c101_topic-8 - 11/7/11 Energy Changes and Calorimetry  ...

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Unformatted text preview: 11/7/11 Energy Changes and Calorimetry   In a calorimetry experiment, the heat flow resul�ng from a chemical rxn is determined by measuring the temperature change of the calorimeter. Then q can be related to E through the 1st law of thermodynamics. ΔE = q + w When the experiment is performed at constant V, we use a subscript “v”. When the experiment is performed at constant P, we use a subscript “p”. Energy Changes and Calorimetry ΔE = qv + wv and ΔE = qp + wp Molar Energy Change Problem   E change is an extensive quan�ty which means that the amount of E released or absorbed depends on the amount of substances that react. Thus, when we report an E change, we must also report the amounts of the chemical substances that generate the energy change. ΔEmolar = ΔE / n   A 1.250 g sample of octane is burned in excess O2 in a constant-­‐volume calorimeter. The T of the calorimeter rises from 21.05oC to 27.78oC. Find the heat transferred to the calorimeter during the combus�on of the octane. Problem Problem   For the previous octane problem what is the ΔEmolar for the combus�on of octane? (A 1.250 g sample of octane is burned in excess O2 in a constant-­‐volume calorimeter. The T of the calorimeter rises from 21.05oC to 27.78oC. Find the heat transferred to the calorimeter during the combus�on of the octane.)   Assume you mix 100.0 mL of 0.200 M CsOH with 50.0 mL of 0.400 M HCl in a coffee-­‐cup calorimeter. The following rxn occurs: CsOH(aq) + HCl(aq) CsCl(aq) + 3H2O() The T of the solu�on before mixing was 22.50oC and it rises to 24.28oC a�er the acid-­‐base rxn. What is the enthalpy of the rxn per mole of CsOH. 1 11/7/11 Enthalpy   In the world around us, most chemical processes occur in contact with the Earth’s atmosphere, at a constant P of 1 atm. For example, plants convert CO2 and H2O into carbohydrates; animals metabolize food; water heaters and stoves burn fuel; and running water dissolves minerals from the soil. All these processes involves E changes at constant P. Enthalpy   Recall, ΔE = qp + wp. Enthalpy is a fundamental thermodynamic property. Let us examine the excellent example of H2O. H2O(l) H2O(g) ΔHvap = +44.0 kJ/mol H2O(g) H2O(l) ΔHcond = -­‐44.0 kJ/mol Some things to remember: Enthalpy Enthalpy   1. When heat transfer occurs (at constant P) from a system to its surroundings the process is exothermic with respect to the system and ΔH has a nega�ve value. Conversely, when heat is absorbed from the surroundings the process is endothermic with respect to the system and ΔH has a posi�ve value.   2. For changes that are the reverse of each other, the ΔH values are numerically the same, but their signs are opposite.   3. The change in energy or enthalpy is directly propor�onal to the quan�ty of material undergoing a change.   4. The value ΔH is always associated with a balanced equa�on for which coefficients are read as moles, so that the equa�on shows the macroscopic amount of materials to which the value of ΔH applies. 2H2O(l) 2H2O(g) ΔHvap = +88.0 kJ Problem Heats of Forma�on   Calcium carbide, CaC2, is manufactured by reducing lime (CaO) with carbon at a high T. Is a rxn endothermic or exothermic? If 10.0 g of CaO is allowed to react with an excess of carbon, what quan�ty of heat is absorbed or evolved by the rxn?   By performing calorimetry experiments, chemists could measure energy and enthalpy changes for most chemical rxns. This would be a lot of work and is not necessary. Instead, the 1st law of thermodynamics make it possible to calculate enthalpy changes for almost any rxn from experimental values for one set of rxns called forma�on rxns.   A forma�on rxn produces 1 mol of a chemical substance from the elements in their most stable forms at 298 K. CaO(s) + 3C(s) CaC2(s) + CO(g) ΔHorxn = +464.8 kJ 2 11/7/11 Heats of Forma�on Heats of Forma�on   Hg(l) + Cl2(g) HgCl2(s)   C(graphite) + 2H2(g) + 1/2O2(g) CH3OH(l)   3. Frac�onal stoichiometric coefficients are common in forma�on rxn because the rxn must generate exactly 1 mol of product.   Forma�on rxns exhibit the following features: 1.  There is a single product with a stoichiometric coefficient of one. 2.  All the star�ng materials are elements and each is in the form that is most stable at T = 298 K and P = 1 atm.   The standard enthalpy of forma�on (ΔHof) is the enthalpy change that accompanies the forma�on of 1 mol of a chemical substance from pure elements in their most stable forms at 209 K and 1 atm P. Every chemical substance has a characteris�c ΔHof expressed in kJ/mole that can be measured and Enthalpy Changes for Chemical Rxns Heats of Forma�on   tabulated. Standard enthalpies of forma�on can be nega�ve, posi�ve or zero.   Standard enthalpies of forma�on are par�cularly useful because they can be used to find the enthalpy change for any rxn that occurs under standard condi�ons. 2NO2(g) N2O4(g) 2NO2(g) N2(g) + 2O2(g) N2(g) + 2O2(g) N2O4(g) Enthalpy Changes for Chemical Rxns   ΔHoreac�on = ΔHodecomposi�on + ΔHoforma�on ΔHodecomposi�on 2NO2(g) N2(g) + 2O2(g) N2(g) + 2O2(g) N2O4(g) ΔHoforma�on ______________________________ 2NO2(g) N2O4(g) ΔHoreac�on Hess’s Law   Hess’s law states that if a rxn is the sum of two or more other rxns, then ΔH for the overall process must be the sum of the ΔH values of the cons�tuent rxns. 3 11/7/11 Problem   Suppose you wish to know the enthalpy change for the forma�on of liquid PCl3 from the elements. What informa�on do you need if the following rxn cannot be carried out directly? Calculate the enthalpy change for the forma�on of 1 mol of PCl3(l) from P4 and Cl2. 4 ...
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This note was uploaded on 11/17/2011 for the course CHEM 101 taught by Professor Scottnickolaisen during the Fall '11 term at California State University Los Angeles .

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