Heat flow: hot-to-cold  stove-to-pot PAUSE CLICK QUESTION: A pot of water at room temperature is placed on a hot stove. If 5000 calories of heat is transferred, q for the pot of water is ______ and q for the stove is ______. A. +5000 calories, +5000 calories B. –5000 calories, -5000 calories C. +5000 calories, -5000 calories D. –5000 calories, +5000 calories By convention, q is assigned a positive value for a system if heat flows into the system (endothermic) q is assigned a negative value for a system if heat flows out of the system (exothermic) Heat flow: hot-to-cold  stove-to-pot A pot of water at room temperature is placed on a hot stove. If 5000 calories of heat is transferred, q for the pot of water is BLANK and q for the stove is BLANK. Which of these best fits the blanks. A. +5000 calories for both blanks B. –5000 calories for both blanks C. +5000 calories for the first blank and -5000 calories for the second, or D. –5000 calories for the first blank and +5000 calories for the second PAUSE CLICK By convention,… that means by agreement among scientists…. CLICK q is assigned a positive value for a system if heat flows into the system. HIGHLIGHT “positive” and “into” (LIGHT RED COLOR) As far as this system is concerned, the process is said to be endothermic. HIGHLIGHT “endothermic” (LIGHT RED COLOR) Conversely, q is assigned a negative value for a system if heat flows out of the system. HIGHLIGHT “negative”, “out of” (LIGHT BLUE COLOR) As far as this system is concerned, the process is exothermic. Let’s examine what happened here. The pot of water is at room temperature. and it is placed on a hot stove. Heat flows from hot to cold. Therefore, heat flows from the stove to the pot. Therefore, q for the pot is positive. Heat flowed into it. CROSS OUT –5000 calories in choices B and D And… q for the stove is negative. Heat flowed out of it… CROSS OUT +5000 calories in choice A The correct answer is choice C. CLICK PAUSE END RECORDING

Video ID: 8-2-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Assisted by Johuan Gasery In partial fulfillment of the requirements for CHEM 481, Fall 2008.