Is Bicarbonate (HCO3-) an acid or base? Conjugate acid vs Conjugate base

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Is bicarbonate (HCO3-) an acid or base?

Bicarbonate is a polyatomic ion made up of one hydrogen, one carbon, and three oxygen with one negative formal charge having a chemical formula HCO3. It is also known as hydrogen carbonate. It is mainly used in pH buffering systems. They are mostly soluble in water at standard temperature and pressure. Bicarbonate is generated when one proton is removed from carbonic acid.

In this article, we will discuss Is Hydrogen carbonate(HCO3) an acid or base? It’s conjugate acid as well as conjugate base, etc.

So, Is HCO3 an acid or base? Bicarbonate(HCO3) is amphoteric in nature means it can act as an acid as well as base, depending on what it is reacting with. It is one of the main components used in a pH buffering system due to its amphoteric nature.

Let’s discuss in more detail how can HCO3 can act as an acid as well as a base?

Name of MoleculeBicarbonate
Chemical formulaHCO3
Molar mass61.0168 g mol−1
NatureAmphoteric
Conjugate acidH2CO3
Conjugate baseCO32-
Acidity(pKa)10.3
Basicity(pKb)7.7

Why HCO3- act as acid?

To get a better perspective of the acidic nature of HCO3. We first look at what the term “Acid” means.

Acid is generally that compound that has a pH value of less than 7 and contains one or more replaceable hydrogen ions. When acidic compounds are dissolved in an aqueous solution, they will release H+ ions.

The more the number of hydrogen ions liberated in an aqueous solution, the higher the strength of that acidic compound.

So, now Why HCO3 act as an acid? HCO3 can act acid only in one condition when the reacting compound is more basic than it e.g. OH. This is because a stronger base has more ability to “accept the proton”.

Hence, HCO3has to donate the proton when reacting with a strong base, and anything that donates the proton is considered acid in nature.

HCO3 act as acid when reacting compound have smaller Ka than it.

⇒ Ka for HCO3 = 4.7 × 10-11

⇒ Ka for OH = > 10-14 Approx

So, reacting compound(OH) have smaller Ka, hence, here, HCO3 will act as acid.

 Therefore, when HCO3 reacted with a stronger base like OH, it left with no choice and have to release proton.

A general rule to distinguish between base and acid compound:-

  • If A compound release more hydrogen ions(H+) than B, then compound A is more acidic than B.
  • If A contains releases more hydroxide ions(OH) than B, then compound A is more basic than B.

So, when HCO3 reacts with a compound having OH ion, then HCO3 will act as an acid and therefore, have to donate a proton. And anything that donates the proton is considered as acid as per Bronsted-Lowry theory.

Is HCO3- an acid?

As we see in the above reaction, HCO3 reacting with a base(OH), therefore donating one proton to hydroxide ion and makes a conjugate base(CO32-).

Bronsted-Lowry’s theory for acid said that “the acid is the substance that donates the proton to reacting species and itself makes a conjugate base.” 

As we see in the above figure, HCO3 is donating the proton to OH and making a conjugate base(CO32-).

Therefore, we can say HCO3 acts as Bronsted-Lowry acid if reacting compound is a stronger base than it.

Also check:- How to tell if something is acid or base or salt or neutral?

Why HCO3- act as base?

A base is defined as a proton acceptor or electron-pair donor. A stronger base liberates more OH when dissolved in an aqueous solution and has a pH value of more than 7.

So, Why HCO3 act as a base? HCO3 acts as a base when reacting with the compound that is more acidic than it, i.e. HCl. This is because a stronger acid has more ability to “donate the proton”.

Hence, HCO3– has to accept the proton when reacting with a strong acid, and anything that accepts the proton is considered base in nature.

HCO3 act as base when reacting compound have larger Ka than it.

⇒ Ka for HCO3 = 4.7 × 10-11

⇒ Ka for HCl = 1.3 × 106

So, reacting compound(HCl) have larger Ka, hence, here, HCO3 will act as base.

