How is Le Chatelier's principle used to treat CO poisoning?

1 Answer
Sep 20, 2014


Breathing pure oxygen shifts the position of equilibrium to remove carbon monoxide from the red blood cells.


The equilibria involved

Dissolved oxygen in the blood binds to the hemoglobin in red blood cells in a reversible reaction.

#"Hb(aq)" + "4O"_2("aq") ⇌ "Hb"("O"_2)_4("aq")#

Carbon monoxide binds even more tightly to hemoglobin than oxygen does:

#"Hb(aq) + 4CO(aq)" ⇌ "Hb(CO)"_4(aq)#

If we reverse the first equation and add the second one, we get the equation

#"Hb"("O"_2)_4("aq") + "4CO(aq)" ⇌ "Hb(CO)"_4("aq") + "4O"_2("aq") #

The bonds between hemoglobin and carbon monoxide are about 300 times as strong as those with oxygen, so the position of equilibrium lies to the right.

The red colour of the predominant #"Hb(CO)"_4# causes the characteristic cherry red skin colour of carbon monoxide poisoning.

The body no longer has enough oxyhemoglobin to maintain life processes.

Applying Le Châtelier's Principle

Giving the patient pure oxygen can reverse the condition.

Le Châtelier's Principle states that if you apply a stress to a system at equilibrium, the equilibrium will shift in the direction that will remove the stress.

#"Hb"("O"_2)_4("aq") + "4CO(aq)" ⇌ "Hb(CO)"_4("aq") + 4"O"_2 ("aq") #

If you increase the concentration of #"O"_2#, the system will try to decrease the concentration of #"O"_2#.

The position of equilibrium will shift to the left and produce more #"Hb"("O"_2)_4#.

The patient may live.

The process is slow.

It takes about 74 min to remove half of the #"CO"# by breathing pure #"O"_2#, compared to 320 min with normal air.

The chances of survival improve greatly if the patient can be transported to a hyperbaric chamber and breathe pressurized #"O"_2#.

This pushes the position of equilibrium even further to the left.