Chem 402 Lecture 14: L14 2:20:17

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University College - Chemistry
University College - Chemistry Chem 402
Barnes Alexander

20 February 2017 L14: Thermochemistry, Enthalpy of Reactions, Condensed Phase Calorimetry I. Thermochemistry A. Basics 1. Goal a. To predict ∆H for every reaction, even if it cannot be carried out in the lab b. ∆H° rxnmeans ∆H at 1 bar (not necessarily at 298.15K) • H = U + pV (at constant p, T) • Heat of reaction is the ∆H for the isothermal reaction at constant pressure c. Enthalpy of reaction is defined with reversible work d. We cannot know 𝐻 values because enthalpy, like energy, is not measured on an absolute scale • We can only measure differences in enthalpy 2. Measuring Enthalpy a. Define a reference scale for enthalpy • 𝐻 (298.15 K, 1 bar) = 0 for every element in its most stable form at 1 bar and 298.15 K b. ∆𝐻 (𝑓98.15K): we can now write reactions to form every compound from its constituent atoms • The heat of reaction is the heat of formation of 1 mole of that compound from the constituent elements in their most stable forms • Remember: ∆H is a state function, doesn’t matter what path is taken ° c. Can tabulate ∆𝐻 (𝑓98.15K) values for all known compounds • We can calculate ∆𝐻 (𝑓) for any reaction (T = 298.15K) d. Enthalpy is a state function, so we can devise whichever path is most convenient to compute an Enthalpy change • Since you always have the same reactants to start and products to finish, a state reaction • Note: iare the stoichiometric coefficients • 1 decompose reactants into elements: − ∑ 𝑖 𝑖𝐻 ° (reactants), negative 𝑓,𝑖 because you are breaking apart each reactant • 2ndput elements together to form products • Use Hess’s Law (since H is a function of state, we can add ∆H’s around paths) 3. Enthalpy Changes a. ∆𝐻 𝑟𝑥= ∑ 𝑖𝑖∆𝐻 𝑓,𝑖(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 −) ∑ 𝑖𝑖∆𝐻 𝑓,𝑖𝑎𝑐𝑡𝑎𝑛𝑡𝑠) b. ∆H at constant p and for reversible process is ∆H = qp c. The heat of reaction is the heat flowing into the reaction from the surroundings • If ∆H < 0, q < 0, heat flows from the reaction to the surroundings, is rx p exothermic • If ∆H rx0, q p 0, heat flows into the reaction from the surroundings, is endothermic • **Since enthalpy is for a defined constant temperature, must give off or absorb heat to remain at constant temperature! ° 4. Temperature Dependence of ∆𝐻 𝑟𝑥 a. Since ( ) = 𝐶 𝜕𝑇 𝑃 𝑝 𝜕∆𝐻 ∑ ° ( ) ∑ ° b. Therefore, ( 𝜕𝑇 )𝑃= ∆𝐶 =𝑝 𝑖 𝑖∆𝐶 𝑝,𝑖 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 − 𝑖 𝑖𝐻 (𝑝,𝑖𝑐𝑡𝑎𝑛𝑡𝑠) c. C pepends on temperature especially during phase transition: 𝑇𝑓𝑢𝑠 𝑠 𝑇 𝑙 • 𝐻 𝑇 = 𝐻 0 +( ) ∫0 𝐶𝑝(𝑇)𝑑𝑇 + ∆ 𝑓𝑢𝑠𝐻 + ∫𝑇𝑓𝑢𝑠𝐶 𝑝` 𝑑𝑇` d. At increased temperature, you may excite vibrational degrees of freedom, which will increase the heat capacity • BUT it may be the same increase for products an
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