LabManuel3 - 1 CHE 303 version Hot Cocktails or Cold The...

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1 CHE 303; version 9/21/10 Hot Cocktails or Cold? The heat of mixing of ethanol and water Introduction Water is the most important molecule to life, and its unique properties affect virtually all of the processes that go on in organisms and the environment. One of the most basic chemical operations occuring throughout the planet is having things dissolve in water. Its powerful solvation properties, stemming from its unusual combination of partial charges and a very open "holey" condensed phase structure, create a driving force for much of the chemistry we see around us. There are various reasons why one substance dissolves in another. Entropy is a major driving force, since a mixture has a more random structure than pure components, but other forces such as neutralization of charge and changes in volume also play a role. In thermodynamic language, if the free energy goes down upon mixing, a solution will be formed. Since the free energy is a linear combination of entropy and enthalpy [ ∆ܩ ], if the entropy change is large and positive, a solution can form for either positive or negative enthalpies of solution. Rather surprisingly, NaCl, which we think of as one of the most soluble of substances, has a positive enthalpy of solution and thus cools a solution when it dissolves. [remember that an enthalpy is negative if heat is given off in a reaction]. ൌ ∆ܪ െ ܶ∆ܵ The sign and magnitude of H 0 soln depends upon the intricate details of the interactions between the solute and solvent molecules in both their pure and mixed states. If the interactions in the pure substances are stronger than those in the solution (the case for NaCl), the mixing can be driven by entropy, even though the reaction is endothermic. In water solutions, hydrogen bonds are important, so molecules such as acetone have a strong propensity to go into solution because strong hydrogen bonds are formed. Normal hydrocarbons, however, do not form hydrogen bonds and therefore do not dissolve ("oil and water don't mix"). Since alcohols are polar on one end and oil-like on the other end, they can go either way. One finds that long-chain alcohols are not soluble, and that methanol has a fairly negative H 0 soln . The question then arises--When you add water to your shot of rum, does it warm up or cool down? [the desirability a particular outcome might vary whether you are on a Caribbean beach or in a ski lodge] If the hydrocarbon part of ethanol fits neatly into a hole in the water structure, heat may be given off-- otherwise the solution might cool down. Our object here is to determine and plot the enthalpy change H 0 soln for the isothermal and isobaric process [ T , P constant] over the complete range of possible compositions:
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2 n A moles H 2 O(l) + n B moles ethanol(l) solution [mole fraction: x A = n A /(n A + n B ); 298.15 K, 1 atm]. This is accomplished by simply measuring the temperature change resulting from mixing
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This note was uploaded on 11/04/2011 for the course CHE 303 taught by Professor Staff during the Fall '08 term at SUNY Stony Brook.

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LabManuel3 - 1 CHE 303 version Hot Cocktails or Cold The...

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