Unit 7: Acid Base Chemistry — Long Questions

Arrhenius concept of acids and bases
Svante Arrhenius, a Swedish Chemist, suggested that acids and bases may be classified in terms of their behavior in water. According to him;

Arrheniius acid
An acid is that substance which dissociates in water to give proton (H⁺) or hydronium ion (H₃O⁺). Some typical Arrhenius acids are HCl, HNO₃, H₂SO₄ and HCN.

Example
HCl(aq) ⇌ H₃O⁺(aq) + Cl⁻(aq) (with H₂O label above arrow)

Arrhenius Base
A base is that substance which dissociates in water to give hydroxyl ions (OH⁻). Some typical Arrhenius bases are NaOH, KOH and Ba(OH)₂.

Example
NaOH(aq) ⇌ Na⁺(aq) + OH⁻(aq) (with H₂O label above arrow)

Role of Water
Water has an essential role to play in Arrhenius concept of acids and bases. Whenever an acid or a base dissociates in water, its molecules participate in reaction by surrounding the resultant proton (H⁺) and hydroxyl ion (OH⁻). Since proton is very small in size and its charge density is very high, it forms a strong bond with the lone pair of water molecule to give hydronium ion (H₃O⁺).
H₂O + H⁺ ⇌ [H-O-H] with H below the bracket

COMPARE

Aspect | Arrhenius concept | Bronsted Lowry concept

Definition of Acid An acid is a substance that donates a proton when dissolved in water. | An acid is a substance that donates a proton but the condition of the presence of water during this donation was however eliminated.

Definition of Base A base is a substance that donates OH⁻ ion in water. | A base is a substance that accepts a proton.

Medium This concept is limited to aqueous medium. | This concept is applicable in any solvent system or may not require any solvent.

Major similarity and difference. All Arrhenius acid are Bronsted Lowry acids. | All Bronsted Lowry bases are not Arrhenius bases.

Limitations This concept is limited to aqueous solutions and does not account for acid-base reactions that occur in non-aqueous medium. Also is limited to substances which contain H⁺ and OH⁻ ions but cannot explain the nature of substances.
For example, Na₂CO₃, K₂CO₃ and NH₃ do not contain any hydroxyl group which will get ionized by water but all these compounds behave as bases and yield OH⁻ in water. | Although this theory is broader than Arrhenius concept, it still has certain limitations.
For example, it can't explain the nature of SO₂ and CaO which are acid and base respectively. But cannot donate H⁺ ion or accept H⁺ ion.

(i) With Ammonium Chloride (NH₄Cl):
When sulphuric acid reacts with ammonium chloride, it produces ammonium sulphate (NH₄)₂SO₄ and hydrochloric acid (HCl). The reaction can be represented as:
H₂SO₄ + 2NH₄Cl → (NH₄)₂SO₄ + 2 HCl

(ii) With Ammonium Nitrate (NH₄NO₃):
The reaction of sulphuric acid with ammonium nitrate results in the formation of ammonium sulphate and nitric acid (HNO₃). The equation for this reaction is:
H₂SO₄ + 2NH₄NO₃ → (NH₄)₂ SO₄ + 2 HNO₃

(iii) With Magnesium Oxide (MgO):
Sulphuric acid reacts with magnesium oxide to produce magnesium sulphate (MgSO₄) and water (H₂O). The balanced reaction is:
H₂SO₄ + MgO → MgSO₄ + H₂O

(iv) With Magnesium Carbonate (MgCO₃):
When magnesium carbonate reacts with sulphuric acid, it produces magnesium sulphate, carbon dioxide (CO₂), and water. The reaction can be written as:
H₂SO₄ + MgCO₃ → MgSO₄ + CO₂ + H₂O

Nature of Reaction.
When a base reacts with a non-metallic oxide, a salt and water are typically formed. This reaction is a type of neutralization reaction. Non-metallic oxides, such as carbon dioxide (CO₂) or sulphur dioxide (SO₂), are generally acidic in nature in this type of reaction.

