General Chemistry by Whitten, Atwood, Morrison Chapter 15 solutions

General Chemistry by Whitten, Atwood, Morrison Chapter 15 solutions

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150 15 Chemical Thermodynamics 15-1 (a) energy : the capacity of a system to do work or transfer heat (b) kinetic energy : energy that a body has because of its motion. (c) potential energy : energy that an object possesses because of its position or composition (e) joule : a unit used to measure amount of energy; defined as one kilogram·meter 2 per second 2 ; equal to the kinetic energy of a 2 kg object moving at one meter per second. 15-3 There are many appropriate examples; one of each is given here. (a) Heating water sufficiently causes it to boil; the steam can push on a piston to do some work. (b) Rubbing two sticks together can cause them to heat up enough to ignite. 15-5 (a) Heat is added to the system when solid ammonium nitrate dissolves in water. The beaker becomes cold for this endothermic process. (b) Work is done by the system when hydrochloric acid and sodium bicarbonate are combined. The products are carbon dioxide gas, water, and aqueous sodium chloride. There is a production of gas in the reaction. Work is done by the system on the surroundings. 15-7 The fuels that we consume (whether fossil fuels or fuels for nuclear reactors) represent concentrated forms of stored (potential) energy. As the fuels are consumed, some of the energy is dispersed as heat, which cannot be completely recovered and converted into useful work. The total amount of energy is not declining, only the amounts of substances from which stored energy can be efficiently released. 15-9 Enthalpy is equal to E + PV. The enthalpy change for a reaction at constant pressure is the heat absorbed or evolved. A negative enthalpy change is one in which the final heat content of the system is less than the beginning heat content and represents an exothermic reaction. In exothermic reactions, the beaker (surroundings) absorb the heat that the system gives off. The reaction of sodium and water is an exothermic reaction, which by definition, has a negative enthalpy change. 15-11 A state function is a variable that defines the state of a system. It is a function that is independent of the pathway by which the state is reached. Examples of state functions are pressure, temperature, volume, and energy. The difference between two state functions is also a state function. For example, if a process involves a pressure change from P 1 to P 2 , then Δ P = P 2 P 1 , no matter how that pressure change occurs; thus Δ P is a state function. Hess ’s Law would not be a law if enthalpy were not a state function because it depends upon the values of enthalpy change being independent of the pathway. 15-13 If Δ H > 0, the reaction is endothermic and absorbs heat from the surroundings.
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151 15-15 (a) heat given off by rxn = heat gained by calorimeter heat ( q ) = 2.48 kJ o C x 0.400 o C = 0.992 kJ for 0.0222 g C 8 H 18 This is given off, so it is assigned a negative quantity in the calculation of Δ H .
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