S11-HW6-CH6-18a

# S11-HW6-CH6-18a - HOMEWORK WEEK 6 CHAPTERS 6 18 6.17 The...

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HOMEWORK WEEK 6 CHAPTERS 6, 18 6.17 The thermodynamic standard state refers to the standard thermodynamic conditions chosen for substances when listing or comparing thermodynamic data: one atm pressure for gases and 1M concentration for solutions. Any specified temperature is acceptable: often listings use 25°C. 6.18 The reference form of an element is the most stable form (physical state and allotrope) of the element under standard thermodynamic conditions. The standard enthalpy of formation of an element in its reference form is zero. This follows from the definition in 6.19. 010. The reaction of C( g ) + 4H( g ) CH 4 ( g ) is not an appropriate equation for calculating the H ° f of methane because the most stable form of each element is not used. Both H 2 ( g ) and C(graphite) should be used instead of H( g ) and C( g ), respectively. 6.81 Write the H° values (kJ/mol) (Appendix C) underneath each compound in the balanced equation: Fe 2 O 3 (s) + 3CO(g) 2Fe(s) + 3CO 2 (g) -825.5 -110.5 0 -393.5 (kJ/mol) = Σ n H°(products) - Σ m H°(reactants) = [2(0) + 3(-393.5)] - [(-825.5) + 3(-110.5)] = -23.5 kJ 18.1 A spontaneous process is a chemical and/or a physical change that occurs by itself without the continuing intervention of an outside agency. Three examples are (1) a rock on a hilltop rolls down, (2) heat flows from a hot object to a cold one, and (3) iron rusts in moist air. Three examples of nonspontaneous processes are (1) a rock rolls uphill by itself, (2) heat flows from a cold object to a hot one, and (3) rust is converted to iron and oxygen. 18.8 The standard free-energy change, G ° , equals H ° - T S ° ; that is, it is the difference between the standard enthalpy change of a system and the product of temperature and the standard entropy change of a system. The word “standard” means that all chemicals in the reaction are at standard physical conditions, i.e. all gases at 1atm pressure and all solutions have a 1 Molar concentration. The standard free-energy change of formation is the free-energy change when one mole of a substance is formed from its elements in their stable states at one atm and at a stated temperature, usually 25 ° C. [Note that the “standard” designation does NOT by itself define the temperature – one can have standard conditions at any stated temperature.]

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18.13 As a spontaneous reaction proceeds, the free energy decreases (i.e. ΔG is negative for a small amount of reaction) until equilibrium is reached at a minimum G (i.e. G = 0 for a small amount of reaction). See the diagram below. [The Solutions Manual includes a long arrow from the top horizontal line to the minimum of the curve. This is INCORRECTLY labeled as ΔG. ΔG represents the SLOPE of the curve at each stage of the reaction.]
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