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Ta practice exam 3 key

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Unformatted text preview: CH301 Fall 2010 Practice Exam 3—Answer Key 1. Which of the following statements is/are true? I. If a gas both expands and releases heat, then the internal energy of the gas must decrease. II. If a gas both expands and absorbs heat, then the internal energy of the gas must decrease. III. If a gas both contracts and absorbs heat, then the internal energy of the gas must increase. 1. I only 2. II only 3. III only 4. I and II only 5. II and III only 6. I and III only correct 7. I, II, and III 8. None of the above The key equation here is E q w which in this case can be thought of as E q pV Examine each statement one at a time. I) releases heat q is negative, expands w is negative. When taken together E must be negative. True. II) absorbs heat q is positive, expands w is negative. When taken together E is ambiguous (it matters how big q and w are). False. III) absorbs heat q is positive, contracts w is positive. When taken together E is positive. True. 2. Why can a bomb calorimeter be used to estimate the enthalpy of a combustion reaction, even though the reaction is conducted at constant volume? 1. The pressure change in the system can be measured 2. The temperature change in the system can be measured 3. The contribution from the change in the volume is very small compared to the change in the energy associated with the reaction. 4. You can’t use a bomb calorimeter to estimate the enthalpy of a combustion reaction Choice 3 is correct, for reasons discussed in class. The change in volume term is around 10 kJ maximum, while the enthalpy of a combustion reaction is thousands of kJ. 3. An ideal gas is compressed adiabatically (no heat is absorbed or released). What are the signs of E , q, and w? 1. 2. 3. 4. 5. 6. E + + 0 0 + ‐ q + ‐ + 0 0 0 w + ‐ ‐ 0 + ‐ correct If no heat is released or absorbed, it means that q = 0. Since E q w , it means that E w for this case. Also, w pV , which implies that w is positive for a compression. Thus, the correct answer is the 5th choice. 4. Which of...
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