1412 Lecture 6 Ch 18 - Entropy Free Energy and Equilibrium...

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Unformatted text preview: Entropy, Free Energy, and Equilibrium Chapter 18 Thermodynamics • Thermodynamics is the study of the relationship between heat and other forms of energy in a chemical or physical process. We have discussed the thermodynamic property of enthalpy, H . We noted that the change in enthalpy equals the heat of reaction at constant pressure . In this chapter we will define enthalpy more precisely, in terms of the energy of the system. Thermodynamics • Thermodynamics can be used to predict if a reaction will occur………. kinetics tell us how fast the reaction will occur. First Law of Thermodynamics • “energy can be converted from on form to another, but it cannot be created or destroyed” • One measure of these changes is the amount of heat given off or absorbed by a system during a constant pressure process, which chemists define as a change in enthalpy (∆H) Second Law of Thermodynamics • “The entropy (S) of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.” • Helps explain why chemical reactions favor a particular direction. Third Law of Thermodynamics • An extension of the Second Law stating: “the entropy of a perfect crystalline substance is zero at absolute 0 K” • This allows us to determine the absolute entropies of substances. Spontaneous Processes and Entropy • A spontaneous process is a physical or chemical change that occurs by itself. Examples include: A rock at the top of a hill rolls down. Heat flows from a hot object to a cold one. An iron object rusts in moist air. These processes occur without requiring an outside force and continue until equilibrium is reached . Spontaneous Physical and Chemical Processes • A waterfall runs downhill • A lump of sugar dissolves in a cup of coffee • At 1 atm, water freezes below 0 C and ice melts above 0 C • Heat flows from a hotter object to a colder object • A gas expands in an evacuated bulb • Iron exposed to oxygen and water forms rust spontaneous nonspontaneous spontaneous nonspontaneous Does a decrease in enthalpy mean a reaction proceeds spontaneously? CH 4 ( g ) + 2O 2 ( g ) CO 2 ( g ) + 2H 2 O ( l ) ∆ H = -890.4 kJ H + ( aq ) + OH- ( aq ) H 2 O ( l ) ∆ H = -56.2 kJ H 2 O ( s ) H 2 O ( l ) ∆ H = 6.01 kJ NH 4 NO 3 ( s ) NH 4 + ( aq ) + NO 3- ( aq ) ∆ H = 25 kJ H 2 O Spontaneous reactions Does a decrease in enthalpy mean a reaction proceeds spontaneously? • We cannot decide whether or not a chemical reaction will occur spontaneously solely on the basis of energy changes in the system. • To make predictions of spontaneity we need another thermodynamic quantity; ENTROPY Entropy and the Second Law of Thermodynamics • The second law of thermodynamics addresses questions about spontaneity in terms of a quantity called entropy ....
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1412 Lecture 6 Ch 18 - Entropy Free Energy and Equilibrium...

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