Ammonium bromide (NH4Br) is an ionic salt of Hydrogen Bromide and Ammonia. It is found in the form of white, odorless granules which turn yellow when exposed to air due to oxidation of Bromide (Br-) ions to Bromine (Br2).
It is the ammonium salt of hydrobromic acid and is composed of ammonium and bromide ions in a 1:1 ratio. It is a strong electrolyte in an aqueous solution and conducts electricity.
It is manufactured by a straight reaction of Hydrogen Bromide (HBr) and Ammonia (NH3). The reaction is written as:
HBr + NH3 —–> NH4Br
So, is NH4Br is acid or base? NH4Br is an acid. It is the salt of Hydrogen Bromide (HBr) and Ammonia (NH3). Here, it can be noted that HBr is a strong acid while NH3 is a weak base. The salts of strong acid and weak base are acidic in nature, therefore, NH4Br is acidic. It has a pKa value of ~5 in the water and a pH value of less than 7.
Why is NH4Br Acidic in Nature?
We know that a reaction between an acid and a base is known as the neutralization reaction and the end-product formed in such reactions is known as the salt.
The salt so formed acquires the properties of its fundamental molecules depending upon their strength and hence, its acidity or basicity is derived from its constituent acid or base.
The simple way out to determine the acidic or basic nature of salt is to look at its key ingredients viz.
The salt of strong acid and strong base are neutral having pH = 7
The salts of a weak acid and strong base are basic having pH > 7
The salts of strong acid and weak base are acidic having pH < 7
In the case of NH4Br, Hydrogen Bromide is a strong acid while Ammonia is a weak base. As already discussed, the salt acquires the properties of the stronger molecule, therefore, NH4Br is acidic.
Further, we can also understand the nature of a substance by looking at its dissociation equation or the ions it forms in an aqueous solution.
The dissociation equation for NH4Br in an aqueous solution can be written as:
NH4Br ——> NH4+ + Br –
Here, Br- is the conjugate base of HBr. We already know that HBr is a strong acid and the conjugate bases of strong acids are weak bases.
The Br- ion is such a weak base that it is unable to hydrolyze any further.
Moreover, NH3 is a weak base and weak bases give rise to strong conjugate acids.
Hence, the NH4+ ion formed is strong in nature.
Therefore, it can be deduced that Br- ions (as they cannot hydrolyze further) do not play any role in determining the acidity or basicity of this solution.
However, the ammonium ions (NH4+) do lose protons in the aqueous solution to form ammonia (NH3) and hydronium ions (H3O+).
These hydronium ions formed owing to the dissociation of ammonium ions are responsible for the acidity of the aqueous solution of NH4Br.
A few days ago, I also wrote an article about KNO3, check out the article is KNO3 an acid or base.
What is an Acid?
The substances that give away their proton when dissolved in an aqueous solution are termed acids. Acids are usually sour in taste and corrosive in nature.
A few examples of acids are HBr, H2SO4, CH3COOH, etc
The ease with which the acids lose their proton determines the strength of an acid, based on which the acids can be characterized as strong acids and weak acids.
The acids that tend to give away their protons effortlessly are called strong acids. For example HBr, H2SO4, etc.
On the other hand, weak acids are those in which the protons are sturdily associated and do not detach easily. For example, CH3COOH, HCN, etc. are weak acids. It requires an ample amount of energy for the complete dissociation of weak acids.
However, the strong acids dissociate or ionize rapidly in an aqueous solution.
Other than being the proton donor the acids display the following characteristics:
It must have a pH of less than 7
It shall turn the blue litmus red.
It shall react with a complementary base to form the salt.
pH of NH4Br
The pH of a solution determines its acidity or basicity. The pH value for any substance ranges from 1 to 14 on the pH scale where pH 7 is neutral (it is the pH of water).
If the pH value of a substance is below 7, it is considered acidic while for a pH value of above 7, a substance is regarded as alkaline.
In the case of NH4Br, we already know that it is an ionic salt of strong acid and a weak base. Therefore, the pH of NH4Br should be less than 7.
