Chemical equation for the decomposition of sodium bicarbonate

Baking soda is an amphoteric inorganic salt with the formula NaHCO ₃ and is as ubiquitous in a chemistry lab as it is in the kitchen. It consists of the sodium salt of the conjugate base of carbonic acid . The latter is a weak acid, which gives bicarbonate a slightly alkaline character.

This compound is a white crystalline substance that can be found in different minerals such as natron and nacolite, also called thermocalite. It is also obtained industrially by passing gaseous ammonia and carbon dioxide through a concentrated solution of sodium chloride or brine.

Why is it called baking soda?

The systematic name of sodium bicarbonate is actually sodium hydrogencarbonate or sodium hydrogentrioxocarbonate(-1). However, even today the common name sodium bicarbonate is preferred because it is shorter and does not present too much ambiguity. The bi- prefix of bicarbonate refers to the fact that this salt contains twice as many carbonates for each sodium ion as sodium carbonate, whose formula is Na ₂ CO ₃ . Note that in the formula above there are two sodium for every carbonate ion, which is equivalent to saying that there are ½ carbonate for every sodium. On the other hand, in NaHCO ₃ there is one carbonate for every sodium, which represents twice ½, which is where the prefix comes from.

Chemical Properties of Baking Soda

Sodium bicarbonate is a relatively inexpensive substance and possesses chemical properties that make it useful in many applications both inside and outside of the laboratory.

• For one, its mildly alkaline character makes it an excellent choice for neutralizing acids when a spill occurs. The neutralization reaction produces an observable fizz that readily indicates when the acid is finished neutralizing. On the other hand, the addition of an excess of bicarbonate is not problematic, since it is a weak base. In fact, in every chemistry laboratory there should always be a bottle with a sodium bicarbonate solution for this purpose.
• In addition, sodium bicarbonate also has an ionizable proton, since carbonic acid is a diprotic acid, so it can also react with bases.
• Finally, this salt has the particularity that it breaks down easily when heated. Such thermal decomposition is the main topic of this article and is described in the next section.

Sodium Bicarbonate Decomposition Equation

As mentioned in the previous section, baking soda breaks down when heated. This reaction produces the release of carbon dioxide (CO ₂ ) in the form of gas and water vapor (H ₂ O), while the alkaline salt sodium carbonate (Na ₂ CO ₃ ) remains in the solid phase. The balanced or adjusted chemical equation is:

NOTE: Below 100°C, if the pressure is 1 atm, the water formed will condense to a liquid state. Above said temperature, steam is generated instead.

This chemical reaction occurs naturally at room temperature, but very slowly. This is the reason why baking soda has an expiration time, which is around 2 to 3 years. However, the reaction speeds up with increasing temperature, occurring rapidly above 80°C. On the other hand, this reaction is catalyzed by acids.

One of the essential features of the thermal decomposition of sodium bicarbonate is the release of carbon dioxide gas and water vapor. This characteristic, along with the fact that neither baking soda nor carbonate is harmful to health, makes baking soda often used as a chemical leavening agent when baking various foods such as cakes, pancakes, and others.

In addition to this, the above decomposition reaction also explains a characteristic feature of the use of this salt in cooking or baking our food: the characteristic metallic flavor that remains in some foods if we add too much bicarbonate to the mixture. This flavor comes from the sodium carbonate that is formed after the decomposition of the baking soda. Sodium carbonate is a much more alkaline compound than bicarbonate since it is its conjugate base, and as an acid, bicarbonate is very weak. Remembering that the strength of the conjugate base is inverse to the strength of the acid, a very weak acid gives rise to a stronger base.

This same reaction is also used in baking powder. This culinary ingredient contains about a third of sodium bicarbonate as a leavening agent; It also contains acidic substances that help in the decomposition of the bicarbonate and that also neutralize the carbonate formed after the reaction.

A second decomposition reaction

If we take a sample of sodium bicarbonate and heat it up, when it gets to around 80°C it will start to decompose rapidly until it is completely converted into sodium carbonate, carbon dioxide and water, as we just saw. However, if we continue to heat, there will come a time when a second decomposition reaction will occur with the release of more carbon dioxide.

This second decomposition reaction occurs at approximately 850°C and produces, in addition to carbon dioxide, sodium oxide (Na ₂ O), as can be seen in the following equation:

Although this decomposition reaction can be carried out by heating sodium bicarbonate to high temperatures, it is not actually a decomposition reaction of sodium bicarbonate, since in this case it is carbonate that is being decomposed of sodium.

References

Buddies, SSL (2017, August 17). Vanishing Baking Soda . Scientific American. https://www.scientificamerican.com/article/vanishing-baking-soda/

Chang, R. (2021). Chemistry (11th ^ed .). MCGRAW HILL EDUCATION.

sciencebit. (2017, February 27). Decomposition reaction of sodium bicarbonate. Chemistry experiment. [Video]. Youtube. https://www.youtube.com/watch?v=NNOrw848tGk

EcuRed. (nd). Sodium Bicarbonate (Substance) – EcuRed . https://www.ecured.cu/Bicarbonato_de_sodio_(Substance)

IUPAC. (2005). NOMENCLATURE OF INORGANIC CHEMISTRY – IUPAC Recommendations 2005 . IUPAC Red Book. http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf