HW 1 TA

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University of Maryland, College Park *

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132

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Chemistry

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Apr 3, 2024

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[SP24] PHYS 132 - 010x/020x HW 1 Problem 1: For the following problems, refer to the table below for needed data. Specific Heat (J/kg/°C) Thermal Conductivity (W/m 2 /°C) Copper 385 375 Water 4182 0.6 Part (a) A copper pot with a mass of 2 kg is sitting at room temperature (20°C). If 200 g of boiling water (100°C) are put in the pot, after a few minutes the water and the pot come to the same temperature. What temperature is this in °C? Numeric : A numeric value is expected and not an expression. T = __________________________________________ °C Part (b) In the transformation that occurred in part (a), how much thermal energy left the water in joules? Numeric : A numeric value is expected and not an expression. E = __________________________________________ J Part (c) In the transformation that occurred in part (a), how much thermal energy entered the copper of the pot in joules? Numeric : A numeric value is expected and not an expression. E = __________________________________________ J Part (d) If there were already 50 g of water in the pot (at room temperature) before the 200 g of hot water was added, what would the common temperature reached have been in °C? Numeric : A numeric value is expected and not an expression. T = __________________________________________ °C Part (e) In solving the above problems, you almost certainly made a number of simplifying but unrealistic assumptions that could affect the final value of the temperature. List three and explain which of the three you think would be most important to include in order to get a more realistic answer (and why you chose the one you did). EssayAdvanced : = __________________________________________ Problem 2: A small electric immersion heater is used to heat 100 g of water for a cup of herbal tea. The heater is labeled "200 Watts". Part (a) Assuming you are doing this problem for an exam, make a number of simplifying assumptions that will allow you to calculate reasonably promptly how much time it would take to heat the water from room temperature (23 C) to just the perfect temperature for making tea (95 C). Specify the assumptions you have made.

EssayAdvanced : = __________________________________________ Part (b) If you really care about the time it takes, which of your assumptions would you think is the most important to take into account? Don't do a new calculation, but explain why you think the assumption you chose is the most significant. EssayAdvanced : = __________________________________________ Problem 3: A gas of potassium chlorate molecules KClO 3 all decompose into potassium chloride, KCl, and diatomic oxygen, O 2 . The products and reactants are in a closed container and can all be treated as ideal gases. Part (a) Fill in the smallest possible integers that allows the stoichiometry of the reaction equation to be correct: ___ KClO 3 → ___ KCl + ___ O 2 What is coefficient for KClO 3 ? Numeric : A numeric value is expected and not an expression. Coefficient = __________________________________________ Part (b) What is coefficient for KCl? Numeric : A numeric value is expected and not an expression. Coefficient = __________________________________________ Part (c) What is coefficient for O 2 ? Numeric : A numeric value is expected and not an expression. Coefficient = __________________________________________ Part (d) If there are N molecules of potassium chlorate in the initial state, how many product molecules are there? Expression : Product molecules = __________________________________________ Select from the variables below to write your expression. Note that all variables may not be required. N Part (e) Select the option that correctly completes this statement: If the reaction takes place as a constant temperature and volume, the pressure ____. MultipleChoice : 1) does not change 2) There is not enough information given in the problem to decide 3) increases 4) decreases Part (f) Select the option that correctly completes this statement: If the reaction takes place at a constant temperature and pressure, the volume ____. MultipleChoice : 1) does not change 2) There is not enough information given in the problem to decide 3) increases 4) decreases Part (g) Select the option that correctly completes this statement: If the reaction takes place at a constant volume in a thermally insulated container

that allows no heat exchange with the outside world (or the container), the gas temperature ____. MultipleChoice : 1) decreases 2) does not change 3) There is not enough information given in the problem to decide 4) increases Part (h) Explain why you chose the answers you did in parts (e), (f), and (g). If you chose "not enough information" for any of the choices, indicate what you would need to know in order to answer. EssayAdvanced : = __________________________________________ Part (i) Select the option that correctly completes this statement: If the reaction takes place as a constant temperature and volume, the work done by the gas on the outside world is ____. MultipleChoice : 1) negative 2) There is not enough information given in the problem to decide 3) zero 4) positive Part (j) Select the option that correctly completes this statement: If the reaction takes place at a constant temperature and pressure, the work done by the gas on the outside world is ____. MultipleChoice : 1) zero 2) There is not enough information given in the problem to decide 3) positive 4) negative Part (k) Select the option that correctly completes this statement: If the reaction takes place at a constant volume in a thermally insulated container that allows no heat exchange with the outside world (or the container), the work done by the gas on the outside world is ____. MultipleChoice : 1) negative 2) There is not enough information given in the problem to decide 3) zero 4) positive Part (l) Explain why you chose the answers you did in parts (i), (j), and (k). If you chose "not enough information" for any of the choices, indicate what you would need to know in order to answer. EssayAdvanced : = __________________________________________ Problem 4: Although all energy is kinetic and potential* it is convenient to break it up into coherent macroscopic kinetic energy (KE), macroscopic potential energy (PE), thermal energy (ThE = kinetic and potential energy of molecules due to random motion), chemical energy (ChE = kinetic and potential energy of electrons in atoms in molecule). Consider a small rocket placed on a pad containing an electrical igniter. The rocket is attached to a small packet of chemical explosive in its tail. The igniter lights a short fuse that ignites the chemical explosive shooting the rocket upward. It rises straight up about 50 feet, then falls to the ground where it bounces and comes to a stop. Consider three times: t 0 = just after the explosion has completed but the rocket has not risen much t 1 = the rocket is just at the top t 2 = the rocket has fallen to the ground and come to a stop Identify what has happened to the various energies of the rocket (not including the explosive packet or fuse) from the beginning to the end of the time segments indicated. Choose "positive, negative, zero, or cannot be determined from the information given" in

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