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Posted by on Apr 18, 2011 in Physical Chemistry | 1 comment

Panting Hot Chickens, Carbon Dioxide and Carbonic Acid

What do hot chickens, pH of blood, bubbly drinks, coral reefs, and thermostating the Earth have in common?  They all make use of one of the most basic processes in Physical Chemistry: the equilibrium between carbon dioxide and carbonic acid,

(1)

Gaseous carbon dioxide readily dissolves in water. In fact at one atmosphere pressure and 25 C about 0.7 liters of carbon dioxide can be dissolved in one liter of water. So all you have to do to make carbonic acid is to bubble carbon dioxide through water. The first thing to notice is that the equilibrium constant is neither too small nor too big at Ka = 0.0017.   This means that the concentration of carbonic acid (at 25 C) is proportional to the concentration of carbon dioxide dissolved in the water,

[H2CO3]=0.0017 x [CO2]

(2)

It is possible to dissolve a lot more than 0.7 liters of CO2 in water by increasing gas pressure above the water.  As the pressure increases, more and more CO2 is dissolved and from Eq.(2), the concentration of carbonic acid increases in tandem.

The law that determines the relationship between concentration of a gas dissolved in a liquid, in this case [CO2], to the pressure above the liquid, P, is called Henry’s Law, P=kCO2[CO2]  where kCO2 is called the Henry Law constant for CO2 .

So it is easy to change the concentration of CO2 in water: just increase or decrease the pressure of CO2 above the liquid.  For example, CO2 in soft drinks is held there by the cap.  Take off the cap and the pressure drops to one atmosphere and the drink bubbles as CO2 is released.

At one atmosphere pressure, about half the CO2 produced by both natural and human activity is dissolved in the oceans because of Henry’s law.  The oceans gobble up a lot of CO2 and in the process they become acidic. This can upset Nature: one consequence is it can hinder the life cycles of krill and the myriad of coral species that depend upon calcium carbonate, (CaCO3) for their shells. Chickens have shells made of calcium carbonate too, but as we will see this depends upon the concentration of CO2 in their blood. So let us look at its chemistry a bit more.

Once carbonic acid has been formed in water, then like all acids it dissociates.

(3)

In this process, the acid equilibrium constant for removing the first proton from carbonic acid is small, ka1 = 4.5 x 10-7, and to remove the second proton,

(4)

the second equilibrium constant is even smaller ka2 = 4.7 x 10-11,.  Since bicarbonate, HCO3- donates a proton to water, it is acting as an Arrhenius Acid.

This means that carbonic acid is a weak acid, but none-the-less, the more carbon dioxide that is added to water, the greater the acidity. If however, the carbon dioxide concentration decreases (take the cap off the soda for example) the bicarbonate associates back to carbonic acid.  That is bicarbonate acts like an Arrhenius base (accepts a proton from water to form hydroxide ions, OH-) ,

(5)

Comparing Eqs.(1) and (5) shows that bicarbonate, HCO3-, acts as both an acid and base. This property is called amphiprotism. Most polyprotic acids e.g., sulfuric acid, H2SO4, are also amphiprotic, having at least one proton that can be donated (acts like an Arrhenius acid) and at least one site at which a proton can be accepted (acts like an Arrhenius base).

Now we can see what is happening by using Le Châtalier’s principle for each chemical equation from Eqs.(1) to (4).  As more carbon dioxide is added to water, the concentration of carbonic acid increases.  This shifts the dissociation of carbonic acid to the right in Eqs.(3) and (4) so the acidity and the concentration of carbonate, CO3-2 increases.  If carbon dioxide is removed from water, the concentration of CO3-2 decreases and the water becomes neutral.  On the other hand if carbonate, CO3-2 is consumed, like when organisms make shells of calcium carbonate, CaCO3(s), then more carbon dioxide is needed to feed this process.

Acid rain, Coral Reefs and Panting Chickens.

It should now be clear that the concentration of carbon dioxide in the atmosphere can change the acidity of our environment.  Besides carbon dioxide, another source of acid is industrial pollutions caused by sulfur dioxide, SO2 and hydrogen sulfide H2S.  Sulfur dioxide, SO2 is similar to CO2 (compare with Eqs.(1) and (3)),

(6)

Sulfurous acid H2SO3 is also weak, and should not be confused with sulfuric acid which is a strong acid.

The IUPAC name for HCO3- is monohydrogen carbonate and, HSO3- it is monhydrogen sulfate, but the common names will be used, bicarbonate and bisulfate.

These gases readily dissolve in rain which leads to acid rain and the erosion of buildings. Calcium carbonate is a common building material and is soluble in acid, just like the shells of crustaceans,

(7)

Not only are buildings damaged, but acid rain also dissolves the salts in rocks, washing them down the rivers and into the oceans. As a result the oceans are salty. Evaporating one cubic meter of sea water leaves about 35 kg of salt (2.2 pounds per cubic foot). That is a lot.  However the oceans do not get saltier over time, so the salts washed down from the land must disappear from the ocean somehow.  I will come back to this later.

In tropical climates, sea salt is obtained by allowing shallow basins to flood at high tide.  As the tide recedes, the water in the basins evaporates in the hot sun, and before the next high tide workers shovel off the sea salt.

