Difference Between Alkali and Base

Introduction

The concept of acids and bases had evolved centuries ago. Initially they were classified on the basis of their taste i.e., acids taste sour and so is their name (Latin word acidus which meant sour) whereas bases taste bitter. Scientific classification of acids and bases was first given by Arrhenius. However, with time, the definition has changed in order to generalize and incorporate all the acids and bases under a single definition respectively. Alkalis are bases only but can be dissolved in the water. In this article our focus would be bases and alkalis.

 

What is Base?

There are 3 different definitions for a base and each one of them defines a base in its own way. These 3 definitions for a base are as follows: 

  1. Arrhenius Concept: Any compound which furnishes OH- ions in its aqueous solution is termed as a base. This definition covers all the hydroxides of alkali metals and alkaline earth metals bases such as NaOH, Ca (OH)2 etc. This is a simplest definition of a base. In general bases are the compounds that react with acids to produce salt. This definition covers some of the bases but leaves out many. So, other definitions of bases were given by different scientists.

  2. Bronsted Lowry Concept: Any compound that is capable of accepting a proton i.e., a H+ ion is termed as a base.  By this definition, even H2O can act as a base as it can acquire a proton to form hydronium ion i.e., H3O+ ion. This definition covers most of the bases but still leaves a few of the bases because not all bases extract H+. AlCl3 acts as a Lewis acid but it has no Hto furnish. So, bases which react with such acids can’t be explained by this definition. So, another definition of a base was given by Lewis.

  3. Lewis Concept: Any compound that can donate a lone pair or can donate a pair of electrons is termed as a base. For instance: NH3 is a Lewis base. It has a lone pair which it can donate. Lewis base usually extract protons and become stable such as NH3, that acquires a proton to form NH4+.

So, according to this definition any compound, ion or element which can donate a pair of electrons to the other species can act as a base. It can be CuO, ZnO. It is a more generalized definition. NaOH, and KOH furnish OH- which can donate a pair of electrons. Therefore, Lewis concept covers most of the aspects of a base.

All these definitions of bases are useful according to the need of the situation. Any compound that follows any of the 3 definitions can act as a base; so, these 3 definitions incorporate all the needed criteria for defining a base.

Bases react with acids to form a salt and/or H2O. Example NaOH reacts with HCl to form NaCl and water. NH3 reacts with HCl to form NH4Cl. 

Bronsted Lowry bases and Lewis bases do not furnish OH- directly; instead when dissolved in water (a protic solvent), they take up H+ from the water, thereby releasing or furnishing OH-indirectly.

 

What is Alkali?

Alkalis are water soluble bases. In other words, bases which can be dissolved in water to furnish OH- ions are termed as alkali.

Alkalis are like a subset of bases. So, we can say that “All alkali are bases but all bases are not alkali”. The term alkali is mainly used for the hydroxides of alkali metals or alkaline earth metals as they easily dissolve in water to furnish OH- ions.

Alkali turn red litmus blue and are bitter to taste. And, pH of a alkali solution is greater than 7.

Some examples of alkali are: 

NaOH, KOH, Be (OH)2, Ca (OH)2

Sodium hydroxide (NaOH) – caustic soda

Potassium hydroxide (KOH) – caustic potash

Calcium hydroxide, {Ca (OH)2 } – limewater

 

Difference Between Alkalis and Bases

In general, people use these two terms interchangeably but there is a basic difference between the two. Alkali are hydroxides of alkali metals and alkaline earth metals. They do not include NH3. So as per this definition, NH3 is not an alkali even though it is a base. Similarly, there’s another definition to it which goes this way: The bases which dissolve in water to furnish OH- are alkalis. This definition also excludes NH3.

So the difference lies in the fact that alkali is like a subset of bases. That’s why we can say, all alkali are bases but not all bases are alkali.

 

Base & Alkali 

The above figure states the same. CuO and ZnO, both are bases but are not alkali as they do not furnish OH- in water by themselves unlike NaOH, KOH which furnish OH- when dissolved in water.

In many organic reactions, specially alkalis are used because OH-  furnished by an alkali acts as a strong nucleophile as well. So, alkalis are preferred when we require a nucleophile and a base at the same time. However, for some of the reactions, specially bases are required which do not act as nucleophiles and in such cases Lewis bases are used such as NH3.

The proper difference between an alkali and a base is observed in organic reactions because most of the reactions are carried out in a solvent medium and according to the need of the reaction, we use a base or an alkali. 

One important difference between a base and an alkali is that alkalis are mostly ionic compounds which can dissociate when dissolved in a polar protic solvent, but bases may or may not be ionic compounds. CuO, ZnO, NH3 these all are covalent compounds and act as base. NaOH, and KOH are important alkalis and are ionic in nature.