2009/07/05

From a problem set...

Problem Set #4, Question 2:

2. Carbon dioxide reacts with ammonia (NH3) in the gas phase to produce formamide (HCONH2) and oxygen. If 15.215g of CO2(g) and 4.139g of ammonia are combined in a 3.50L vessel and allowed to react equilibrium, the formamide concentration is found to be 24.8mM. What is the equilibrium constant for this reaction? Is this reaction product-favored or reactant-favored?

Balanced equation:
2 CO2(g) + 2 NH3(g) <--> 2 HCONH2(g) + O2(g)

[CO2]initial = 15.215g / 44.009g/mol / 3.50L = 0.098778M
[NH3]initial = 4.139g / 17.031g/mol / 3.50L = 0.069436M
[HCONH2]initial = 0M
[O2]initial = 0M
{NOTE: Once again, I'm carrying too many sig figs here on purpose because this is the middle of the calculation. I'll round later.}

Set these up in a table, I don't have tables active in the blog so....

Change in:
[CO2] = -2x [NH3] = -2x [HCONH2] = +2x [O2] = +x

So at equilibrium,
[CO2]eq = (0.098778-2x)M
[NH3]eq = (0.069436-2x)M
[HCONH2]eq = (0+2x)M
[O2]eq = (0+x)M

The problem states that the equilibrium concentration of formamide is 24.8mM, so we can use that number to find the value of "x".
[HCONH2]eq = (0+2x)M = 24.8mM
x = 12.4mM = 0.0124M

Plugging in to find all equilibrium concentrations:
[CO2]eq = (0.098778-2x)M = (0.098778-0.0248)M = 0.07398M
[NH3]eq = (0.069436-2x)M = (0.069436-0.0248)M = 0.04464M
[HCONH2]eq = (0+2x)M = 0.0248M
[O2]eq = (0+x)M = 0.0124M

Kc = { (0.0248)2(0.0124) } / { (0.07398)2(0.04464)2 } = 0.699
Slightly reactant-favored.

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