Ho for the reaction below. N 2 O (g ) + NO 2 (g ) → 3 NO (g ) ∆H o = ? 8 1/7/2014 Experimental techniques to measure ∆H: CALORIMETRY • Coffee-cup calorimeter used to measure the amount of heat produced (or absorbed) in a reaction at constant P CALORIMETRY • Equations For heat transfer involving homogenous systems - PURE substances (eg. water) q = m • c • ∆T = mass • specific heat capacity • temperature change = g • ( J/g-oC) • oC – Method to measure qP for reactions in solution. – Constant-Pressure Calorimetry is used to measure enthalpy of reactions. For heat transfer involving heterogenous systems (eg. entire calorimeter) q = Ccal • ∆T = ( J/ oC) • oC • Styrofoam-based calorimetry in general, measures ∆H Calculating Enthalpy of Reaction using Calorimetry • Main Condition: No heat flows out of the calorimeter. The system is composed of the calorimeter, the main...
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