Lecture 3 Using and Calculating Enthalpy

Lecture 3 Using and Calculating Enthalpy -...

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Summary from Last Lecture Enthalpy (H) is a useful, measurable chemical quantity. Enthalpy is a state function , so H depends only on the difference between the final and initial values of H. (Path does not matter) The change in enthalpy equals the heat absorbed or released at constant pressure. The heat capacity ( c , C ) specifies how much heat is required to raise a substance by 1 °C (or 1 K). Calorimetry can be used to determine H rxn and c . 1 H < 0 exothermic H > 0 endothermic
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Today’s Topics: Using and Calculating Enthalpies Finish Chapter 6 Intensive vs. Extensive Properties Hess’s law Standard Enthalpies of Reaction ( ௥௫௡ ) Standard Enthalpies of Formation ( ) Connection between H and bond energies (BE) (section 9.4) CHM139 F2011 Jockusch , Thermochemistry L3 2
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Extensive vs. Intensive Properties Extensive Property: A property whose value depends on the amount of material present (e.g., size, volume, mass) Intensive Property: A property whose value does not depend on the amount of material present (e.g., temperature, melting point, density) Enthalpy is an extensive property Iceberg Ice cube
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Hess’s Law For the reaction, 3 H 2 ( g ) + N 2 ( g ) 2 NH 3 ( g ) H = –92.2 kJ Enthalpy is a state property , which bring us to Hess’s Law: the enthalpy change for a process is equal to the sum of the changes for individual steps of the process. In practice, this reaction follows a two-step path: (1) 3 H 2 ( g ) + N 2 ( g ) H 2 ( g ) + N 2 H 4 ( g ) (2) H 2 ( g ) + N 2 H 4 ( g ) 2 NH 3 ( g ) If H (step 1) is 95.4 kJ… H (step 2) must be -187.6 kJ H overall = H 1 + H 2 + ………. + H n Enthalpy
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The industrial degreasing solvent methylene chloride (CH 2 Cl 2 ) is prepared from by reacting methane with chlorine: CH 4 ( g ) + 2 Cl 2 ( g ) CH 2 Cl 2 ( g ) + 2 HCl ( g ) Use the following data to calculate H (in kilojoules) for the above reaction : CH 4 ( g ) + Cl 2 ( g ) CH 3 Cl( g ) + HCl( g ) H = –98.3 kJ CH 3 Cl( g ) + Cl 2 ( g ) CH 2 Cl 2 ( g ) + HCl( g ) H = –104 kJ Hess’s Law: Example
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