Calculating the pH for 1 M NH4Br solution:
The concentration of hydrogen ions is calculated as:
[H+] = √Ka * M
As the value of Ka = NH4Br is 5.6 * 10-10
Also, we are calculating the hydrogen ion concentration for 1 M NH4Br solution
Therefore, [H+] = √5.6 * 10-10 * 1
= 2.3 * 10-5
The pH value for 1 M solution can now be calculated as:
pH = – log [H+]
= – log [ 2.3 X 10-5 ]
= – [ log 2.3 – 5 log 10]
As the pH value is less than 7, therefore, it is clear that NH4Br is acidic in nature.
Why NH4Br is not a Base or Basic Salt?
Several theories provide us with the definition of the base. Three of the important ones define base as follows:
Lewis theory: A molecule that has an unshared pair of electrons available to be given away to another molecule.
Bronsted-Lowry theory: A molecule that easily takes up the proton given away by another molecule.
Arrhenius theory: A molecule that produces hydroxide ion (OH-) in a solution.
NH4Br does not fit into any of the above-mentioned definitions.
It does not have an available pair of electrons, it does not take up a proton from a solution, and lastly, instead of hydroxide ions (OH-), it produces hydronium ions (H3O+) in a solution.
Further pH value for NH4Br is less than 7, a characteristic of acids. Therefore, NH4Br cannot be considered a base.
Below is the image of the molecular geometry of NH4Br.
Why NH4Br is Acidic in an Aqueous Solution?
We have already discussed that as NH4Br is an ionic salt of strong acid and weak base, it is acidic in nature. But, we are still unclear on which ions determine the acidity of NH4Br in an aqueous solution.
Let’s try to understand this.
First, let us recall that on dissociation in an aqueous solution.
• A weak acid forms a strong conjugate base
• A strong acid forms a weak conjugate base
• A weak base forms a strong conjugate acid
• A strong base forms a weak conjugate acid
Coming to NH4Br, when dissolved in water it breaks down into ammonium (NH4+) and bromide ions (Br-) ions.
The equation can be written as:
NH4Br (aq) —–> NH4+ + Br –
Here, Br – is a weak conjugate base of a strong acid (HBr) and which is lacks the strength to hydrolise further and therefore, has no impact on the pH of the solution.
However, the NH4+ ion is the strong conjugate acid of a weak base NH3. It hydrolysis further in the solution to form hydronium ions and ammonia.
The equation is written as:
NH4+ (aq) —-> NH3 + H3O+
The presence of these hydronium ions is responsible for the acidic nature of the NH4Br solution.
Some properties of NH4Br are listed in the table given below:
|Molar mass||97.94 g/mol|
|Boiling Point||452 °C|
|Melting Point||235 °C|
|Solubility||78.3 g/100 g water at 25 °C|
|Density||2.42 g/cm3 at 25 °C|
|Vapour Pressure||1 mm Hg at 198.3 °C|
|Refractive index||1.712 at 25 °C|
• A few important uses of Ammonium Bromide are as follows:
• Ammonium Bromide is used in the manufacturing of photographic films, plates and, papers.
• It is used for making wooden articles fireproof.
• NH4Br is used in lithography and for engraving.
• Pharmaceutical industries also make use of ammonium bromide in medicinal preparations
• It is also used as a corrosion inhibitor.
• Ammonium Bromide is used in photochemical reactions.
• Also, it is used for the precipitation of silver salts.
• Ammonium Bromide is acidic in nature. It is an ionic salt of strong acid (HBr) and weak base (NH3).
• The ammonium ions are formed after dissociation of NH4Br in an aqueous solution which further ionizes to release proton. This proton is responsible for the acidity of the solution.
• It has a pH value of less than 7.
• It is a strong electrolyte that dissociates into ammonium (NH4+) and bromide (Br-) ions in an aqueous solution.
Hope the article helped you clear your doubts. In case, you need any kind of explanation regarding the chemistry topics, let me know through your valuable comments.