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Chemical reaction in a cylinder

Page history last edited by Joe Redish 3 years, 2 months ago



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, help in place by two pins that can be moved up and down in a channel if desired 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 equilibrium


(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.


(b) 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.


Suppose that the cylinder initially had a volume of 2 liters and contained 1 mole of NO2.


(c) 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 energy is absorbed (if the change in chemical energies is negative) or released (if the change in chemical energies is positive)?





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