Example For example, when sodium hydroxide (a base) reacts with carbon dioxide (a non-metallic oxide), the following reaction occurs:
2NaOH + CO₂ → Na₂CO₃ + H₂O

By Product Nature & Non-Metallic Oxides
In this case, sodium carbonate (Na₂CO₃) is formed along with water.
We can infer that non-metallic oxides tend to exhibit acidic properties when they react with bases. This characteristic is due to their ability to react with bases to produce salts and water, indicating that they can act as acid in a chemical reaction. Thus, non-metallic oxides are often classified as acidic oxides.

The acidic character of both dry hydrogen chloride (HCl) gas and HCl solution in water can be explained by their ability to donate protons (H⁺) in a chemical reaction.

(i) Dry HCl Gas: Even in its gaseous form, HCl can act as an acid. When it comes into contact with moisture or water vapor in the environment, it can dissociate to release H⁺ ions. Although dry HCl gas is not an acid in the traditional sense, it can still exhibit acidic properties when it interacts with suitable substance.

(ii) HCl Solution in Water: When HCl is dissolved in water, it completely dissociates into H⁺ ions and Cl⁻ ions. The presence of H⁺ ions in the solution gives the solution its acidic character. The more H⁺ ions present, the stronger the acidity of the solution.

An acid and its conjugate base are related but represent different forms of a chemical species in a proton transfer reaction.

Definitions An acid is a substance that can donate a proton (H⁺) to another substance.
A conjugate base is a species which is formed by donating a proton by an acid.

Example When hydrochloric acid (HCl) donates a proton, it becomes its conjugate base, which is the chloride ion (Cl⁻).
The reaction can be represented as follows:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻

Acid Base Conjugate acid conjugate base

Strength Acids can vary in strength, with strong acids completely dissociating in water, while weak acids partially dissociate.
The conjugate base of a strong acid is typically weak and does not readily accept protons, while the conjugate base of a weak acid is stronger and can accept protons more easily.

Role in Reactions Acids are proton donors, while conjugate bases act as proton acceptors in acid-base reactions. This relationship is essential in understanding the concept of acid-base equilibria.

Acids play significant roles within the human body is. Following are the some importance of acids in various physiological processes are as:

(i) Digestion: One of the most well-known acids in the human body is hydrochloric acid (HCl), which is secreted by the stomach lining. It creates an acidic environment that helps break down food, activates digestive enzymes, and kills harmful bacteria that may enter with food.

(ii) pH Balance: Acids are crucial for maintaining the body's pH balance. The human body operates optimally at a specific pH range and various acids help regulate this balance. For example, carbonic acid plays a key role in maintaining blood pH.

(iii) Metabolism: Many metabolic processes involve acids. For instance, lactic acid is produced during anaerobic respiration when glucose is broken down for energy in the absence of oxygen. This process is essential during intense exercise when the body requires quick energy.

(iv) Cellular Functions: Acids are involved in various cellular functions, including the synthesis of DNA and RNA. Nucleic acids, which are essential for genetic information, contain acidic components.

(v) Electrolyte Balance: Acids contribute to the balance of electrolytes in the body, which is vital for nerve function and muscle contraction. For example, the dissociation of acids in solution can release ions that help conduct electrical signals.

When carbon dioxide (CO₂) is passed through lime water, which is a dilute solution of calcium hydroxide Ca(OH)₂, different reactions occur depending on the duration of exposure.

1. For a Short Duration: When CO₂ is bubbled through lime water for a short time, a reaction occurs where carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO₃), which is a white precipitate. The solution will turn milky due to the formation of this precipitates. The reaction can be represented as:
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(ℓ)

2. For a Long Duration: If CO₂ is passed through lime water for an extended period, the initial precipitate of calcium carbonate will eventually dissolve in excess CO₂, forming calcium bicarbonate Ca(HCO₃)₂, which is soluble in water. As a result, the milky appearance will disappear, and the solution will become clear again. The reaction for this process can be represented as:
CaCO₃(s) + CO₂(g) + H₂O(ℓ) → Ca(HCO₃)₂(aq)

Bronsted – Lowry Acids An acid is a substance that donates a proton (H⁺).
This definition requires that to behave as an acid a compound must have a proton to donate. The condition of the presence of water during this donation was, however, eliminated. All Arrhenius acids are, Bronsted-Lowry acids as well for example, HCl/ It dissociates in water to give H⁺ and Cl⁻ . It also donates H⁺ to H₂O forming H₃O⁺.

