Chapter 7 - Thermochemistry

Chapter 7 - Thermochemistry - THERMOCHEMISTRY → 1st Law...

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25 THERMOCHEMISTRY 1 st Law of Thermodynamics Enthalpies of Reaction - Hess' Law , etc. Bond Energies Why Thermochemistry? We've discussed some physical processes - eg. dissolution of ionic solids (salts) in water - & some chemical reactions - precipitation, acid-base & redox . .. in terms of changes of state and changes in molecular formula - making/breaking of bonds, transfer of electrons, etc. Energy flow also accompanies these processes . THERMODYNAMICS provides the framework to understand energy flow & its role in determining whether a process actually occurs or not. eg. 2 Na(s) + 2 H 2 O(l) 2 NaOH(aq) + H 2 (g) occurs spontaneously , liberating a good deal of heat energy However, the reverse reaction never happens.
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26 More egs. MgSO 4 (s) & NH 4 NO 3 (s) dissolve spontaneously in water. When MgSO 4 (s) dissolves in water, heat is liberated. used in hot packs When NH 4 NO 3 (s) dissolves in water, heat is consumed. used in cold packs Thermochemistry deals with the accounting of heat transfer . We defer the question of what makes a process spontaneous till later when we take up entropy & Gibbs free energy . Thermodynamics hinges on being very precise in our description of a " system " and its processes eg. Types of Systems: isolated system closed system open system system is sealed off outside world - no heat or matter flow across boundary - rigid walls no mechanical work done on or by system system has impermeable walls, but is not thermally insulated - heat flow , but - no matter flow - walls can be rigid or non-rigid system has permeable, non-thermally- insulating walls - heat and matter flow - walls can be rigid or non-rigid
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27 Properties of Systems 1. Discrete labels: - chemical species identity - phase identity eg. CH 4 (g) Zn(s) Br 2 (l) 2. Continuous variables: p, T, n, V pressure, temperature, # of moles, volume U, H, S, G energy, enthalpy, entropy, Gibbs free energy State Functions - they depend on the state of the system eg. Processes cause changes in system state & corresponding changes in system state functions . Change in a state function depends only on the initial & final state - not on how the change was achieved p = p f p i = 2 atm T = T f T i = 150 K n = n f n i = 0 mol V = V f V i = 5 m 3 p i = 1 atm T i = 300 K n i = 1 mole V i = 10 m 3 U i , H i , S i , G i p f = 3 atm T f = 450 K n f = 1 mole V f = 5 m 3 U f , H f , S f , G f some process initial system state final state
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28 Work - mechanical or electrical We will focus on mechanical work Apply a force to a spring to compress it work is done on the spring work = force × distance The spring stores this energy. Work is done on the system , which increases the energy of the system . Compressing a gas in a cylinder is similar: compress spring distance through which spring is compressed V i V f apply force on gas in cylinder via piston Compressed gas has higher energy
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29 Temperature & Heat Hot vs. Cold Temperature measures the average kinetic energy per atom Heat flows from system 2 (on the right) to system 1 (on the left).
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Chapter 7 - Thermochemistry - THERMOCHEMISTRY → 1st Law...

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