18 - Thermodynamics - Directionality of Chemical Reactions

18 - Thermodynamics - Directionality of Chemical Reactions...

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Chapter 18: Thermodynamics: Directionality of Chemical Reactions 18.1Reactant-Favored and Product-Favored Processes Product-favored process is one in which products predominate over reactants; most or all of reactants will eventually be converted to products without continuous outside intervention Reactant-favored process is one in which reactants predominate over products at equilibrium; requires continuous outside intervention to product large quantities of products Reactant-favored process is exact opposite of a product-favored process Product- / reactant-favored indicates a direction in which a chemical reaction will take place 18.2Chemical Reactions and Dispersal of Energy Fundamental rule that governs whether a process is product-favored is that energy will spread out (disperse) unless it is hindered from doing so Chemical system is usually defined as collection of atoms that make up reactants (and also products but bonded in different ways) Most exothermic reactions are product-favored at room temperature because chemical potential energy stored in bonds between relatively few atoms can be dispersed over many more atoms as surroundings are heated Probability and Dispersal of Energy o Dispersal of energy occurs because probability is much higher that energy will be spread over many particles than that it will be concentrated in a few o If energy can be dispersed over a very much larger number of particles, it will be Dispersal of Energy Accompanies Dispersal of Matter o Energy becomes more dispersed when a system consisting of atoms or molecules expands to occupy a larger volume; gas expands until it fills a container o Dispersal of energy refers to spreading of energy over a greater number of different energy levels of atoms and molecules at given temperature, more different energy levels are accessible in larger volume than smaller volume – why? o When matter spreads out, number of ways of arranging energy associated with matter increases; energy is more dispersed so spreading out of matter is product-favored o Compressing of a gas is reactant-favored because work must be put into compressing gas and energy stored in gas could be used later o Any physical or chemical process in which energy is dispersed over many energy levels in final state is product favored because the final distribution of energy will be more probable Exothermic reaction disperses potential energy of chemical bonds to thermal energy of a much larger number of atoms or molecules Process where matter spreads out, which disperses energy as well as matter o A physical or chemical process is reactant-favored if neither energy nor matter will be reactant- favored o If one of these situations apply but not the other, quantitative information is needed to decide which effect is greater 18.3Measuring Dispersal of Energy: Entropy The nanoscale dispersal of energy in a sample of matter is called entropy (S) Entropy changes can be measured with calorimeter; at constant temperature and pressure, ΔS = S
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18 - Thermodynamics - Directionality of Chemical Reactions...

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