[Chemistry Class Notes] on Acids and Bases Pdf for Exam

The acid definition is given as any hydrogen that contains a substance capable of donating a proton (a hydrogen ion) to the other substance. A base is an ion or molecule that is able to accept a hydrogen ion from an acid.

Usually, the acidic substances are identified with their sour taste. Basically, an acid is a molecule that can donate an H+ ion and also can remain energetically favorable after a loss of H+ ion. Acids are much known to turn blue litmus into the red.

On the other side, bases are characterized by a slippery texture and a bitter taste. A base that is dissolved in water is known as an alkali. When these substances react chemically with acids, they further yield salts. Besides, the bases are much known to turn red litmus into blue.

Acid-Base Theories 

To define acids and bases, three alternative hypotheses have been proposed. The Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases are among these hypotheses. This subsection provides a brief overview of each of these theories. There are three hypotheses that can be used to define acids and bases.

1. “An acid generates H+ ions in a solution, whereas a base generates an  OH– ion in its solution,” according to the Arrhenius theory of acids and bases.

2. “An acid is a proton donor, while a base is a proton acceptor,” according to the Bronsted-Lowry theory.

3. Finally, the Lewis definition of acids and bases depicts acids and bases as “electron-pair acceptors” and “electron-pair donors,” respectively.
   

Arrhenius Theory of Acids and Bases 

According to the Arrhenius theory, acid is a chemical that when dissolved in water produces the H+ ion. It raises the H+ ion concentration in the solution. The base is a chemical that dissolves in an aqueous solution and ionises the OH– ion. Thus the solution contains a high concentration of OH– ions.

In an aqueous solution, Arrhenius acid increases the concentration of protons or H+ ions. 

Arrhenius Acids 

Definition of acids according to Arrhenius: Svante Arrhenius, a Swedish chemist, first presented the Arrhenius hypothesis of acids and bases in 1884. He proposed categorising certain substances as acids or bases depending on the type of ions generated when they were combined with water.

Hydrochloric acid in the water, for example. As stated below, HCl undergoes a dissociation reaction to produce H+ and Cl– ions. The formation of hydronium ions raises the concentration of  H+ ions.

HCl (aq) → H+(aq) + Cl–(aq)

HCl (aq) + H₂O(l) → H₃O+(aq) + Cl–(aq)

Arrhenius Base

A substrate called an Arrhenius base raises the concentration of hydroxide ions in an aqueous solution. The extremely soluble sodium hydroxide molecule in water, which dissociates to generate sodium ion and hydroxide ion, is an Arrhenius base example.

To increase the concentration of hydroxide ions in an aqueous solution, NaOH entirely dissolves to give hydroxide ions and sodium ions.

NaOH(aq) → Na+(aq) + OH–(aq).

Bronsted Theory of Acids and Bases 

Bronsted Lowry Acid

An acid is defined as a donor of protons by the Bronsted-Lowry theory.

According to this hypothesis, a base is a proton acceptor (or an  H+ ion acceptor).

Bronsted acids dissociate to release protons, resulting in a higher concentration of  H+ ions in the solution.

Acid ⇌ Proton + Conjugate base

Bronsted Lowry Base 

Bronsted bases, on the other hand, take protons from the solvent water to produce hydroxide ions.

The capacity to explain the acidic or basic character of ionic species is one advantage of the Bronsted-Lowry definition of acids and bases.

This theory has a significant flaw in that it does not explain how chemicals that lack hydrogen, such as BF₃ and AlCl₃, display acidic characteristics.

Base + Proton ⇌ Conjugate acid

Strong Bronsted-Lowry acids are those that have a strong inclination to donate a proton but have a weak conjugate base.

Weak Bronsted-Lowry acids have a slight tendency to give a proton, while their conjugate base is strong.

Lewis Concept of Acids and Bases

Lewis acid: An acid, according to Lewis’ definition, is a species with an empty orbital and hence the ability to take an electron pair.

A Lewis base is a species that has a single pair of electrons and hence can operate as an electron donor.

The hydrogen atom is not included in this theory’s description of acids and bases.

