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Name: rita w
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: Around 1999


Question:
I'm confused! I'm teaching balancing equations and chemical reactions to my students in elementary school. I used some baking soda. Sodium bicarbonate is NaHCO3(can't do subscripts on this) but bicarbonate says two carbonate or 2CO3(subscript) How can I explain this to my students after I just taught them bi means two??? The box says sodium bicarbonate is baking soda. My text says baking soda is sodium hydrogen carbonate What's the explanation"?


Replies:
Dear Rita,

There are several naming systems used in chemistry. One of them is the system used to name NaHCo3 "sodium hydrogen carbonate." This is the modern method, and is called the Stock system, after Alfred Stock, a GErman chemist who invented it. This name arises from the universal name of "carbonate" given to CO3 2-.

Now, the name "sodium bicarbonate" is also accepted; it is an older, historically used name. However, the prefix "bi" does not refer to "two" of anything, as your students rightly observed. Unfortunately I do not know the origin of the name bicarbonate. I do know that the prefix "bi" is not commonly used in inorganic chemistry nomenclature, at least not for simple compounds, to indicate "two." One uses "di" instead; H2S is dihydrogen sulfide, not bihydrogen sulfide. So in the end, you can call NaHCO3 baking soda, sodium bicarbonate, or sodium hydrogen carbonate, and all 3 are correct.

There's another example of this "bi" business' HSO4-, or hydrogen sulfate ion, is also called bisulfate.

Hope this helps.

Best Regards,

Prof. Topper
Dept of Chemistry
The Cooper Union New York, NY


The reason that HCO3- is called "bicarbonate" is historical. Basically, it dates to the days before atomic theory was developed (which actually wasn't very long ago). Let's look at calcium carbonate and calcium bicarbonate, for example. Calcium carbonate is CaCO3, and the bicarbonate is Ca(HCO3)2. Calcium bicarbonate has TWO carbonates (derived from carbon dioxide) per calcium, and calcium carbonate has one carbon dioxide. It's fairly easy to lose the first carbon dioxide from Ca(HCO3)2; all you need to do is heat an aqueous solution of calcium bicarbonate: Ca(HCO3)2 = CaCO3 + CO2 + H2O. This is the "boiler-scale reaction," which is responsible for a lot of the white scale in hot-water heaters, and is also responsible for building up those beautiful terraces of Mammoth Hot Springs in Yellowstone National Park. It's a lot more work to lose the second carbon dioxide; this requires the elevated temperatures in cement kilns: CaCO3 = CaO + CO2.

So, bicarbonate is called that because a molecule of sodium, ammonium, calcium, etc. bicarbonate contains twice as many units of carbon dioxide than the corresponding carbonate does. "Sodium hydrogen carbonate" is a more descriptive name for baking soda than "sodium bicarbonate." Otherwise, you just have to know that HCO3- is called "bicarbonate."

Richard Barrans Jr., Ph.D.
Chemical Separations Group
Argonne National Laboratory



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