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Topic: how to do equilibrium calculations  (Read 6994 times)

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Offline ethylenediamine

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how to do equilibrium calculations
« on: October 19, 2006, 03:23:12 AM »
Why does the amount of an inert gas affect the equilibrium conversion? I had a problem in which sulfur dioxide reacts with oxygen gas to form sulfur trioxide. However, air was involved to supply the oxygen. Hence, about 79% of the air was nitrogen.

I had to do an expression for the equilibrium constant:

SO2 + 0.5 O2 = SO3

K = ySO3*P/[(ySO2*P) (yO2*P)1/2]
K = ySO3/[(ySO2) (yO2*P)1/2]

However, in order to calculate the partial pressures and/or percent compositions (the y values), I had to account for the amount of amout of material coming out, I had to do a calculation for amount coming in and amount coming out with the conversion.

Got that...

However, I had to find percent composition. I could find the expression for the amount that comes out for each compound. However,  I had to divide that by the total amount of moles to get percent composition. The problem was that I also had to include the nitrogen gas (which does not take part in the reaction) in the total moles.

This seems incredibly against what I have always believed in what I learned in chemistry. In a constant volume, additon of an inert  (inert substance that takes no part in reaction) should have no affect on the equilibrium states. Can anyone answer this, or should I specify with the actual example I had to do???

Thanks in advance... I hope....

Offline Borek

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Re: how to do equilibrium calculations
« Reply #1 on: October 19, 2006, 03:46:40 AM »
Equlibrium expression doesn't contain percent composition, but the partial pressures. Addition of inert gas changes total pressure - but partial pressures remains intact.

No idea if it addresses your problem, but at least it points out things I don't get in your problem description :)
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Offline Donaldson Tan

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Re: how to do equilibrium calculations
« Reply #2 on: October 19, 2006, 07:46:15 AM »
Nitrogen has an effect on the temperature of the system, but this is a physical process.

Assuming sulphur dioxide is added to the reactor at stoichiometric quantity, then Nitrogen is the major component in the system. Its effect on the chemical equilibrium is a physical process, not a chemical one. Nitrogen absorbs the net amount of heat released by the chemical reaction. The temperature of the system depends on the heat capacity of Nitrogen, thus the amount of Nitrogen acts as a control for the temperature of the system. By increasing the nitrogen content of the system, the temperature of the system is less likely to increase, but to stay at steady state or decrease. The ratio of products to reactants will thus change as the change in temperature brings about a change in the value of the equilibrium constant.
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