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**Chpt 6 - Thermochemistry**

Energy and framework (definitions) Internal energy, heat & work Enthalpy & Calorimetry Hess’s Law Standard Enthalpies Applications? HW: Chpt 6 - pg , #s 14, 16, 24, 27, 28, 32, 34, 38, 41, 42, 44, 49, 54, 62, 72, 73, 80, 84, Due Wed Oct. 7

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**Thermodynamic Framework**

Energy - capacity to do work or produce heat Law of conservation of energy PE (generally chemical potential energy) and KE Heat involves a transfer of energy - Very different from temperature (a measure of motions of particles) Heat is NOT a substance Work is force acting over a distance

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**State Functions Energy = work + heat**

Ball rolling down hill example - heat and work are different depending on pathway

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**State Functions - con’t**

State function or state property only depends on current state not pathway Energy is state function Heat and work are not state functions Another example, Elevation vs. Distance

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**Chemical Energy System and surroundings Exothermic and endothermic**

Chemical PE <--> thermal energy Recall energy reaction diagrams Only concerned with Energy of Reactants and Products (not pathway) Internal Energy E = q + w q=heat, w=work 1st Law of thermodynamics Energy of Universe is constant

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**Universe Energy is Constant**

Exothermic or Endothermic?

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**Chemical Energy - con’t**

Quantities have sign and magnitude System’s perspective for sign, thus endo is flow into system so q is positive (gaining heat) E <0 exothermic, E>0 endothermic Work done on system is positive, w>0 Work done by system is negative, w<0

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PV Work Common types of work are expansion by a gas and compression on a gas PV work, w = - PV if volume is expanding w = PV if volume is compressing V = Vfinal - Vinitial,

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**PV Work - derivation Example that shows Pressure=force/area = F/A**

so F = P x A Work is force x dist = F x h so W = P x A x h volume of cylinder = A x h Thus W = PV The sign is (-) for expanding gas, since work is done by system on surroundings

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Enthalpy, H H = E + PV, since E, P and V are state functions H is also a state function At constant pressure H = qp (qp is heat at const p) In general, for open laboratory chemical reactions pressure is constant, so the change in enthalpy is used interchangeably with the heat of a reaction. For a chemical reaction H = Hproducts - Hreactants Exothermic means enthalpy, H < 0 Endothermic means enthalpy, H > 0

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Calorimetry The science of measuring heat. Substances absorb heat differently; heat capacity, C, measures this C = heat absorbed / increase in temp Specific heat capacity is per gram substance A calorimeter measures heat change. q = C x m x T our text uses ‘s’ for C The AP test will use C, actually Cp

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**Calorimeter Simple styrofoam cup**

calorimeter is easily used for lab measurements and constant pressure measurements.

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Bomb Calorimeter A “Bomb” calorimeter schematic - it only looks like a bomb. It is used when constant volume measurements are needed.

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Hess’s Law Since enthalpy is a state function, heat of reactions can be calculated from a known set of simple chemical rxns combined together to get the final rxn. One step: N2(g) + 2O2(g) --> 2NO2(g) H = 68kJ Two distinct steps N2(g) + O2(g) --> 2NO(g) H = 180kJ 2NO(g) + O2(g) --> 2NO2(g) H = -112kJ Total these reactions

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Hess’s Law schematic The overall reaction enthalpy is independent of pathway

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**Hess’s Law rules Characteristics of H for a rxn**

If a reaction is reversed, the sign of H is also reversed. The magnitude of H is directly proportional to the quantities of reactants and products in the rxn i.e. if the coefficients are multiplied by an integer, H is multiplied by same integer.

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Example 6.8 pg. 254 We want to calculate the H for the synthesis of diborane, B2H6, from its elements. 2B(s) + 3H2(g) --> B2H6 (g) H = ? Use the following data: 2B(s) + 3/2O2(g) --> B2O3(s) H = kJ B2H6(g) + 3O2(g) --> B2O3(s) +3H2O(g) H = kJ H2(g) + 1/2O2(g) --> H2O(l) H = kJ H2O(l) --> H2O(g) H = kJ Hints - 1) work backward from the required/desired reaction, 2) reverse any reaction as needed to align reactants and products, 3)multiply any reaction as necessary to get correct coefficients. Recall H is a state function (independent of pathway)

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**Standard Enthalpies of Formation**

Hfo of a substance is the change in enthalpy that accompanies the formation of 1 mole of the substance from its elements in their standard states. Standard state: gas is 1 atm, liquid or solid is pure substance, solutions are 1M. Elements are state at 1atm and 25oC.

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**CalculatingH from Hfo**

Since enthalpies are state functions independent of pathway, in chemical reactions, the reactants can be taken apart into their elements and the products can be constructed from their elements. For a chemical reaction H = npHfo (products) - nrHfo (reactants) Data found in Appendix pg A19 - A22

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**Example H from Hfo 4 NH3(g) + 7 02(g) --> 4 NO2(g) + 6 H2O(l)**

NH3(g) -46 kJ/mol NO2(g) 34kJ/mol H2O(l) kJ/mol O2(g) 0 kJ/mol H = Products - Reactants… 4x34kJ/mol + 6x(-286kJ/mol) x(-46kJ/mol) H = -1396kJ/mol

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Greenhouse Effect

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