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5 Thermochemistry Solutions to Exercises experiment neglects other pathways by which the kinetic energy is transferred. It would be difficult to make enough different careful measurements to completely confirm our answer to (c). 5.17 Analyze. Given: heat capacity of water = 1 Btu/lb - °F Find: Btu Plan. heat capacity of water = 4.184 C; g J °C J Btu J This strategy requires changing °F to °C. Since this involves the magnitude of a degree on each scale, rather than a specific temperature, the 32 in the temperature relationship is not needed. 100 °C = 180 °F; Solve. g °C 453.6 lb 5 1 Btu = 1054 J/Btu 5.18 (a) Analyze. Given: 1 kwh; 1 watt = 1 S = 1J. Find: conversion factor for joules and kwh. Plan. kwh wh WS J Solve. 1 kwhx 1000 1 kw W 60 min h min 60 1 1J = 3.6 10⁶ J 1 kwh = 3.6 10⁶ J (b) Analyze. Given: 100 watt bulb. Find: heat in kcal radiated by bulb or person in 24 hr. Plan. 1 watt = 1 kcal = 4.184 10³ J; watt J/s J kcal. Solve. 100 watt = 100 60 min sec 60 hr min 24 hr 1 kcal = 2065 = 10³ kcal 24 hr has 2 sig figs, but 100 watt is ambiguous. The answer to 1 sig fig would be 2 10³ kcal. 5.19 (a) In thermodynamics, the system is the well-defined part of the universe whose energy changes are being studied. (b) A closed system can exchange energy but not mass with its surroundings. (c) Any part of the universe not part of the system is called the surroundings. 5.20 (a) The system is open, because it exchanges both mass and energy with the surroundings. Mass exchange occurs when solution flows into and out of the apparatus. The apparatus is not insulated, so energy exchange also occurs. Closed systems exchange energy but not mass, while isolated systems exchange neither. (b) If the system is defined as shown, it can be closed by blocking the flow in and out, but leaving the flask full of solution. 5.21 (a) Work is a force applied over a distance. (b) The amount of work done is the magnitude of the force times the distance over which it is applied. w = Fxd. 111