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CHM 116 Ch 19-Thermodynamics

CHM 116 Ch 19-Thermodynamics - Chapter 19 Thermodynamics...

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1 1 1 Chapter 19 Thermodynamics z Study of energy changes in chemical reactions – useful to understand the nature of chemical changes z Invented to try to improve steam engines z Review Thermochemistry (study of heat changes): Chapter 5 2 Types of processes z Spontaneous - occurs without external intervention z Nonspontaneous - does not occur unless energy is added from an external source z At Equilibrium - not all reactions go to completion; reversible reactions (can move back and forth along the same path). 3 19.1 Spontaneous Processes z Spontaneous processes z hot object cools z gases expand z iron rusts z Cs reacts with H 2 O z What do all these processes have in common? z They happen with no external intervention. z A process that is spontaneous in one direction is not spontaneous in the reverse direction. 4 What makes a process spontaneous? z Tendency to go to a state of lower energy. z Enthalpy: Δ H < 0 (exothermic), but can be endothermic z Tendency to become more disordered or random z Entropy: Δ S > 0 (increase in randomness) Gummy bear 5 Review: Enthalpy and the First Law of Thermodynamics z 1st Law = Law of Conservation of Energy: energy is neither created nor destroyed in a process z Types of energy of interest: internal energy (E), heat (q), work (w) z Internal energy (E) = energy stored within a system 6 In open systems, w is zero or so small that it is negligible z So Δ E q z q = Δ H at constant pressure z So Δ E ≈ Δ H Review
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