Collecting Sea Salt

Our air is made up mostly of oxygen, (20%) and nitrogen (80%), but there are trace amounts of other gases, like helium and carbon dioxide which is only 0.039%.  Now helium (Molecular Weight (MW) of 4 is light and generally floats up and escapes into space, like a helium balloon.  Nitrogen has a MW = 28 and oxygen is similar at 32, so these gases tend to mix together.  Carbon dioxide has a MW of 44, and is heavier, and so tends to sink to the surface (where the oceans absorb it), but even so, these gases all mix by the effects of climate. The carbon dioxide not absorbed by the oceans remain in the atmosphere.

Acid rain explains why buildings and rocks erode, and sea shells get dissolved, but what about panting chickens?  We keep cool by perspiration that evaporates from our skin, but chickens do not perspire and, like most dogs, pant to keep cool.  As they pant, they eliminate carbon dioxide and as a direct consequence of Eqs.(1) to (4), the concentration of carbonate dissolved in their blood decreases. For this reason, tropical chickens have egg shells that are thinner and more fragile than those in more temperate climates.

When an egg is immersed in vinegar, after a few days the calcium carbonate shell dissolves, leaving the thin protein membrane sack surrounding the white and yolk.

If the tropical chickens could drink carbonated water to keep cool, then the carbon dioxide would dissolve in the digestion fluids and carry carbonate to the blood stream.

Would you expect the the egg shells of penguins to be thick or thin?

Hot Chickens

Buffering Blood

You might think that adding more and more acid increases the acidity of a solution, and this is generally true.  However sea water, blood and many fluids are buffered, so adding more acid does not significantly change the pH unless large amounts of acid swamp the system.  A buffer is very simple.  All you need is a weak acid to be present in the solution.  Let us write a weak acid as HA,

(8)

(a strong acid is simply

(9)

Because the acid equilibrium constant of a weak acid is small, the equilibrium lies to the left of Eq.(8), whereas a strong acid is 100% dissociated.  Then if acid is added, the H+, associate with the base, A- to form HA (Le Châtalier’s principle again). Hence the pH does not change much and the solution is buffered.  It is important that the pH of blood is maintained constant otherwise blood acidosis can occur.  The normal pH of blood is slightly basic between 7.35 to 7.45.

The ability of the body to maintain stable equilibrium between parts is called homeostasis. Buffers in the blood provide an effective way of maintaining the pH constant. The end products of many metabolic processes are, however, acidic. After all, metabolism is oxidation and this produces carbon dioxide. The kidneys remove H+ ions that can build up, but if the kidneys fail, then acidosis is one result. Sometimes, however, the pH of the blood changes too rapidly for the kidneys to keep up even in healthy people.  The panting-chicken mechanism works faster.  As we exercise for example, acid builds up in our muscles.  But at the same time we are breathing rapidly and this eliminates carbon dioxide from the blood, lowers the concentration of carbonic acid, and quickly counteracts acidosis.

Bases can of course be used to neutralize excess stomach acid, also called heart burn. Calcium carbonate (“Tums”), magnesium hydroxide (“Milk of Magnesia”) aluminum hydroxide (“Amphojel”) and sodium bicarbonate (“baking soda”) are common remedies. When baking soda neutralizes acid, carbon dioxide gas is formed. We have seen this mechanism before when we discussed the formation of carbonic acid,

(10)

This equation makes it clear that the acid in the stomach, H3O+, reacts with bicarbonate and form carbon dioxide, which can make us burp.

Panting Planet Earth

The Earth “pants” through volcanoes that burp out a lot of carbon dioxide. We have all heard that carbon dioxide is a greenhouse gas. The structure of carbon dioxide is O=C=O.  It is a linear molecule and its two bonds vibrate with a frequency that lies in the infrared (heat).  So energy from the sun is captured in the O=C bonds. This raises the temperature of Earth.  However, as discussed above, the carbon dioxide dissolves in water to form acid rain. This, along with the increased temperature, increases erosion, thereby washing the salts into the salty sea.

Erosion, therefore, removes carbon dioxide from the atmosphere and so the average climate temperature decreases; acid rain decreases, erosion is reduced and the Earth cools.

The sea should therefore get saltier, but it does not.  Throughout our oceans, there are deep sea volcanic vents into which sea water pours. There it evaporates, leaving sea salts behind, many of them being carbonates.  Once in the hot magnum of the Earth, the carbonate is converted back to carbon dioxide, which escapes from volcanoes back into the atmosphere and the cycle starts again.

“Panting” volcanoes therefore thermostat the Earth, i.e. they maintains global temperatures within a given range which happily is conducive to life. When it is hot, acid rain causes more erosion and more salts are washed into the sea. This decreases the carbon dioxide in the atmosphere and so cools the Earth.  When it is cold, the opposite occurs.  Erosion decreases and carbon dioxide accumulates in the atmosphere and heats up the Earth.

So in the end, carbon dioxide is not the villain, but human activity is.  We have interfered with this thermostat by the enormous combustion of fossil fuels that started with the industrial revolution. Industrial production of carbon dioxide and other greenhouse gases can only lead to global warming; upsets the balance; and threatens the biodiversity of our planet.

A great book with this type of information is by Bill Bryson, “A Short History of Nearly Everything

You can find similar topics and their explanations, along with  interactive multimedia animations, in the latest edition of Physical Chemistry e-book by Laidler, Meiser, Sanctuary

~ Bryan

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1 Comment

  1. Yes, exactly so. CO2 raises the temperature of the Earth, produces acid rain and increases erosion. But in contrast to chickens, panting by Earth through volcanoes raises the temperature of the Earth and increases erosion. Rain becomes acidic, and removes CO2 from the atmosphere (as does dissolution in the oceans), and the Earth cools. Hence the cycle helps to thermostat the planet.

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