Sodium hydroxide is a metal hydroxide. It is commonly known as caustic soda.
NaOH is a corrosive solid and is soluble in water.
The most common method of preparation of NaOH is the electrolysis of brine solution.
NaOH is a very useful compound.
Substances can be classified as acids, bases, and salts based on their various properties.
In this article, we will understand various theories explaining the meaning of acids and bases and study the nature of sodium hydroxide in detail.
So, is NaOH an acid or a base? NaOH is a strong base as it completely dissolves in water to release hydroxide ions (along with sodium ions) which are responsible for the basic nature of an aqueous solution. A Compound that releases OH- ions in an aqueous solution is basic in nature. The pH value of NaOH is around 12 classifying it as a strong base.
Why is NaOH a Base?
Different scientists gave different definitions for acids and bases. We will focus on three main theories.
According to this theory, the substance which loses H+ ions in an aqueous solution is an acid, while the substance that loses OH– ions in an aqueous solution is a base.
When NaOH is dissolved in water, NaOH breaks down to release Na+ and OH– ions. Due to the release of hydroxide ions in water, it is classified as an Arrhenius base.
This theory is valid when the solvent is water.
According to this theory, a substance that gains an electron pair is a Lewis acid, while the substance that loses an electron pair is called a Lewis base.
Lewis acids are electron-deficient compounds (like BF3), and Lewis bases are electron-rich compounds. (like NH3).
In the case of NaOH, hydroxide ion is released, which has lone pair of electrons, and other electron-deficient compounds can accept these electrons.
According to this theory, a substance that loses a proton is an acid, while the substance that gains a proton is called a base.
When NaOH reacts with an acid, it accepts the proton lost by the acid and behaves like a Bronsted-Lowry base. In NaOH, OH– accepts the proton. Thus, a higher concentration of hydroxide ions means a stronger base.
Thus, according to all three major theories, NaOH has been classified as a base.
All bases are not soluble in water.
Bases that are soluble in water are called alkali. Sodium hydroxide is an alkali.
What makes NaOH a Strong Base?
A base can be classified as strong or weak (according to Arrhenius theory) depending on the extent of hydroxide ions released on dissolving in water.
A compound is considered a strong base if it dissociates completely in water to release hydroxide ions and the counterions.
For instance, if 1 mole of a strong base is taken, 1 mole of hydroxide ions and 1mole of counter ions would be released. No amount of strong base is left undissociated.
In the case of a weak base, complete dissociation does not occur in water. For instance, if 1 mole of a weak base is taken and dissolved in water, much less than 1 mole of hydroxide ions and counter ions would be released.
Some undissociated base would be left, which is supposed to be in equilibrium with the undissociated base.
When NaOH is dissolved in water, it is dissociated to form Na+ ions and OH– ions. This happens for both strong and weak bases. The difference lies in the extent of dissociation.
NaOH (aq) 🡺 Na+ (aq) + OH–(aq)
Keq = [Na+][OH–]/[NaOH]
Keq * [NaOH] = kb = [Na+][OH–]
For a strong base, the concentration of hydroxide ions is very large, and the concentration of undissociated form is significantly less. This implies that kb is large for strong bases.
NaOH dissociates entirely, and no molecule is left in an undissociated form. Hence, it is a strong base.
Weak Base Vs. Strong Base
|Strong base||Weak base|
|It dissociates completely into hydroxide ions and the counterions (cations) on dissolving in water.||It does not dissociate entirely into hydroxide ions and the counterions on dissolving in water.|
|All the molecules of the base are dissociated.||Some base molecules are dissociated, and others remain in equilibrium with the undissociated base.|
|For a given concentration, they are more reactive.||For a given concentration, they are less reactive.|
|pH value is more than 7 and nearer to 14.||pH value lies in the range of 7-14. The exact value depends on the concentration.|
|It has a high pka value and low pkb value.||It has a low pka value and a high pkb value.|
|It is a strong electrolyte.||It is a weak electrolyte.|
|For example, NaOH, KOH, etc.||For example, NH4OH, NH3, etc.|
For weak bases like NH4OH, the equilibrium is represented as-
Keq = [NH4+][OH-]/[ NH4OH]
kb = Keq * [ NH4OH] = [NH4+][OH-]
Kb is relatively small for weak bases as all the base is not dissociated to form hydroxide ions.
Why is NaOH not an Acid?
NaOH dissociates in water to release hydroxide ions which can accept a proton.
According to Arrhenius concept, acid dissociates to release H+ ions in an aqueous solution, but NaOH releases hydroxide ions instead.
According to the Bronsted-Lowry concept, acid releases a proton in an aqueous solution, while in NaOH, hydroxide ions gain a proton instead of losing.
According to Lewis concept, acids accept an electron pair and are electron deficient, while NaOH loses an electron pair and is electron-rich.