Bronsted-Lowry Base A base is a substance that accepts a proton.
For example, OH⁻, NH₃ and Cl⁻ are all bases because they have the ability to accept a proton. Note that except OH⁻ all other species are not Arrhenius bases. All Arrhenius bases are, however, Bronsted-Lowry bases as well.

Bronsted-Lowry and Arrhenius Acid and Base:
All Bronsted-Lowry acids and bases are not Arrhenius acids and bases. NH₄⁺ is not Arrhenius acid and NH is not Arrhenius base.
According to Bronsted-Lowry, an acid base reaction is that reaction in which a proton is transferred from a proton donor to its acceptor. This reaction may take place in gas phase or in the presence of any solvent.

Example (i) Consider the following reaction between hydrogen chloride gas and liquid water:
HCl(g) + H₂O(l) ⇌ H₃O⁺(aq) + Cl⁻

Acid Base

In this reaction, HCl gas acts as an acid because it donates its proton to water which acts as a base.

(ii) Similarly when ammonia gas dissolves in water, a proton is transferred from water to ammonia and ammonium ion is formed.
NH₃(g) + H₂O(l) ⇌ OH⁻(aq) + NH₄⁺(aq)

Base Acid Conjugate base Conjugate acid

Ammonia is a base while water is an acid in this reaction. Water has the ability to act both as an acid or a base depending upon the other compound with which it reacts.

Amphoteric Compound Water is therefore called an amphoteric compound which means a compound that can behave both as an acid and a base.
In the reverse reaction, OH⁻ is a base because it accepts a proton donated by the acid NH₄⁺. In order to differentiate, OH⁻ is called the conjugate base while NH₄⁺ the conjugate acid.

Examples Some other examples of Bronsted-Lowry acids and bases.
HCN(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CN⁻(aq)

Acid Base Conjugate acid Conjugate base

(a) Properties of Acids:

Reaction of Acid with metal oxide and Alkalis:
(i) With alkalis or metal oxides, they form salts and water.
2HNO₃(aq) + CaO(s) ⟶ Ca(NO₃)₂(aq) + H₂O(ℓ)
H₂SO₄(aq) + 2KOH(aq) ⟶ K₂SO₄(aq) + 2H₂O(ℓ)

(ii) Reaction of Acid with metals:
With reactive metals (Mg, Zn) they form salts and evolve hydrogen gas.
Mg(s) + 2HCl(aq) ⟶ MgCl₂(aq) + H₂(g)
Zn(s) + H₂SO₄(aq) ⟶ ZnSO₄(aq) + H₂(g)
The unreactive metals Cu, Ag, Au and Pt do not evolve hydrogen gas with acids.

(iii) Reaction of acid with carbonate and hydrogen carbonate:
They decompose metal carbonates and hydrogen carbonates evolving carbon dioxide gas.
CaCO₃(s) + 2HCl(aq) ⟶ CaCl₂(aq) + H₂O(ℓ) + CO₂(g)
2NaHCO₃(s) + H₂SO₄(aq) ⟶ Na₂SO₄(aq) + 2H₂O(ℓ) + 2CO₂(g)

(b) Properties of Alkalis
Alkalis are those bases which are soluble in water. Examples of alkalis are NaOH, KOH etc. Ca(OH)₂ is sparingly soluble in water while Cu(OH)₂ is insoluble.

(i) Reaction of Alkalis and Acid:
Alkali react with acid to form salt and water
NaOH(aq) + HCl(aq) ⟶ NaCl(aq) + H₂O(ℓ)
Ca(OH)₂(aq) + H₂SO₄(aq) ⟶ CaSO₄(aq) + 2H₂O(ℓ)

(ii) Reaction of Alkalis with Ammonium salt:
Alkali react with ammonium salt and to liberate NH₃ gas.
Ca(OH)₂(aq) + 2NH₄Cl(aq) ⟶ CaCl₂(aq) + 2H₂O(ℓ) + 2NH₃(g)
NaOH(aq) + NH₄NO₃(aq) ⟶ NaNO₃(aq) + H₂O(ℓ) + NH₃(g)