Is HBr Acid : Weak or strong, Why, How and Detailed Facts

So is hbr acid or base? In this article we shall have a closer approach towards the acidity of HBr. We know it is an inorganic compound and a diatomic molecule having hydrogen and bromine as atoms.

Now lets try to answer the above questions and whether is hbr acid or not .

Why HBr is acid?

In order to predict whether HBr is acid we first need to know what are criteria, properties a substance should follow in order to be termed as an acid. So what are acids in general? An acid is a substance ( molecule or ion)which has a potential to accept a proton or it can also be said as capable of accepting a pair of electrons ( in the reactions).

Or we can also say that an acid when dissolved in solution (aqueous) dissociates and produces H+ ion giving H3O+ ion. So now coming to HBr , it is observed that when HBr is dissolved / mixed in a solution ( aqueous), dissociation of the ions into H+ and Br- takes place. And after that the H+ (ions) readily combine with the H2O molecule giving H3O+ .

Well, it can be said that HBr is increasing the H+ concentration in the dissolved solution. According to Arrhenius an acid will increase concentration of H+ (ion) when dissolved in a solution (aqueous) . So, isn’t it following the above discussed acid theory ( Arrhenius)? Yes , it does follow so we can say that HBr is an acid.

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Is HBr always an acid?

There are different concepts of acids and bases, but we are going to have a look at few them :

According to solvent system definition an acid is a molecule ( or species)which increases cationic concentration of a solvent and a base is the molecule which increases the anionic concentration of a solvent, so what do you think that HBr will fall in which of the above discussed category?

As we know H+ is the smallest cation and on dissolving in aqueous solution there is dissociation of its species ( H+ and Br-) , so yes it will increase the concentration of cation and it is an acid.

Considering the Usanovich concept of acid and bases : According to this concept an acid is a molecule ( or species) that upon reacting with base, gives cations or has the potential to accept electrons.
And base is said to be molecule that reacts with acid and gives anions. So we know HBr reacts with base and gives up H+ ions ( which are cations),so yet it is an acid.

So till now we have seen Arrhenius theory, solvent system concept, Usanovich concept and in all of them HBr is an acid, hence we we can conclude HBr is always an acid. And in further topics other methods such as Lewis acid and Bronsted acid have been discussed which will give a more clear picture about HBr’s acidity .

Is HBr strong or weak acid ?

HBr is a strong acid and not a weak acid. So what are the conditions for an acid to be a strong one ?

It is said that if any acid is able to undergo complete dissociation and ionization when dissolved / mixed in an aqueous solution it is a strong acid. A acid will have high potential of loosing it’s proton (H+) and then the water takes the proton and forms hydronium ion.

Strong acids are said to have small pka values (logarithmic constant) but a higher Ka value ( i.e dissociation constant of an acid). The reasons for HBr being a strong acid are discussed below:

The pka value( i.e it’s logarithmic constant)is -9 and hence it is able to completely ionize and give a proton ( in aqueous form).The strong acid character arises as a result of the covalent bond( weak in nature) between H and Br atoms .
Another reason for strong acidic behavior of HBr is due to the overlapping of the atoms of H and Br is quite small reason being the difference in size of orbital (i.e 1s and 4p) which decreases the strength of the bond between H-Br ( which can be easily broken).
NOTE : the negative charge is observed to be diffused /spread over the orbital ( 4p orbital), hence the Br- ion is found to be comparatively stable. And this leads to the reduction of the charge density which causes the dissociation constant (for HBr) to be quite high.

Below are some of the factors which determine the strength of acid :

With increase in atomic radius, acidity also increases. The atomic radius of HBr is found to be around 0.80 +- 0.11 which is quite high.
If the conjugate base is more electronegative then acidic character also increases.

Hence HBr is a strong acid or we can say that possesses strong acidic behavior.

Is HBr Lewis acid or Bronsted acid ?

Well in order to predict whether HBr is Lewis acid or Bronsted acid, we first need to know about Lewis acid and Bronsted acid in general.