An acid reacts with a base to form salt and water according to a neutralization reaction but does not react with another acid to show a neutralization reaction.
We know that HCl is acid and KOH is a base. NaOH reacts with acid like HCl to form NaCl and water but does not neutralize with another base like KOH.
This implies that NaOH is not an acid.
Conjugate Acid of NaOH
When an acid loses a proton, the species so formed is referred to as the acid’s conjugate base. A very strong acid has a weak conjugate base and vice-versa.
When a base gains a proton, the species so formed is referred to as the base’s conjugate acid. A very strong base has a weak conjugate acid and vice-versa.
We can say that conjugate acid-base pairs differ by proton.
Acid + Base —–> the conjugate base of acid + conjugate acid of the base
NaOH is a strong base. Base gains a proton. In NaOH, hydroxide ions are responsible for the basic nature, and sodium ion is just a spectator ion.
Spectator generally refers to audience. So, sodium ions do not participate; they are just present in the solution to balance the charges.
On gaining a proton, NaOH forms water along with sodium ions. Hydroxide ions accept a proton to form water which is the conjugate acid of NaOH.
As sodium hydroxide is a very strong base, its conjugate acid is supposed to be weak, and water is a weak acid only.
The conjugate acid of NaOH is Water ie; H2O
The Conjugate acid-base concept determines the relative strength of acid and base.
Why is NaOH a Lewis Base?
Lewis base is an electron-rich substance and can donate its electron pairs to another electron-deficient compound or lewis acid. Solvent need not be water for Lewis theory.
NaOH has hydroxide ions. There are lone pairs and formal charges on hydroxide ions that can be donated to an electron-deficient species like a proton (H+) to form water.
Sodium-ion acts as a spectator ion. Thus, in a simple neutralization reaction between NaOH and HCl, NaOH acts as a Lewis base, and HCl acts as a Lewis acid.
pH of NaOH
pH helps in the determination of [H+] in a given solution. It is calculated using the formula-
A high concentration of H+ means that the solution is highly acidic, and pH would be lower.
A solution with a pH of 2 is more acidic than a solution with a pH of 4.
At 25°C, the pH scale ranges from 0 to 14-
• When pH=7, the substance is neutral.
• When pH<7, the substance is acidic.
• When pH>7, the substance is basic.
NaOH is a strong base, and therefore its pH is more than 7 and nearer to 14.
For a base, we need to know the concentration of hydroxide ions, so we have pOH.
pH + pOH=14
if the pH of a substance is 13, pOH would be 1.
We can easily find the strength and concentration of hydroxide ions by measuring the pH of the solution.
For instance, a 0.1 M of NaOH has a pOH of 1 (pOH= -log (0.1)) and pH=13. Alternatively, if we know that a solution has a pH of 13, we can use antilog to find pOH and pH.
What Factors affect the Strength of a Base?
1. Atomic Size: A large atom with high polarizability can donate electron pairs easily and is more basic than a smaller atom.
For example, KOH is more basic than NaOH as K can be easily ionized due to its larger size, and hydroxide can be released faster.
2. Availability of Electrons: The availability of electrons for donating to another species decreases when the electrons are in conjugation with other atoms. This reduced availability decreases the basic strength of a compound.
For example, methylamine is more basic than aniline as lone pairs of nitrogen are delocalized in aniline.
3. Electronegativity: Electronegativity of the atom donating electrons affects the basic strength as electronegative atoms do not donate electrons easily and are less basic.
For example, CH3– is a stronger base than F–.
Acidity of NaOH
The acidity of a base is defined as the number of hydroxide ions that can be produced from one molecule of a base on ionisation.
The basicity of an acid is defined as the number of hydrogen ions that a particular acid can produce on ionisation.
One molecule of NaOH releases one molecule of OH– and can react with one H+ ion to form one water molecule. Thus, sodium hydroxide is a monoacidic base.
The Litmus Test of NaOH
Litmus is one of the natural indicators used to check whether a given substance is acidic, basic, or neutral. Litmus solution is obtained from lichens.
The litmus dye is purple when the solution is neither acidic nor basic.
Generally, red and blue litmus papers are available. Bases turn red litmus blue while acids turn blue litmus red. Acids do not affect red litmus paper, while bases do not affect blue litmus paper.
NaOH is a strong base and turns the colour of red litmus to blue and has no effect on blue litmus paper.
NaOH is a strong base in water as it completely dissociates to form hydroxide ions which can accept protons to form water.
The pH of NaOH lies above 7 and near to 14.
Conjugate acid of NaOH is water.
NaOH is monobasic.
A weak base does not dissociate entirely, and the pH lies in the range of 7 to 14.
Litmus paper can be used to check whether a compound is acidic or basic, or neutral. NaOH turns red litmus blue.