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Lec06_drobny_11 - Lecture 6 Standard Enthalpies Reading...

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Lecture 6: Standard Enthalpies • Reading: Zumdahl 9.6 Recommended Problems:9.61, 9.63, 9.67, 9.69 • Outline – What is a standard enthalpy? – Defining standard states. – Using standard enthalpies to determine Δ rxn
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Definition of Δ f We saw in the previous lecture the utility of having a series of reactions with known enthalpies. •What if one could deal with combinations of compounds directly rather than dealing with whole reactions to determine Δ H rxn ?
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Definition of Δ f (cont.) Standard Enthalpy of Formation, Δ f : “The change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances at standard state .” We envision taking elements at their standard state, and combining them to make compounds, also at standard state.
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What is Standard State? Standard State: A precisely defined reference state. It is a common reference point that one can use to compare thermodynamic properties. Definitions of Standard State: For a gas: P = 1 atm. For solutions: 1 M (mol/L). Note : different convention in some biochemistry applications… For liquids and solids: pure liquid or solid For elements: The form in which the element exists under conditions of 1 atm and 298 K.
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Definitions (cont.) Standard elemental states (cont.): – Hydrogen: H 2 (g) (not atomic H) – Oxygen: O 2 (g) – Carbon: C (gr)..graphite as opposed to diamond We will denote the standard state using the superscript “°”.
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