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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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