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Chemical reaction in a cylinder - Enthalpy

Page history last edited by Joe Redish 7 years, 1 month ago

7.2.2.P3

 

Suppose we have a cylinder with a volume, V0, containing 1 mole of NO2 at pressure p0, 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 p0 >> atmospheric 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 NO2 in the cylinder undergoes the reaction

 

2 NO2 (g) → N2(g) + 2 O2 (g).


After the reaction, the gases 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.)

 

(a) 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 liters and contained 1 mole of NO2.

 

(b) Given the binding energies of the following bonds:

  • N≣N      -945 kJ/mole
  • N-O      -631 kJ/mole
  • ON-O   -305 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? 

 

(c) 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.

 

(d) Did the gas do work on the piston or vice versa? How much? Did that change the internal thermal energy of the gas?

 

(e) What is the change in enthalpy in the reaction?

 

 

 

Joe Redish 1/29/17

 

 

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