In the accompanying cylinder diagram a chemical process occurs at constant temperature and pressure. (a) Is the sign of w indicated by this change positive or negative? (b) If the process is endothermic, does the internal energy of the system within the cylinder increase or decrease during the change and is ∆E positive or negative?

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Irving HeathcoteLv2

26 May 2020

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Consider the accompanying energy diagram. (a) Does this diagram represent an increase or decrease in the internal energy of the system? (b) What sign is given to ∆E for this process? (c) If there is no work associated with the process, is it exothermic or endothermic?

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Suppose we have a cylinder with a volume, V_0, containing 1 mole of NO_2 at pressure p_0. and sitting on a heat sink that can maintain the temperature of the gas in the cylinder at a temperature T, whatever happens inside the cylinder. (Assume that p_0 >> atmosphere pressure so that we can ignore the air on top.) On top of the cylinder there is a piston, held in place by two pins. If desired, the pins can be moved up and down in a channel to change the volume in the piston. Now suppose that for some reason, all the NO_2 in the cylinder undergoes the reaction 2 NO_2 (g) rightarrow N_2 (g) + 2 O_2 (g). After the reaction, the gates are allowed to come to thermal equilibrium. (We assume the chemical reaction stays where it is. We are not looking at a real reaction seeking chemical as well as thermal equilibrium. This is just a model to see what would happen to a gas if chemical energy were made available to its thermal degrees of freedom.) Given that the temperature and volume is the same as it was before, what has happened to the pressure in the cylinder? Explain your thinking. Suppose that the cylinder initially had a volume of 2 Biers and contained 1 mole of NO_2. Given the binding energies of the following bonds: N II N -945 kJ/mole N-O -631 kJ/mole ON-O -304 kJ/mole O=O -498 kJ/mole is the reaction exothermic or endothermic? How much chemical energy is absorbed from the thermal energy (if the change in chemical energies is negative) or released into the thermal energy (if the change in chemical energies is positive) as a result of the chemical reaction alone? How much does that change internal energy of the gas in the cylinder? Now let the piston rise slowly by ratcheting the pins upward until the pressure inside the cylinder is the same as it was before the reaction took place. What's the new volume? Explain. Old the gas does work on the piston or vice versa? How much? Did that change the internal thermal energy of the gas? What is the change in enthalpy in the reaction?

Imagine a container placed in a tub of water, as depicted in the accompanying diagram. (a) If the contents of the container are the system and heat is able to flow through the container walls, what qualitative changes will occur in the temperatures of the system and in its surroundings? What is the sign of q associated with each change? From the system’s perspective, is the process exothermic or endothermic? (b) If neither the volume nor the pressure of the system changes during the process, how is the change in internal energy related to the change in enthalpy?