Lewis acids have electrophilic properties, while Lewis bases have nucleophilic properties.

Cu²⁺, BF₃, and Fe³⁺ are examples of Lewis acids.  F–, NH₃, and C₂H₄ are examples of Lewis bases (ethylene).

A Lewis acid absorbs an electron pair from a Lewis base, resulting in the formation of a coordinate covalent bond. Lewis adduct is the name given to the resultant chemical.

This notion has the advantage of allowing various substances to be classified as acids or bases. However, it provides little information about the acid and base’s strength.

One of the theory’s flaws is that it doesn’t account for acid-base reactions that don’t involve the development of a coordinate covalent bond.

pH of Acids and Bases

To find the numeric value of the acidity or basicity level of a substance, the pH scale (pH stands for ‘potential of hydrogen’) can be used. Here, the pH scale is the most common and trusted procedure to measure how acidic or basic a substance is. Also, a pH scale measure can differ from 0 to 14, where 14 is the most basic, and 0 is the most acidic a substance can be.

The other way to check if a substance is acidic or basic is by using a litmus paper. There exist two types of litmus paper available, used to identify the acids and bases. They are the red litmus paper and the blue litmus paper. The blue litmus paper changes red under acidic conditions, whereas the red litmus paper turns blue under alkaline or basic conditions.

Properties of Acids and Bases

Properties of Acids

• Acids are good conductors of electricity.

• They are corrosive in nature.

• When reacted with metals, acid substances produce hydrogen gas.

• Always, their pH values are less than 7.

• Acids are sour-tasting substances.

• Examples of acids are Hydrochloric acid [HCl], Sulfuric acid [H₂SO₄], Acetic acid [CH₃COOH], and more.

Properties of Bases

Some of the properties, such as a bitter taste, are owned by all bases. The bases also feel slippery. You can dream about what slippery soap looks like, and this is a foundation. Also, bases conduct electricity when immersed in water because they consist of charged particles in the solution.

• Bases are found to have a soapy texture when we touch them.

• When dissolved in water, these substances release hydroxide ions (OH– ions) 

• Bases act as good conductors of electricity in their aqueous solutions

• Always, the pH values corresponding to the bases are greater than 7.

• Bases are bitter-tasting substances, having the ability to turn red litmus paper into blue.

• Examples can be given as milk of magnesia [Mg(OH)₂], Sodium Hydroxide [NaOH], Calcium Hydroxide [Ca(OH)₂], and more.
  

Neutral Substances

These are the substances, which have no properties of either acid or base, which has a similar amount of hydroxyl ions and hydrogen ions, and they do not modify the color of the litmus surface.

• Neutral substances do not display any acidic or basic characteristics.

• Their pH values approximately 7.

• Neutral substances have no effect on blue or red litmus paper.

• pH of pure water is exactly 7.

• Examples are Common salt (NaCl), Water, and more.
  

Uses of Acids and Bases

Various uses of acids and bases can be listed as follows:

Uses of Acids

• Vinegar, which is a diluted solution of acetic acid, has different household applications. It is used primarily as a food preservative.

• Citric acid is an integral part of orange juice and lemon juice. It is also used as a food preservative.

• Sulfuric acid is more widely used in batteries. Commonly, the batteries used to start automobile engines contain this acid.

• The industrial production of dyes, explosives, paints, and fertilizers involves the use of nitric acid and sulfuric acid.

• Phosphoric acid is a primary ingredient in various soft drinks.
  

Uses of Bases

• The manufacturing of paper and soap involves the use of sodium hydroxide. Also, NaOH is used in the manufacturing of rayon.

• Ca(OH)₂, which is also called calcium hydroxide or slaked lime, is used to manufacture the bleaching powder.

• Dry mixes used in decoration or painting are made using a limited amount of calcium hydroxide.

• Magnesium hydroxide, also called the milk of magnesia, is most commonly used as a laxative. It also reduces if there is any excess acidity in the human stomach and is, thus, used as an antacid substance.

• Ammonium hydroxide is an important reagent that is used in laboratories.

• Any excess acidity in soils is neutralized by employing the slaked lime.