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Ch.6 - Thermochemistry
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All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 41

Chapter 6, Problem 41

A system releases 622 kJ of heat and does 105 kJ of work on the surroundings. What is the change in internal energy of the system?

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Welcome back, everyone to another video. A system releases 896 kilojoules of heat and thus 254 kilojoules of work on the surroundings. What is the change in internal energy of the system? We are given for answer choices. A 1874 kilojoules B negative 1150 kilojoules C 954 kilojoules and D negative 2241 kilojoules. So let's solve this problem using the first law of thermodynamics delta E equals Q plus W which tells us that the change in internal energy is equal to the amount of heat exchanged between the system and its surroundings. Plus the work done on or by the system. The most important thing for us is to determine the signs of Q and W. So first of all, if the heat is released, not only we take 896 kilojoules, but we also add the negative sign because the heat is released. If the heat is released by the system, it's negative. If it's consumed, it's positive. Now for work, notice how the problem states that the work is done on the surroundings. So it takes 254 kilojoules. And we also add a negative sign because the work is done on the surroundings. Whenever work is done on the system, it's positive. Whenever work is done on the surroundings, it's negative. So it would be more accurate to say that the work is done on the surroundings, right? So let's take this into account, this is why we're taking a negative sign. And therefore delta E is simply negative 896 kilojoules plus negative 254 kilojoules. Now that we have our signs, we can calculate the actual correct result. In this case, that would be negative 1150 kilojoules and this would correspond to the change in internal energy. So the correct answer to this problem would be option B negative 1150 kilojoules. Thank you for watching.
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Identify each energy exchange as primarily heat or work and determine whether the sign of ΔE is positive or negative for the system. a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.)

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