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Unformatted text preview: 1 1 Chapter 16 Chapter 16 Thermodynamics Thermodynamics • Study of energy changes in chemical reactions ¡ useful to understand the nature of chemical changes • Recall thermochemistry (study of heat changes) from Chapter 8 • You are responsible for information from CHM 151, specifically sections 2, 5, 7, 9, and 10 (except “work”). Thermochemistry Review • 1st Law of Thermodynamics: Energy is conserved, it cannot be created or destroyed ¡ If the system gives off heat, the universe must absorb it. ¡ Heat flow (enthalpy, Δ H) always defined with reference to the system • System absorbs heat, Δ H > 0, endothermic • System gives off heat, Δ H < 0 , exothermic Calculating Δ H o rxn • Δ H o rxn = Σ Δ H o f products - Σ Δ H o f reactants ¡ Heat of formation values, Appendix B ¡ Δ H o f is reported as kJ/mol The o symbol refers to the standard state 1 00 • The symbol refers to the standard state, 1.00 atm pressure, 25.0 o C, 1.00 M for solutions • Review heat of formation reactions (making 1 mole of substance from its elements in the standard state) ¡ Δ H o f for elements is 0 (no energy needed!) • Write heat of formation rxn for MgCl 2 (s), H 2 O(l). Thermodynamics • How do we know if a chemical reaction will occur? • How can we tell how far a reaction will go? • Thermodynamics helps to answer these questions. • Enthalpy is only one part of the equation. • Entropy is another. Spontaneous Processes • In chemistry, we define a spontaneous process as one that proceeds on its own without any external influence. C id th i f i t ¡ Consider the expansion of a gas into a vacuum. This happens spontaneously. The reverse process does not! • Figure 16.1 2 2 Spontaneous Chemical Reactions Spontaneous Chemical Reactions • Spontaneous processes ¡ hot object cools ¡ gases expand ¡ iron rusts ¡ Cs reacts with H 2 O • What do all these processes have in common? • They happen with no external intervention. ¡ Nature tends toward lower energy! Usually exothermic reactions are spontaneous (but not always)!! Spontaneous Processes • Spontaneous does not mean the same thing as “fast” ¡ Rusting of iron is slow but spontaneous • Thermodynamics just tells us whether or not a reaction will occur, not how fast it will occur ¡ Kinetics (Ch. 12) looks at that! • Problem 16.1 Entropy (S) • Section 8.13: Nature tends toward disorder (increase in entropy). ¡ Who wants to play 52 card pick-up? • Entropy is a measure of disorder or randomness. ¡ A process is usually spontaneous if it increases the disorder (change in entropy is positive: Δ S > 0) ¡ S: J/K Which has the greater entropy? Which has the greater entropy?...
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