Hydroxides

In this section, we will have an introduction to hydroxides. Hydroxide is a chemical compound. It is represented by OH-. It is a diatomic molecule. It is a combination of oxygen and hydrogen in the ratio of 1:1. A covalent bond is created between hydrogen and oxygen atoms. It is overall a negatively charged molecule. The most commercial hydroxide is sodium hydroxide. Water constitutes hydroxide ions. When dissociated in water, give out solvated hydroxide ions. Arrhenius, a Swedish scientist, defined bases with respect to hydroxide ions. He said hydroxide presence in a solvent makes it a strong base. Any compound with a pH above 7 is called bases. 

Salts are formed by hydroxide ions. Hydroxide ions are negative charge carriers. Hence, they form bonds with positively charged ions. This bonding gives rise to ionic compounds. 

Properties

•  Name – Hydroxide 
• Chemical formula – OH-
• Molecular mass – 17 g/mol 
• The conjugate base of hydroxide – anionic oxide 
• The conjugate acid of hydroxide – H2O
• pH – high above 7 
• Solubility – most of them are insoluble except alkali 
• Synonyms – hydroxide ion, hydroxyl ion or oxidanide 
• Common examples – Sodium hydroxide, Calcium hydroxide, and Ammonium hydroxide 

Most of the inorganic materials which contain hydroxide in their names are not ionic compounds. Hydroxy groups are responsible for this. Hydroxy groups are -OH groups. Hydroxy groups are neutral atoms. Hydroxide ions bear a negative charge. Both hydroxy and hydroxyl groups are nucleophiles. Both are capable of acting as catalysts.

Structure 

This is the basic figure of a hydroxide ion. There is a presence of covalent between oxygen and hydrogen. Covalent bonds are created when there is sharing of electrons. 

Hydrogen has a valency one. Oxygen has valency two. One electron from hydrogen and one electron from oxygen form a covalent bond. The residual charge in oxygen is carried over in the compound.

Functions  

The main functions of hydroxide ions are:

1. Hydroxide ions act as a Lewis base. 
2. It functions as a catalyst. In organic reactions, hydroxide ions behave as a base catalyst.
3. It can act as a ligand. This happens due to the presence of lone pairs of electrons.
4. It can function as a nucleophile as well.
5. They are involved in neutralization reactions.

Basic nature of hydroxide ions

Arrhenius defined bases with respect to hydroxide ions. He said hydroxide presence in a solvent makes it a strong base. Arrhenius bases are any compound that is capable of producing hydroxide ions upon dissolving in aqueous solutions. 

Let us consider ammonia dissolving in water. The equation governing this reaction is, 

NH3 + H2O → NH4+ + OH– 

When ammonia is dissolved in water, it gives

1. Ammonium ion
2. Hydroxide ions

Salts that contain hydroxide ions are called basic salts. These salts give rise to one or more hydroxide ions. It also gives a cation. These basic salts, when reacting with acids, undergo neutralization reactions.

Let us see how water dissociates. 

H2O = H+ + OH–

The above equation shows how water is dissociated. It gives one hydroxide and one hydrogen ion. 

Amphoteric nature of hydroxide ions

The ability of a compound to behave as an acid or base is called an amphoteric compound. Hydroxide ions are capable of exhibiting amphoteric properties. They are capable of acting like Bronsted-Lowry bases and Lewis acid. 

Amphoteric nature is seen in metal hydroxides that have highly charged central metal atoms. In this segment, we will see how aluminium hydroxide reacts with acidic and basic solutions.

• Amphoteric hydroxide reacting with Acid Solutions

Aluminium hydroxide[ Al(OH)3] is one of the most common examples of an amphoteric hydroxide. Al(OH)3 is largely insoluble in water. But, it is soluble with strong acidic solutions. Let us consider a reaction of HCl with Al(OH)3:

HCl(aq) + Al(OH)3(aq) → AlCl3(aq) + 3H2O(l)

• Amphoteric hydroxide Reacting with basic Solutions

Now we will see how aluminium hydroxide reacts in a basic solution. Here, we will consider strong bases like NaOH. 

This is the governing equation for the reaction 

Al(OH)3(aq) + OH–(aq) →Al(OH)4–(aq)

In this reaction Al(OH)3 takes a hydroxide ion from the solution. This makes it a Lewis acid 

Inorganic Hydroxide ions

Alkali Metal hydroxide 

Other than NaOH and KOH, the hydroxide ions of other alkali metals can also be useful. For example, lithium hydroxide has a pkb value of -0.36. It is a strong base. It can be used in the purification of breathing gas for submarines. In a spacecraft, it can remove CO2.

The reaction equation for LiOH is 

2LiOH + CO2 → Li2CO3 + H2O

Alkaline metal  Hydroxide

Beryllium hydroxide is an alkaline metal hydroxide. It has the chemical formula of Be(OH)2. It is an amphoteric hydroxide. It forms a soluble hydrolysis product after adding acids. The hydroxide ions are trimeric here. 

 [Be3(OH)3(H2O)]3+ 

Boron Group Hydroxide

Boric acid is a simple hydroxide in the boron group. Contrary to alkaline earth metal hydroxides, boric acid does not dissociate in water. It releases protons when it reacts with water. This makes it a lewis acid.

Boric acid with the chemical formula – B(OH)3    

The equation is for reaction with water is 

B(OH)3   + H2O  →B(OH)4– + H+

Carbon Group hydroxide

Carbon elements form complex hydroxides. Let us take a hypothetical compound, methane tetraol. The formula methane tetraol is C(OH)4. Compounds like methane tetraol and orthocarbonic acid are unstable in aqueous solutions. Let us see methane tetraol reacts with water:

C(OH)4 → HCO3–+ H3O+

HCO3–+ H+ → H2CO3

Carbon anhydride is an alias for CO2. It is formed by the dehydration of carbonic acid. Carbonic acid has the formula: 

H2CO3(OC(OH)2)

Transition metal hydroxide 

Transition and post-transition hydroxides have +2 oxidation states. They are insoluble in water. They are poorly defined. These hydroxides undergo condensation to form oxides. This process is called olation. Let us take the example of silver hydroxide. It has the chemical formula Ag(OH). Ag(OH) decomposes to Ag2O spontaneously. Transition metals with oxidation state +1 are also unstable. The same applies to post-transition metals.

The fundamental equation for transition and post-transition metal hydroxides is 

LnM(OH2) + B ⇆ LnM(OH) + BH+ (L= Ligand ,B= Base)

Where M is the transition metal.

Applications 

• Fuel cells are produced using hydroxide ions.
• They can be used as disinfectants. 
• They are also utilized as food preservatives.
• NaOH, a strong basic hydroxide, is used in many processes in industries. Example: Soap manufacturing.
• In agriculture, KOH is used.
• It helps to extract alumina from ores.

Conclusion  

Hydroxides are a simple group of atoms. Even though it is simple, it is the fundamentals of chemistry. It is denoted by OH-. Arrhenius said that the basic nature of a compound depends on the number of hydroxide ions in it. Hydroxide ions can behave as ligands, bases, catalysts, and nucleophiles.  Hydroxide ions also take part in neutralization reactions. Some hydroxides can have both acid and basic properties. They are called amphoteric hydroxides. Aluminium hydroxide is one example of amphoteric hydroxide. They react differently in acidic and basic solutions. Some applications of hydroxide ions are used as a disinfectant, a strong base to produce fuel cells.