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Earlier, acids, bases, and salts were characterized by the experimental testing of their aqueous solutions. An acid is defined as a substance whose water solution tastes sour, turns blue litmus red and neutralizes bases. A substance is called base if its aqueous solution tastes bitter, turns red litmus blue or neutralizes acids.
Salt is a neutral substance whose aqueous solution does not affect litmus. According to Faraday: acids, bases, and salts are termed as electrolytes. Further, Liebig proposed that acids are compounds which contain hydrogen that can be replaced by metals.
Acids
Acidity is a characteristic property of acids. Acidic substances are usually very sour. Apart from hydrochloric acid, there are many other types of acids around us. Citrus fruits like lemons and oranges contain citric and ascorbic acids while tamarind paste contains tartaric acid.
In fact, the word ‘acid’ and ‘acidity’ are derived from the Latin word ‘acidus’ which means sour. If you dip a blue litmus paper into an acid, it will turn red while a red litmus paper will not change colour. Acids also liberate dihydrogen when they react with some metals.
Bases
Bases turn red litmus paper blue while the blue litmus paper stays blue. They taste bitter and also feel soapy. Some other common examples of bases include sodium bicarbonate that is used in cooking and household bleach.
Image: Litmus paper test. [Source: Wikimedia Commons]
Salts
Apart from sodium chloride, other common salts are sodium nitrate, barium sulfate etc. Sodium chloride or common salt is a product of the reaction between the hydrochloric acid (acid) and sodium hydroxide (base). Solid sodium chloride is made of a cluster of positively charged sodium ions and negatively charged chloride ions held together by electrostatic forces.
Electrostatic forces between opposite charges are inversely proportional to the dielectric constant of the medium. In other words, we can say that a compound that has acidity in its nature and a compound that has basicity as its nature, may yield salts when combined together.
The universal solvent, water, has a dielectric constant of 80. Therefore, when sodium chloride is dissolved in water, the dielectric constant of water reduces the electrostatic force, allowing the ions to move freely in the solution. They are also well-separated due to hydration with water molecules.
Image: Dissolution of sodium chloride in water [Source: Wikimedia Commons]
These are some examples of acids and bases:
Ionization And Dissociation
Dissociation is the separation of ions from an ionic crystal when a solid ionic compound dissolves in water. On the other hand, ionization is the process where a neutral molecule breaks into charged ions when dissolved in a solution. The extent of ionization depends on the strength of the bonds between ions and the extent of solvation of ions.
The three most important modern concepts of acids and bases are:
Arrhenius Concept
According to Arrhenius concept, Substances which produce H+ ions when dissolved in water are called acids while those which ionize in water to produce OH– ions are called bases.
HA → H+ + A– (Acid)
BOH → B+ + OH– (Base)
Arrhenius proposed that acid-base reactions are characterized by acids if they dissociate in aqueous solution to form hydrogen ions (H+) and bases if they form hydroxide (OH–) ions in aqueous solution.
Limitations of Arrhenius Concept
Bronsted-Lowry Concept
Bronsted and Lowry in 1923 independently proposed a more general definition of acids and bases. According to them, an acid is defined as any hydrogen-containing material (molecule, anion or cation) which can donate a proton to other substance and a Base is any substance(molecule, cation or anion) that can accept a proton from any other substance. Therefore, acids are proton donor whereas bases are proton acceptor.
Conjugate Acid-Base Pairs
Consider a reaction
Acid1 + Base2 → Acid2 + Base1
H2O + HCl ⇔ H3O+ + Cl–
In this reaction, HCl donates a proton to H2O and is, therefore an acid. Water, on the other hand, accepts a proton from HCl, and is, therefore, a base. In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction, the H3O+ ions donate a proton to Cl– ion, hence H3O+, an ion is an acid. Cl– ion, because it accepts a proton fromH3O+ ion, is a base.
Acid-base pairs in which the members of reaction can be formed from each other by the gain or loss of protons are called conjugate acid-base pairs.
Limitations of Bronsted Lowry Concept
Lewis Concept
According to Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. Thus, it can be said that a Lewis acid is electron-pair acceptor.
The advantage of the Lewis theory is that complements the model of oxidation-reduction reactions. Oxidation-reduction reactions take place on a transfer of electrons from one atom to another, with a net change in the oxidation number of one or more atoms.