Taking into account Lewis concept : An acid is a molecule ( species) which accepts a pair of electrons. In order to be called Lewis acid a molecule (or species) should possess one empty orbital (at least one) in the valence shell of its atom.

Summing up Lewis acid :

Species having an incomplete octet in its central atom or we can also say that this compounds are electron deficient.
Species whose central atom has vacant d-orbital
All cations are considered to be Lewis acids among which the smallest cation H+ (ion) acts as the strongest Lewis acid.

Now taking into account Bronsted acid :

It is considered that a species which has the potential of donating a proton is called Bronsted acid. And it’s conjugate base is the species which is formed once the acid donates proton .

After understanding the concept of Lewis acid and Bronsted acid it can be said HBr is a Lewis acid as it has potential to accept a lone pair (one) in the process of combination with molecule of water and also it is a Bronsted acid as it is capable to lose H+ (ion) and form a base (which is known as conjugate base of acid ).

Did I just confuse you? Let me come to the point HBr can act as a Bronsted acid and Lewis acid , as it is observed that most of the times Bronsted acid is Lewis acid as well . But not always Lewis acid can be a Bronsted acid.

Is HBr a binary acid?

Binary acids (or also known as hydracids) are molecules ( or compounds)which has a hydrogen-bonded (or we can say combined) with another element which is mostly nonmetallic in nature. So, yes HBr is a binary acid as in the HBr molecule hydrogen is bonded to bromine which has nonmetallic character.

Is HBr a strong acid in aqueous solution?

As we know that HBr is a diatomic molecule in aqueous solution and on dissolving in water gives a strong acid.

After a period of boiling (constantly) we get aqueous solution of HBr (which is observed to distill = 124.3 degree Celsius). The pH value being 1.602 (considering the solution of HBr is 0.025 M). So, we can conclude that yes HBr is a strong acid in aqueous solution.

Is HBr stronger than HCl ?

As we know HBr and HCl both are binary acids ( meaning hydrogen atom is bonded to a element which is nonmetallic in nature).

It has been observed that the bond between H-Br is more (longer) as compared to H-Cl bond , also Br is much larger than Cl hence HBr is more stronger acid than HCl .

So what happens is HBr is able to ionize completely and quite easily as compared to HCl (the reason for this is long bond length). Meaning HBr is capable of producing more of H+ (ions) in comparison to HCl. So we can conclude HBr is stronger than HCl.

Is HBr a strong or weak nucleophile?

In order to act as a nucleophile a molecule should be (or act ) as a Lewis base . So , what is a Lewis base ? A Lewis base concept is that molecules containing unshared pair of electrons in their energy levels:

Taking into consideration almost all of the anions , the larger is the charge density more will be the strength of the base.
( Alkenes or the alkynes ) can form bonds ( coordinate) with metal ions and CO, C6H6 etc. which are observed to form π complexes with transition metal.

Now after understanding Lewis base concept it is clear that HBr does not follow any of the criteria and it is a Lewis acid. There is less or no scope for HBr to act as Lewis base , which is the primary condition for a molecule to act as a nucleophile.

So it is not possible for HBr to donate electrons before the donation of proton. Hence we can conclude HBr will not act as a nucleophile.

Is HBr not an electrophile ?

Yes HBr is said to be ( observed to be) an electrophile . So, now let us analyze how.

In the reaction between HBr and an alkene ,the HBr will act like an electrophile , so what happens is the π electrons of the reacting alkene react with H+(ion) of the HBr.

The reason for this reaction is that the Br atom atom is capable to withdraw electrons from hydrogen as it is partially negatively charged. Hence hydrogen becomes electrophilic. In conclusion HBr is an electrophile and not a nucleophile.

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Sania Jakati

This is Sania Jakati from Goa. I am an aspiring chemist pursuing my post graduation in organic chemistry. I believe education is the key element that moulds you into a great human being both mentally and physically. I’m glad to be a member of scintillating branch of chemistry and will try my best to contribute whatever I can from my side and Lambdageeks is the best platform where I can share as well as gain knowledge at the same time.