Ch 202 Problem 5.26

5.26 For the following processes, calculate the change in internal energy of the system and determine whether the process is endothermic or exothermic.

(a) A balloon is heated by adding 850 J of heat. It expands, doing 382 J of work on the atmosphere.

here, heat flowed into the system (the balloon), so we have q = +850 J. the system expanded against the surroundings, and in doing so did work, so w = -382 J. by the first law,

since q is (+), heat flowed into the system and the process is endothermic.

(b) A 50 g sample of water is cooled from 30oC to 15oC, thereby losing approximately 3140 J of heat.

since heat is flowing from the system (the water) to the surroundings, q is (-) and the process is exothermic. There really isn't any work done to speak of here, so DE = q = - 3140 J.

(c) A chemical reaction releases 6.47 kJ of heat and does no work on the surroundings

since q = (-), the heat flows from the system to the surroundings, and the process is exothermic. We have

DAC 7/8/08