The Lewis theory further suggested that acids react with bases and share a pair of electrons but there is no change in the oxidation numbers of any atoms. Either an electron is transferred from one atom to another, or the atoms come together to share a pair of electrons.
Al(OH)3 + 3H+ → Al3+ + 3H2O (Aluminium hydroxide is acting as a base)
Al(OH)3 + OH– → Al(OH)4- (Aluminium hydroxide is acting as an acid)
These reactions are showing clearly: When Aluminium hydroxide accepts protons, it acts as a base. When it accepts electrons, it acts as an acid. This Lewis acid-base theory also explains why non-metal oxides such as carbon dioxide dissolve in H2O to form acids, such as carbonic acid H2CO3.
CO2(g) + H2O(l) → H2CO3(aq)
Limitations of Lewis Concept
Introduction
Litmus Solution
Acid or Base in a Water Solution
pH Scale
Importance of pH in Everyday Life
Bleaching Powder
Baking Soda
Washing Soda
Plaster of Paris
All acids generate hydrogen gas in reaction with metals. Hydrogen is common to all acids. In fact,
Properties of Acids Bases and Salts
Properties of Acids
On the basis of strengths or capacity to donate hydrogen ions, acids can be described as:
Properties of bases
2NaOH + Zn →
→Na2ZnO2 + H2
Strength of bases
⇌
Na+(aq) + OH–(aq)
⇌
NH4+(aq) + OH–(aq)
On the basis of the number of hydroxide groups present in a base, they can be classified as monobasic (one OH–), dibasic (two OH–), tribasic (three OH–) bases and so on.
Arrhenius Concept of Acid and bases
Arrhenius concept is the oldest concept to explain acids and bases. According to this concept.
HCl(g) →H2O
→H2O H+(aq) + Cl-1(aq)
Such types of acids are called Arrhenius acids, like nitric acid (HNO3), acetic acid (CH3COOH), etc.
NaOH(s) →H2O
→H2O Na+(aq) + Cl-1(aq)
Properties of Salts
→ NaCl + H2O
→ NH4Cl + H2O
→ CH3COONa + H2O
→ CH3COONH4 + H2O
The pH of a salt solution depends on the strength of acids and bases combined in the neutralization reaction.
Is Salt An Acid or A Base?
Acids form hydrogen ions (H+) in aqueous solution. In place of hydrogen ions, it’s better to write hydronium ions (H3O+) which form due to reaction of H+ with water. For example, Hydrochloric acid (HCl) in water.
HCl + H2O →
→ H3O+ + Cl–
At the same time bases are the substances that produce hydroxide ion OH- in water. Acids and bases react with one another to yield two products: water, and an ionic compound known as salt. The reaction is called as neutralization reaction.
NaOH + HCl →
→ H2O + Na+Cl–
Neutralization reactions are always exothermic in nature, which means they liberate the same amount of heat. This amount is fixed for any acid and base that is 57.7kJ per mole of H+. For any acid and base combination the net reaction is
H+(aq) + OH– (aq) →
→ H2O
Because salt produced in neutralization reaction is ionic in nature and exists in the form of ions just before the reaction. Hence salts are the main products of a neutralization reaction.
Formation of Ammonium
When ammonia is dissolved in water it forms ammonium ions (NH4+) and hydroxyl (OH–) ions by the formation of a co-ordinate covalent bond.
The acidic property of an acid is due to the presence of hydrogen ions (H+) while that of a base or alkali is due to the presence of hydroxyl (OH–) ions in them.When strong acids and bases (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.
H+(aq) + OH–(aq) →
→ H2O(l)
Neutralization can be viewed as a reaction in which an acid combines with a base, neutralizing the positively charged hydrogen ion and the negatively charged hydroxyl ion, to form a molecule of water and the respective salt.
Addition of Acids or Bases to Water
The process of dissolving an acid or a base in water is a highly exothermic one. As this reaction generates a lot of heat, care must be taken while mixing concentrated acids with water, specially nitric acid or sulfuric acid with water.
As a rule: The acid must be added slowly to the water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns.
The glass container may also break due to excessive local heating and cause damages! Mixing an acid or base with water results in dilution. It decreases the concentration of ions (H3O+/OH–) per unit volume thereby dissipating the heat effect easily. The list of acids and bases are shown below. Naming of acids and bases is done on the acid base scale.
ASSIGNMENT : ACIDS, BASES AND BASES ASSIGNMENT S.6 MARKS : 10 DURATION : 1 week, 3 days