Is HNO3 an acid or base? - Strong vs Weak (Nitric acid)
Nitric acid is a strong oxidizing agent with the chemical formula HNO3. It is made up of three elements hydrogen, oxygen, and nitrogen. The pure compound of nitric acid is colorless and has a suffocating odor. . It is used in the manufacture of various organic and inorganic nitrates.
In this article, we will discuss Is HNO3 an acid or base, Is it strong or weak? its conjugate base or acid, etc.
So, Is HNO3 an acid or base? HNO3 is an acid. Because on dissolving in an aqueous solution, it dissociates into two ions (H+ and NO3–), and anything that releases an H+ ion in a solution is considered acid in nature.
| Name of Molecule | Nitric acid |
| Chemical formula | HNO3 |
| Nature | Strong acid |
| Conjugate base | NO3– |
| pH value | 3.01 |
| Acidity (pKa) | -1.4 |
| Solubility in water | Completely miscible |
Why HNO3 act as acid?
According to various acid-base theories, an acid is something that releases a proton or H+ ion on dissolving in an aqueous solution.
In the case of HNO3, it dissociates completely in an aqueous solution to give H3O+ and NO3– ions.
⇒ HNO3 + H2O → H3O+ + NO3−
Here in this reaction, HNO3 donates the proton to H2O and formed NO3– conjugate base. And H2O acts as a base because it accepts the proton from HNO3 and forms H3O+ conjugate acid.
Theories to check whether HNO3 is an acid or base.
1. Arrhenius’s theory for acid:
Arrhenius’s theory state that acid is a substance that gives the H+ ions or increased the amount of hydrogen ion in an aqueous solution.
So, the HNO3 molecule, when dissolving in an aqueous solution it dissociates into H+ ions which quickly form hydronium ions (H3O+). So, we can say, HNO3 is Arrhenius acid.
2. Bronsted-Lowry theory for acid:
It states that a compound is an acid when it donates a proton to another compound and itself forms the conjugate base. And the base is something that accepts the proton from another species and itself forms conjugate acid.
Let’s consider the reaction of HNO3 with H2O.
As per the above reaction, HNO3 donates the proton to H2O and itself forms a conjugate base which is NO3–. Therefore, we can say, in the above reaction, HNO3 acts as Bronsted-Lowry acid, and H2O acts as a Bronsted-Lowry base.
Is Nitric acid (HNO3) strong or weak?
To check If Nitric acid(HNO3) a strong or weak, first we have to take a clear understanding of the differences between strong and weak acids.
A strong acid is generally a compound that dissociates completely or is 100% ionized in a solution to yield H+ ions which means no undissociated acid remains in the solution, all moles of acid completely break off and release a lot of H+ ions.
Example- HCl, HBr, H2SO4, HI, HClO4, etc.
Also Read:-
- Is HBr a strong acid?
- Is H2SO4 a strong acid?
- Is HCl a strong acid?
- Is HI a strong acid?
- Is HClO4 a strong acid?
And weak acid is generally a compound that is partially or not ionized completely in a solution which means some moles of acid remain undissociated in the solution and do not break completely into ions.
Examples of weak acids- Hydrocyanic acid(HCN), Acetic acid(CH3COOH), Ammonium ion (NH4+), Phosphoric acid(H3PO4), HNO2, HF, HCOOH, H2CO3, etc.
Also, Read:-
- Is CH3COOH a weak acid?
- Is HF a weak acid?
- Is HCN a weak acid?
- Is HNO2 a weak acid?
- Is HCOOH a weak acid?
- Is H3PO4 a weak acid?
- Is H2CO3 a weak acid?
