Chem 1320 1st Edition Lecture 36Outline of Last Lecture I. Endothermic & ExothermicII. State FunctionIII. EnthalpyIV. Heat CapacityOutline of Current LectureI. Hess’s LawCurrent Lecture I. Combining Reactions: Hess’s Law- Use component reactions- Target to obtain the ΔH that is unknown for a target reaction - Works because enthalpy is independent of path (state functions)Standard Enthalpy (heat) formation: Calculate the heat of a reaction for the fermentation of 1 mol of glucose, to form ethyl alcohol, and carbon dioxide Method one: use enthalpy to formation of data: requires a balanced chemical equationMethod two: use combustion data and Hess’s law (how it is actually done)50 grams of water is cooled from 30 degrees to 15 degrees. How much heat is absorbed by the surroundings? Eq=0Qsurroundings + qsystem = 0CΔT=qS x mΔT=q These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.612 J of heat is removed from 37.2L of gas at 0.80atm and the volume becomes 20.0L. what is the change in the internal energy of the gas? Δu=q+wW=-Pext +ΔVq= -612 J Vi=37.2LVf=20LPext=0.8atm Relate the energy stored in bonds stored in bonds to energy released in reactions “chemical energy”. Forming bonds releases energy and breaking bonds requires an input of energy Rationale: associated with exothermic reactions and energy absorption is associated with the endo-thermic reactions. If making and breaking bonds is related to energy transfer it seems reasonable that careful counting of bonds broken and made could be related to a heat of reaction ΔH reactionComplication: not all bonds have the same chemical environment so not all bonds have the same bond enthalpyCompromise: bond enthalpies in tables are always over many different chemical
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