# thermo

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Chapter 15 (not much on E) Thermodynamics: Enthalpy, Entropy & Gibbs Free Energy Thermo 2 Thermodynamics: thermo = heat (energy) dynamics = movement, motion Some thermodynamic terms chemists use: System: the portion of the universe that we are considering open system: energy & matter can transfer closed system: energy transfers only isolated system: no transfers Surroundings: everything else besides the system Isothermal: a system that is kept at a constant temperature by adding or subtracting heat from the surroundings. Heat Capacity: the amount of heat energy required to raise the temperature of a certain amount of material by 1°C (or 1 K). Specific Heat Capacity: 1 g by 1°C Molar Heat Capacity: 1…show more content…
Thermo 9 Standard Enthalpy of Formation -- H  f The amount of heat absorbed (endothermic) or released (exothermic) in a reaction in which one mole of a substance is formed from its elements in their standard states, usually at 298 K (25°C). Also called heat of formation. H  = 0 for any element in its standard state (the f natural elemental form at 1 atm or 1 M) at 298 K. EXAMPLES: C(graphite, s) + O 2 (g) H° = 0 kJ/mol rxn CO2 (g) 0 kJ/mol elements in their standard states negative sign heat released -- exothermic rxn 393.5 kJ/mol product (one mole) H° (CO2 ) = f 393.5 kJ/mol Thermo 10 2H2 (g) + O 2 (g) H° = 0 kJ/mol rxn 0 kJ/mol 2H2 O (g) 483.6 kJ/ 2 mol elements in their standard states product (two moles) divide by 2 to put on per mole basis!! negative sign heat released -- exothermic rxn H° (H2 O) = f 241.8 kJ/mol Note that I usually will not have you calculate Hfº on homeworks or tests – so you generally don’t have to worry about normalizing your answer to a per mole basis. Hess 's Law -- Adding Reactions The overall heat of reaction (Hrxn) is equal to the sum of the Hf (products) minus the sum of the Hf (reactants): H° = rxn  (# eqiv) H° (products)  (# eqiv) H° (reactants) f f Therefore, by knowing Hf of the reactants and products, we can determine the Hrxn for any reaction that involves