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CH 301 hw11 solution

CH 301 hw11 solution - cha(dyc97 Homework 11 Sutclie(53770...

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cha (dyc97) – Homework 11 – Sutcliffe – (53770) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The combustion of a certain amount of methane gives off 100 kJ of energy. Dur- ing this process the amount of energy in the universe 1. The answer cannot be determined with- out knowing the heat capacity of methane. 2. increases by 100 kJ. 3. decreases by 100 kJ. 4. does not increase or decrease. correct Explanation: The First Law of Thermodynamics states that the total amount of energy in the uni- verse is constant. The energy change occurs in the system. 002 10.0 points How much total work is done when 6 . 3 of H 2 expands from 25 liters to 50 liters against a constant external pressure of 1 atm, then ex- pands further from 50 to 100 liters against a constant external pressure of 0.1 atm? An- swer in calories. Correct answer: - 726. Explanation: w = - P Δ V w is not a state function. The pathway must be taken into consideration. Calculate w for each step. Step 1 : P = 1 atm V i = 25 L V f = 50 L Δ V = 25 L w = ( - 1 atm)(25 L) parenleftbigg 101 . 33 J atm · L parenrightbigg × parenleftbigg 1 cal 4 . 1867 J parenrightbigg = - 605 . 0708 cal Step 2 : P = 0 . 1 atm V i = 50 L V f = 100 L Δ V = 50 L w = - (0 . 1 atm)(50 L) parenleftbigg 101 . 33 J atm · L parenrightbigg × parenleftbigg 1 cal 4 . 1867 J parenrightbigg = - 121 . 0142 cal summationdisplay w = - 605 . 0708 cal - 121 . 0142 cal = - 726 . 085 cal 003 10.0 points An ideal gas is allowed to expand isothermally from 2.00 liters at 5.00 atm to 5.00 liters at 2.00 atm against a vacuum ( i.e. , no outside pressure). Which of the quantities q , w , Δ E , and Δ H are zero for this process? 1. w , q , and Δ E 2. w 3. w and Δ E , 4. Some other combination 5. w , q , Δ E , and Δ H correct Explanation: P = 0 so w = - P Δ V = 0 . For isothermal expansion, T = const. There is no change in molecular kinetic en- ergy or potential energy due to intermolecu- lar attraction (which is zero for ideal gases). Therefore Δ E = 0. Δ E = q + w 0 = q + 0; q = 0 Δ H = q = 0 Therefore w , q , Δ E , and Δ H all equal zero. 004 10.0 points

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cha (dyc97) – Homework 11 – Sutcliffe – (53770) 2 Which of the following is true of a general thermodynamic state function? 1. The change in the value of a state func- tion is always negative for a spontaneous re- action. 2. The change of the value of a state func- tion is independent of the path of a process. correct 3. The value of the state function remains constant. 4. The change in the value of the state func- tion is always positive for endothermic pro- cesses. 5. The value of a state function does NOT change with a change in temperature of a process. Explanation: A change in a state function describes a difference between the two states. It is inde- pendent of the process or pathway by which the change occurs. 005 10.0 points For a certain reaction at constant pressure, Δ U = - 70 kJ, and 51 kJ of expansion work is done by the system. What is Δ H for this process?
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CH 301 hw11 solution - cha(dyc97 Homework 11 Sutclie(53770...

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