- Is NH4+ a weak acid?
| Strong acid | Weak acid |
| They ionize completely. | They do not completely ionize. |
| They have high conductivity. | They have less conductivity. |
| The value of pH lies between 0 to 3. | The value of pH lies between 3 to 7. |
| They dissociate completely to produce an H+ ion. | They dissociate partially to produce an H+ ion. |
| They are strong electrolytes. | They are weak electrolytes compared to strong acids. |
| Example – HNO3, H2SO4, HBr, etc. | Example – C2H2O4, HNO2, HCOOH, etc. |
Now, Is HNO3 (Nitric acid) strong or weak? Nitric acid (HNO3) is a strong acid. Because when it is dissolved in an aqueous solution, it completely dissociates into H+ and NO3– ions in the solution.
As on dissolving HNO3 in an aqueous solution, no hydrogen remains bound, all are dissociated and converted into H+ ions which means the concentration of hydrogen ions increased in the solution. Hence, we can say, HNO3 is a strong acid or act as a strong acid.
Note: In diluted form, HNO3 is weak but in concentrated form, it is a very dangerous or strong acid.
As per the above picture, when HNO3 dissolves in an aqueous solution, it completely ionizes or dissociates into H+ and NO3– ions. No undissociated parts of it remain in the solution, all parts completely break off and are ionized in an aqueous solution. the presence of high proton ions in the solution shows the strong ability of HNO3 acid.
H3O+ and H+ are the same things.
The single arrow used in the above reaction shows that only a forward reaction will take place. As NO3– is the very weak conjugate base of HNO3, so it has no power to reform with H+ splitting ion in an aqueous solution and produce an HNO3 molecule again.
Hence, only splitting ions(H+ and NO3–) of HNO3 will be available in the aqueous solution that correspondingly shows its strong acid nature.
Also, the acid dissociation constant value(Ka) for HNO3 is 2.3 × 101.
The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution
⇒ If the value of the dissociation constant of acid is greater than 1 (Ka >> 1), then the nature of the compound is a strong acid.
⇒ If Ka << 1, then the nature of the compound is a weak acid.
Trick to remember the acidity strength of oxyacids
“An oxyacid is an acid that contains oxygen, hydrogen, and one other element(metal or non-metal)”. Examples of oxyacids:- H2SO4, HNO3, H3PO4, etc.
Trick– If oxyacids have 2 or more oxygen than hydrogen, then oxyacid is a stronger acid.
∴ HNO3 oxyacid has three oxygen and one hydrogen, so (3 -1 = 2), we have two oxygen left, Hence, it is a strong acid as per rule.
∴ H2SO4 oxyacids have four oxygen and two hydrogens, so (4 – 2 = 2), we have two oxygen left, hence, it is a strong acid.
∴ H3PO4 oxyacid has four oxygen and three hydrogens, so (4 – 3 = 1), we have only one oxygen left, hence, it is a weak oxyacid.
Here’s the list of some common strong or weak acids.
Also Read:
Is HNO3 also act as base?
When HNO3 faces or reacts with superior to it, then in this condition it acts as a base. What does it mean? It simply means, when HNO3 reacts with the less reactive element that accepts the proton, it will act as an acid.
And when HNO3 reacts with a more reactive compound like sulfuric acid or super acids that must donate the proton, it will act as a base. Does it make sense?
Let’s more elaborate on it, to understand in which condition HNO3 acts like acid and in which base.
HNO3 acts as an acid:
- When the surrounding compound is less reactive or bases in nature.
- When the surrounding compound is less acidic than HNO3.
Just remember these two conditions to understand when HNO3 acts as an acid. Let’s take an example to crystal clear your doubt.
Here, HNO3 is acting as an acid because it donates the proton to H2O and forms the conjugate base.
H2O is very less reactive and very less acidic than HNO3, so, it has to accept the proton from a strong acid like HNO3.
HNO3 acts as a base:
- When the surrounding compound is more reactive.
- When the surrounding compound is more acidic in nature than HNO3.
Example – Sulfuric acid is more reactive and more acidic than HNO3. So, when reacting with sulfuric acid, HNO3 accepts the proton from it and forms the conjugate acid.
