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Unformatted text preview: 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: H < 0 (exothermic), but can be endothermic z Tendency to become more disordered or random z Entropy: 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 E q z q = H at constant pressure z So E H Review 2 2 7 Review Enthalpy Change ( H) H H 8 19.2 Entropy and the Second Law of Thermodynamics z Matter tends to change spontaneously to a state of lower energy and greater disorder z Use H to measure energy changes. z Consider the expansion of a gas: z This is at constant temperature , so no heat is transferred. Thus H = 0. There is no opposing pressure, so no work is done. Why does the gas expand spontaneously? 9 Why does the gas expand? z Which is more likely - a random distribution of molecules throughout the volume of the gas, or an ordered distribution with all the gas molecules on one side? z A disordered or random system is more probable. (More possible different arrangements) 10 Which has more microstates? Which is more likely? 11 Entropy z How to measure disorder? Use entropy (S). z The more disordered the system, the larger the value of the entropy. z Entropy is a state function: S = S final- S initial z S > 0 corresponds to an increase in randomness or freedom of motion. 12 19.3 The Molecular Interpretation of Entropy z The entropy of a substance is related not only to the number of possible arrangements of the atoms and molecules. z The greater the motion, the greater the number of possible arrangements....
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