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Unformatted text preview: choudhury (fac489) – Post-lab 2 Enthalpy – lyon – (51155) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Your numerical answer should have four sig- nificant figures. When fuels are burned in the presence of limited oxygen, carbon monoxide is formed. Given the heats of reaction for these two com- plete combustion reactions, CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(l) ΔH =- 890 kJ 2CO(g) + O 2 (g) → 2CO 2 (g) ΔH =- 566 kJ Calculate the heat of reaction for the incom- plete combustion of methane: 2CH 4 (g) + 3O 2 (g) → 2CO(g) + 4H 2 O(l) ΔH = ? kJ Correct answer:- 1214 kJ. Explanation: To get the final reaction, multiply the first equation (and its ΔH) by 2, then reverse the second equation (and change the sign on its ΔH) and add the ΔH values: 2CH 4 (g) + 4O 2 (g) → 2CO 2 (g) + 4H 2 O(l) ΔH =- 1780 kJ 2CO 2 (g) → 2CO(g) + O 2 (g) ΔH = 566 kJ Add those two equations to get the desired equation and ΔH =- 1214 kJ. 002 10.0 points The amount of heat required to raise the tem- perature of one gram of a substance one de- gree Celsius (also one Kelvin) is called 1. specific heat capacity. correct 2. Hess’s Law. 3. enthalpy of formation. 4. standard state of the substance. Explanation: 003 10.0 points Enthalpy is a state function because it is de- fined by the initial and final states of the sys- tem, not by the pathway or process by which the change of enthalpy was accomplished. 1. True correct 2. False Explanation: 004 10.0 points Why is it important to continuously stir the reaction mixture while taking temperature readings? 1. It is important to continuously stir the mixture because the temperature probes we will be using to measure temperature auto- matically shut themselves off if they are not kept in constant motion. 2. It is important to continuously stir the mixture in order to keep the reaction going. If the stirring slows or stops, the reaction itself will stop. 3. It is important to continuously stir the mixture in order to make sure the tempera- ture of the solution is the same throughout the entire volume. correct 4. It is important to continuously stir the mixture because the unknown metal in Part 2 will not dissolve without continuous stirring. 5. It is important to continuously stir the mixture because temperature is a measure of the kinetic energy of the particles and stirring provides kinetic energy to the mixture. 6. Stirring is not important because all the heat is trapped inside the cup whether you stir or not. Explanation: choudhury (fac489) – Post-lab 2 Enthalpy – lyon – (51155) 2 In Part 1 the hot water and cold water will mix almost immediately, but in the other three parts, the source of the heat is localized....
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- Fall '08