As H2SO4 is more acidic, it must donate the proton to HNO3 to gain stability, hence, it formed HSO4– conjugate base. Whereas, HNO3 has to accept the proton as H2SO4 is superior to it, hence, forming NO2+ conjugate acid.
⇒ H2SO4 + HNO3 → H2NO3+ + HSO4–
H2NO3+ is a really unstable ion, hence, it dissociates into H2O and NO2+.
∴ H2SO4 + HNO3 → NO2+ + H2O + HSO4–
Therefore, in this case, HNO3 acts as a base since it accepts the proton from H2SO4.
Is HNO3 lewis acid or base?
In short, HNO3 acts as Lewis acid Because as per Lewis’s theory for acid, a compound that accepts the electron pair from another compound is classified as Lewis acid since HNO3 has an H+ ion that can accept the electron pairs from a compound donor.
⇒ Lewis acid → electron pair acceptor
⇒ Lewis base → electron pair donator
Note: Lewis acid is a superset of Bronsted-Lowry acids which means every compound that falls into the category of Bronsted-Lowry acid is Lewis acid.
As we already discussed, HNO3 is Bronsted-Lowry acid because it donates the proton when reacts with less acidic compounds than it such as H2O and forms the conjugate base. Therefore, HNO3 also acts as lewis acid.
Technically, HNO3 acts as lewis acid. Because the nitrogen central atom has a deficiency of electrons and it can accept the electron pair from another compound in order to release the proton from it.
And we know anything that accepts the electron pair from other compounds is Lewis acid in nature.
What is the Conjugate acid and base of HNO3
Conjugate acid is formed when one H+ has been added to the parent base and the conjugate base is formed when one proton or H+ ion is removed from the parent acid.
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.
As we know, HNO3 sometimes acts as the base and sometimes as an acid. So, obviously, its conjugate acid-base pair also vary with its nature.
- When HNO3 acts like acid, it will make the conjugate base.
- When HNO3 acts like a base, it will make the conjugate acid.
So, when the HNO3 acts as acid its conjugate base is NO3– and when it acts as the base then its conjugate acid is NO2+.
Is HNO3 stronger acid than HNO2?
Yes, HNO3 is a stronger acid than HNO2, one reason is that HNO2 does not dissociate completely in an aqueous solution to liberate proton or H+ ions whereas HNO3 is nearly 100% ionized in an aqueous solution and completely dissociates into H+ and NO3– ions.
Or check the conjugate base of acids to know the stability of the compound. Here, the HNO3 conjugate base is NO3– which is more stable than the conjugate base of HNO2 which is NO2–.
Because in the case of NO3–, the electron can delocalize over three oxygen but in the case of NO2–, the electron can delocalize over only two oxygen.
So, NO3– electron has a greater volume to stabilize the charge than the NO2–. Therefore, NO3– is more stable than NO2–.
Note: If the conjugate base of A acid is more stable than the conjugate base of B acid, then A is a stronger acid than B acid.
Uses of Nitric acid
- It is used as an oxidizing agent.
- It is widely used in the pharmaceutical industry.
- It is mainly used to produce Aqua Regia.
- It is used in the making of nitrate fertilizer.
- It is also used for making explosive substances like TNT.
Conclusion
Here comes the last part! It’s time to rewind this article on HNO3 acid or base with some highlighted or important points.
- Is HNO3 an acid or base? HNO3 is an acid. Because it has a proton donating ability. Also, the value of the acid dissociation constant of HNO3 is very high, close to 2.3 × 101.
- Nitric acid (HNO3) acts as Lewis acid. Because it has an H+ ion that can accept the electron pairs from a compound donor.
- The conjugate base of HNO3 is NO3– and conjugate acid is NO2+.
- In presence of strong acids like sulfuric acid, HNO3 acts as the base because it has to accept the proton from the stronger acid.
- HNO3 (Nitric acid) is a strong acid. Because it is 100% ionized or completely dissociates ions in an aqueous solution.
About the author
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