Therefore, when HCO3 reacts with stronger acid like HCl, it is left with no choice and has to accept the proton from HCl. And we know anything that accepts the proton act as a base.

is hco3- base

Illustration of above reaction:-

  • HCl donating a proton to HCO3 and formed conjugate base(Cl). Hence, it is a Bronsted-Lowry acid.
  • Bicarbonate(HCO3) accepting the proton from HCl and formed conjugate acid(H2CO3). Hence, it is a Bronsted-Lowry base.

As per Bronsted-Lowry base theory, a compound is said to base when it accepts the proton from reacting species and forms a conjugate acid by adding one proton to itself.

Therefore, we can say HCO3 can act as a Bronsted-Lowry base if react with strong acid.

Also Read:- How to tell if the acid or base is strong or weak?

What is the conjugate base and conjugate acid of HCO3-?

To know what is the conjugate acid-base of HCO3, first, we have to through the concept of conjugate acid-base pairs.

The concept of conjugate acid-base pair.

  • A very strong acid always forms a weak conjugate base.
  • A very strong base always forms a weak conjugate acid.
  • A very weak acid always forms a strong conjugate base.
  • A very weak base always forms a strong conjugate acid.

concept of conjugate acid-base pairs

Conjugate acid:-

A conjugate acid is a species formed by gaining one electron by base compound or you can say when the proton is added to the parent base then the compound is formed which is called conjugate acid. Example of conjugate acid-

⇒ NH3 + H+ → NH4+

Here in this reaction, NH3 is a weak base that accepts the proton when dissolved in an aqueous solution and formed conjugate acid(NH4+).

So, NH4is the strong conjugate acid of NH3.

Conjugate base:-

A conjugate base is a species formed by losing one electron by an acid compound or you can say when the proton is removed from the parent acid then the compound is formed which is called the conjugate base. Examples of conjugate base-

⇒ HCl → H+ Cl

Here, in this reaction, HCl is a strong acid that donates the proton when dissolved in an aqueous solution and formed a conjugate base(Cl).

So, Cl- is the weak conjugate base of HCl.

⇒ H2SO4 → H+ + HSO4

Here, in this reaction, H2SO4 is a strong acid that donates the proton when dissolved in an aqueous solution and formed a conjugate base(HSO4).

So, HSO4 is a weak conjugate base of H2SO4.

Now, what is the conjugate acid and base of HCO3? When HCO3 acts as an acid it will make a conjugate base and when it acts as the base it will make conjugate acid.

⇒ When HCO3 react with stronger acid than it such as HCl, it accepts the one proton from HCl and formed a conjugate acid(H2CO3).

⇒ When HCO3 reacts with a stronger base than it such as OH, it donates the one proton to OH and formed a conjugate base(CO32-).

What is the conjugate base and conjugate acid of HCO3-?

Also Read:– Is H2CO3 an acid or base?

Uses of Bicarbonate ion

  1. Bicarbonate is used to maintain acid-base (pH buffering system) in the human body to counteract the activity of highly acidic digestive juices.
  2. The salts of bicarbonate ions are used as leavening agents.
  3. It is one of the important sinks in the carbon cycle.

Summary

“Bicarbonate is an intermediate form in the deprotonation of carbonic acid.” Its acidity value is 10.3 and its basicity is 7.7. The nature of HCO3 is amphoteric means it either acts as an acid or as a base depends on some condition.

  • The conjugate acid of HCO3 is Carbonic acid(H2CO3).
  • The conjugate base of HCO3 is Carbonate ion(CO32-).
  • Is bicarbonate (HCO3) acid or base? HCO3 can act as acid as well as base depends on the strength of reacting compound.
  • In presence of a strong base like OH, HCO3 acts as an acid.
  • In presence of a strong acid like HCl, HCO3 acts as a base.
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