Hydroxide: Structure, Ion, Properties & Uses

Hydroxide is a diatomic ion with the IUPAC name Oxidanide and chemical formula OH⁻. The anion is composed of one atom of oxygen and hydrogen bonded by a covalent bond.

• Hydroxide is a chemical compound that carries a negative electric charge.
• The compound is used as a base, a ligand, a nucleophile, and a catalyst.
• It is also known as Hydroxyl radical, Hydroxyl, or hydroxide ion.
• Some of the salt of the compound is dissociated in aqueous solution by liberating hydroxide ions.
• Hydroxide ion is amphoteric in nature with a valency of -1.
• Some real-life applications of the compound include using the compound as food preservation and disinfectant.

Key Terms: Hydroxide, Anion, Oxygen, Hydrogen, Covalent Bond, Ion, Alkali Metal, Alkaline Earth Metal, Boron Group, Carbon Group, Potassium Hydroxide, Sodium Hydroxide, Ammonium Hydroxide

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What is Hydroxide?

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The hydroxide ion is a negatively charged molecule composed of one oxygen and one hydrogen covalently bound together. It is an extremely strong base when dissolved in water.

• As per the Arrhenius definition, the presence of a hydroxide ion defines a chemical as a base.

Thus, a base is a chemical with a high pH and which has:

• A soapy taste
• A slippery feel
• A favourable reaction to acid

Hydroxide has a negatively charged ion. It is frequently found in ionic compounds covalently attached to positively charged ions. Some hydroxide-containing ionic compounds, such as the harmful bases sodium hydroxide and potassium hydroxide, disperse well in water.

• The neutral compound of the hydroxide anion is the hydroxyl radical. 
• When a hydroxide ion is attached to an electropositive atom, it can ionise itself and liberate hydrogen cation (H+).
• Many organic and inorganic compounds have the word hydroxide in their name.

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Hydroxide Ion

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Hydroxide is a polyatomic ion where hydrogen and oxygen atom carries a negative charge. The base of the compound is a group of hydroxide ions. When it undergoes a self-ionisation reaction, it will form a natural part of water.

• It can be represented by the reaction:

H₃O⁺ + OH^- -------→ 2H2O

• The equilibrium reaction of the compound is given as:

K_w = [H⁺] [OH^-]

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Hydroxide Structure

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The structure of a hydroxide compound consists of a single covalent bond between oxygen and hydrogen atoms. The valency of the compound is -1. 

• Oxygen has a valency of two, and hydrogen has a valency of one.
• It will form a cylindrical symmetry shape when combined with various divalent metals.
• Hydroxides have the formula OH^-.
• In this compound, oxygen and hydrogen form a two-electron bond.
• Since it has obtained an electron, hydrogen has a negative charge.

Hydroxide

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Properties of Hydroxide

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The properties of hydroxide are as follows:

• The molecular mass of the hydroxide compound is 17g/mol.
• They are insoluble in water.
• However, alkali hydroxides are highly soluble in water.
• The conjugate acid and base of the compound are H₂O and anionic oxide, respectively.
• It acts as a catalyst in many organic chemistry reactions.
• pH of the compound is greater than seven.

A short information about Hydroxide is tabulated in the following table:

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Formation of Hydroxide

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The most common method used for the formation of hydroxide involves the hydration reaction of alkali and rare-earth metals. The process involves the addition of sodium metal to water, which results in the formation of sodium hydroxide.

2Na+2H₂O→2NaOH+H₂

• We can also form calcium hydroxide with a similar process.
• Similarly, copper hydroxide can be formed by the exchange reaction between copper sulphate and sodium hydroxide.

CuSO₄+2NaOH→Cu(OH)₂+Na₂SO₄

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Function of Hydroxide

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The function of hydroxide are as follows:

Hydroxide as a Base

A significant proportion of hydroxide-containing compounds are chemical bases. As a result, hydroxide ions are involved in a wide range of acid-base reactions, including the neutralisation reaction.

• An arrhenius base is a substance that produces hydroxide ions when dissolved in aqueous solution.

Hydroxide as a Ligand

A nucleophile or ligand is an ion of hydroxide. Each oxygen atom in the OH ion has a "lone pair" or sometimes referred to as unshared pairs of electrons in its outermost shell, which it donates to an appropriate receiving atom to form bonds.

• A hydroxide ion behaves as a Lewis base.
• Ions that do not participate in acid-alkali reactions are Spectator Ions.

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Inorganic Hydroxides

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The inorganic hydroxides are divided into four categories which are as follows:

Alkali Metals

There are many useful alkali metals, including sodium hydroxide and potassium hydroxide. Lithium hydroxide is also a useful alkali metal, which has a pK_b of -0.36. 

• The compound is used in the submarine purification system for breathing gas.
• It is used in spacecraft for the removal of carbon dioxide and CO₂ from exhaled gas.

LiOH + CO₂ ------→ Li₂CO₃ + H₂O

Alkali Earth Metal

It includes compounds like beryllium hydroxide Be(OH₂), which is amphoteric in nature. When acid is added to it, then it will form a product like trimeric ion [Be₃(OH)₃(H₂O)₆]³⁺, which is the hydroxy (OH) group.

Boron Group

Boric Acid is considered the simplest of hydroxides, which doesn’t disconnect in an aqueous solution. The compound reacts with molecules of water as lewis acid by releasing protons.

B(OH) ₃ + H ₂O ⇌ [BO(OH) ₂] ⁻ + H ₃O

Carbon Group

The carbon group forms a complex category of hydroxide. Some carbon compounds like methanetetraol or orthocarbonic acid form an unstable aqueous solution.

C(OH)₄ → HCO⁻₃ + H₃O⁺

HCO⁻₃ + H⁺ ⇌ H₂CO₃

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Hydroxide in Organic Reaction

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There are two ways in which hydroxide is used in organic reaction which are as follows:

Base Catalysis

Hydroxide is a form of anion that is used as a catalyst in many organic reaction. It will release a proton in case of weak acid which include releasing carbon acids, amines phenols and alcohols.

Nucleophilic Reagent

Hydroxide is used as an intermediate in any nucleophilic reaction. It include compounds like amido ion (NH-) and fluoride ion F^–.

R¹C(O)OR² + OH⁻ ⇌ R¹CO(O)H + ⁻OR² ⇌ R¹CO₂⁻ + HOR²

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Uses of Hydroxide

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There are numerous uses of hydroxide which are as follows:

• Sodium hydroxide (NaOH) is being used in the production of a wide variety of household products.
• It includes paper, infomercial drain cleaners, oven cleaners, soap, detergents, and aluminium.
• Many mineral ores, including bauxite ores and limonite, are hydroxides.
• It is used in industry as a strong base, whereas potassium hydroxide is used in agriculture.
• The compound is used as an anticoagulant for blood and stored buffer.
• It helps in preventing kidney stones.
• Hydroxide is used in organic compounds like sugar, soaps, and lipids.
• Many industries use the compound to produce disinfectants.

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Things to Remember

• Hydroxide is also known as Hydroxy or Hydroxyl radical or hydroxide ion.
• It is made up of a hydrogen and an oxygen atom retained together by a covalent bond.
• The hydrogen atom is negatively charged.
• Hydroxide is commonly used as a flavour enhancer.
• It is one of the most fundamental diatomic ions known.
• The term "hydroxide" also refers to a group of compounds containing the ion hydroxide.
• It functions as a linker, catalyst, base, and positive ion.

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