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

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Hess’ Law of Heat Summation The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps.
Background image of page 2
This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

This note was uploaded on 04/13/2008 for the course CHM 2045 taught by Professor Mitchell during the Fall '07 term at University of Florida.

Page1 / 2

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

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online