Unformatted text preview: ess’s Law – Balance the reaction first! • Hess’s Law of Heat Summation – Arrange reactions  and  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:  2 Fe(s) + O2(g) → 2 FeO(g)
 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 : 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 : retain ∆H o = 2(+544 kJ/mole)
∆H o = +1088 kJ/mole)  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
• Coffee-cup calorimeter used to measure the amount
of heat produced (or
absorbed) in a reaction at
constant P CALORIMETRY
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
q = Ccal • ∆T
= ( J/ oC) • oC • Styrofoam-based calorimetry in
general, measures ∆H Calculating Enthalpy of Reaction using
• Main Condition: No heat flows out of the calorimeter. The system is composed of the calorimeter, the main...
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