CHEMICAL THERMODYNAMICS 1 chem 16 4 slides per page

# if a particular reaction is carried out in a series

This preview shows page 1. Sign up to view the full content.

This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online