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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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 Fall '07
 Mitchell
 Chemistry, Enthalpy, Stoichiometry, Chemical reaction, target equation, Equations Thermochemical Equation

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