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Energy+Part+3

# Energy+Part+3 - Calculating an unknown ∆H involves three...

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Energy and Chemical Reactions – Part 3 Stoichiometry of Thermochemical Equations Thermochemical Equation – a balanced equation that also states the heat of reaction (∆H rxn ) for that specific equation. Sign – The sign of H depends on whether the reaction is exothermic ( ) or endothermic (+). A forward reaction has the opposite sign of the reverse reaction. 2H 2 O( l ) 2H 2 ( g ) + O 2 ( g ) H rxn = 572 kJ Reverse reaction H rxn = 575 kJ Magnitude – The magnitude of H is proportional to the amount of substance reacting. Formation of 1 mol of water from its elements: H 2 ( g ) + 0.5O 2 ( g ) H 2 O( l ) H rxn = 286 kJ Problem: If it takes 90.8 kJ of heat to decompose 1 mol of HgO, how much heat is required to produce 1 short ton (2000 lb or 907 kg) of Hg? HgO( s ) Hg( l ) + 0.5O 2 ( g ) H = 90.8 kJ Heat (kJ) = (907 kg)(10 3 g / 1 kg)(1 mol Hg / 200.6 g Hg)(90.8 kJ / 1 mol Hg) = 4.11 x 10 5 kJ 1

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Hess’ Law of Heat Summation The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps.
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Unformatted text preview: Calculating an unknown ∆H involves three steps: • Identify the target equation, the step whose ∆H is unknown, and note the number of moles of reactants and products. • Manipulate the equations for which ∆H is known so that the target numbers of moles of reactants and products are on the correct sides. Remember to:-Change the sign of ∆H when you reverse an equation.-Multiply number of moles and ∆H by the same factor. • Add the manipulated equations to obtain the target equation. All substances except those in the target equation must cancel. Add their ∆H values to obtain the unknown ∆H. Problem: Nitrogen oxides undergo many interesting reactions. Calculate ∆H for the overall equation 2NO 2 ( g ) + 0.5O 2 ( g ) N → 2 O 5 ( s ) from the following information. N 2 O 5 ( s ) 2NO( → g ) + 1.5O 2 ( g ) H = 223.7 kJ ∆ NO( g ) + 0.5O 2 ( g ) NO → 2 ( g ) H = 57.1 kJ ∆ − 2...
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Energy+Part+3 - Calculating an unknown ∆H involves three...

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