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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: user1000 on October 31, 2014, 05:01:04 PM

Title: How to approach this? (partial pressure at given temperature)
Post by: user1000 on October 31, 2014, 05:01:04 PM
2CO(g) + O2(g) --> 2CO2(g) at 1000Kelvin

I have found the equilibrium constant at 1000K, and now I need to calculate the partial pressure of O2 when mixing CO2 and CO in a ratio of 2:1. The total pressure is 1atm.
Hint: The eqilibrium constant is so large, we  use that CO2:CO is unchanged after the equilibrium have settled. 

How do I go about this?

Help would be appreciated
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: Borek on October 31, 2014, 05:13:13 PM
Write the formula for K. What are knowns? What are unknowns, and how many of them?
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: user1000 on October 31, 2014, 05:17:24 PM
Quote from: Borek on October 31, 2014, 05:13:13 PM
Write the formula for K. What are knowns? What are unknowns, and how many of them?

Kp=Kc/RT

Kp=pCO2^2/(pO2*pCO^2)

Is this correct?
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: Borek on October 31, 2014, 06:25:41 PM
So far, so good.
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: user1000 on October 31, 2014, 07:04:42 PM
Quote from: Borek on October 31, 2014, 06:25:41 PM
So far, so good.

pO2=4/Kp   ?
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: Borek on October 31, 2014, 07:34:39 PM
Looks OK to me.

Final step should be to calculate all partial pressures, plug them into Kp and see if they check out, and if the final pressure is as expected.
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: user1000 on November 02, 2014, 08:09:18 AM
Quote from: Borek on October 31, 2014, 07:34:39 PM
Looks OK to me.

Final step should be to calculate all partial pressures, plug them into Kp and see if they check out, and if the final pressure is as expected.

This is where I am stuck. Can someone help me with this?
Title: Re: How to approach this? (partial pressure at given temperature)
Post by: Enthalpy on November 02, 2014, 10:06:02 AM
If the equilibrium favours much one compound, you know in advance how much there is of this one, rather accurately. You can deduce the others and check the assumption.

If the assumption were slightly optimistic, you can subtract a bit from the favoured compound and loop the computation once or twice for improved accuracy.

If no compound is much favoured, you have to write the sums (reaction equation) and ratios (equilibrium constants) of all partial pressures and solve for good - by hand for CO2 vs CO, with a software if burning aluminium and polybutadiene in ammonium perchlorate.

For that, you don't want to compute the equilibrium between every pair of possible products, but rather refer the constants of the products to a limited set of compounds. The elements would have seemed a natural set, but they lead to impractical equilibrium constants that may overflow or underflow computer capabilities.