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| Ex 21.5 |
For the reaction 2NH 3  N 2 +3H 2 the equilibrium partial pressures at 298 K are 0.9 bar, 0.1 bar and 0.2 bar respectively for ammonia, nitrogen and hydrogen. Calculate Keq. How does Keq change with the total pressure on the system? |
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| Solution |
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| Keq = .1 x (0.2) 3 /(0.9)2
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| = 4.94 x 10-3 |
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Keq is dimensionless. In the above calculation, each partial pressure is divided by the pressure in the standard state, Po which equals 1 bar for all gases and hence only the dimensionless numbers are given in the equation. If the total pressure of the system is changed, the partial pressures readjust to give the same Keq as per the Le Chatelier's principle and Keq remains unchanged, as it is determined by  G0 , which is the difference in the standard state free energies at 1 bar. Keq depends on temperature. |
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