Write short notes on any two. [2x5=10] i) Characteristics of homologous series ii) Laboratory preparation of ethyne. iii) Relation between Kp and Kc. iv) Advantages of modern periodic table.

i) Characteristics of Homologous Series

A homologous series is a group of organic compounds that share the same general formula, similar chemical properties, and a gradual change in physical properties due to the addition of a CH₂ unit. Below are the key characteristics of a homologous series:

1. Same General Formula

• Compounds in a homologous series follow a common molecular formula.
• Example:
  • Alkanes: CₙH₂ₙ₊₂ (e.g., methane CH₄, ethane C₂H₆, propane C₃H₈, etc.)
  • Alkenes: CₙH₂ₙ (e.g., ethene C₂H₄, propene C₃H₆, etc.)
  • Alkynes: CₙH₂ₙ₋₂ (e.g., ethyne C₂H₂, propyne C₃H₄, etc.)

Methane (CH₄)Methane (CH₄)Ethane (C₂H₆)Ethane (C₂H₆)Propane (C₃H₈)Propane (C₃H₈)Ethene (C₂H₄)Ethene (C₂H₄)Propene (C₃H₆)Propene (C₃H₆)Ethyne (C₂H₂)Ethyne (C₂H₂)

2. Similar Chemical Properties

• Compounds in the same series undergo similar chemical reactions because they have the same functional group.
• Example:
  • All alkanes undergo combustion and substitution reactions.
  • All alkenes undergo addition reactions (e.g., hydrogenation, halogenation).

3. Gradual Change in Physical Properties

• As the molecular mass increases (due to the addition of CH₂), there is a gradual change in physical properties such as:
  • Melting and boiling points increase.
  • Density increases.
  • Solubility in water decreases.
• Example:
  • Methane (CH₄) is a gas at room temperature, while hexane (C₆H₁₄) is a liquid.

4. Difference in Molecular Mass by CH₂ Unit

• Each successive member differs by a CH₂ unit, leading to an increase in molecular mass by 14 u (atomic mass of CH₂).
• Example:
  • Ethane (C₂H₆) → Propane (C₃H₈): Increase by CH₂ (mass = 14 u).

5. Same Type of Bonding

• Compounds in a homologous series have the same type of bonding (e.g., all alkanes have only single covalent bonds).

6. Identical Functional Group

• All members have the same functional group, which determines their chemical behavior.
• Example:
  • Alcohols (–OH group), carboxylic acids (–COOH group), etc.

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ii) Laboratory Preparation of Ethyne (C₂H₂)

Ethyne (acetylene) is prepared in the laboratory by the hydrolysis of calcium carbide (CaC₂). The reaction involves treating calcium carbide with water.

CaC₂+H₂OWaterethyneethyne+Ca(OH)₂

Reaction:

$$\mathrm{CaC}{}_2+2\mathrm{H}{}_2\mathrm{O}\to \mathrm{C}{}_2\mathrm{H}{}_2+\mathrm{Ca}(\mathrm{OH}){}_2$$

Procedure:

1. Materials Required:

   • Calcium carbide (CaC₂)
   • Water (H₂O)
   • Delivery tube
   • Gas jar
   • Beaker

2. Steps:

   • Take a small amount of calcium carbide in a beaker.
   • Slowly add water dropwise to the calcium carbide.
   • Ethyne gas is evolved, which can be collected by the downward displacement of water (since ethyne is less dense than air).
   • The gas is passed through a wash bottle containing water to remove impurities (like ammonia and hydrogen sulphide).
   • The purified ethyne gas is collected in a gas jar.

Chemical Explanation:

• Calcium carbide reacts with water to produce ethyne (C₂H₂) and calcium hydroxide (Ca(OH)₂).
• The reaction is exothermic, meaning it releases heat.

Purification of Ethyne:

• The crude ethyne gas may contain impurities like NH₃, H₂S, and P₄H₂ (phosphine).
• To purify, the gas is passed through:
  • Acidified copper sulphate (CuSO₄) to remove ammonia.
  • Lead nitrate (Pb(NO₃)₂) to remove hydrogen sulphide.
  • Concentrated sulphuric acid (H₂SO₄) to dry the gas.

Collection:

• Ethyne is collected by the downward displacement of water because it is sparingly soluble in water and lighter than air.

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iii) Relation Between Kp and Kc

The equilibrium constants Kp (equilibrium constant in terms of partial pressures) and Kc (equilibrium constant in terms of concentrations) are related for gaseous reactions. The relationship is derived from the Ideal Gas Law and depends on the number of moles of gaseous products and reactants.

General Reaction:

For a general gaseous reaction: $$a\mathrm{A}(\mathrm{g})+b\mathrm{B}(\mathrm{g})\rightleftharpoons c\mathrm{C}(\mathrm{g})+d\mathrm{D}(\mathrm{g})$$ The equilibrium constants are defined as: $$K_c=\frac{[C{]}^c[D{]}^d}{[A{]}^a[B{]}^b}$$ $$K_p=\frac{(P_C{)}^c(P_D{)}^d}{(P_A{)}^a(P_B{)}^b}$$

Relationship Between Kp and Kc:

Using the Ideal Gas Law ($PV=nRT$), the partial pressure of a gas is related to its concentration by: $$P_i=C_{i}RT$$ where:

• $P_i$ = partial pressure of gas $i$,
• $C_i$ = concentration of gas $i$,
• $R$ = universal gas constant,
• $T$ = temperature in Kelvin.

Substituting into $K_p$: $$K_p=\frac{(C_{C}RT{)}^c(C_{D}RT{)}^d}{(C_{A}RT{)}^a(C_{B}RT{)}^b}$$ $$K_p=\frac{[C{]}^c[D{]}^d}{[A{]}^a[B{]}^b}\times (RT{)}^{\Delta n}$$ where $\Delta n=(c+d)-(a+b)$ (change in the number of moles of gas).

Thus, the relationship is: $$K_p=K_c(RT{)}^{\Delta n}$$

Special Cases:

1. If $\Delta n=0$ (equal moles of gaseous products and reactants): $$K_p=K_c$$ Example: $\mathrm{N}{}_2(\mathrm{g})+3\mathrm{H}{}_2(\mathrm{g})\rightleftharpoons 2\mathrm{NH}{}_3(\mathrm{g})$ (here, $\Delta n=2-4=-2$, but if we consider only gases, it depends on the reaction).

2. If $\Delta n\ne 0$:

   • For reactions where the number of moles of gaseous products is greater than reactants, $K_p>K_c$.
   • For reactions where the number of moles of gaseous products is less than reactants, $K_p<K_c$.

Example:

Consider the reaction: $$\mathrm{N}{}_2\mathrm{O}{}_4(\mathrm{g})\rightleftharpoons 2\mathrm{NO}{}_2(\mathrm{g})$$ Here, $\Delta n=2-1=1$. $$K_p=K_c(RT{)}^1=K_{c}RT$$

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iv) Advantages of the Modern Periodic Table

The Modern Periodic Table (also called the Long Form of the Periodic Table) was developed by Dmitri Mendeleev and later refined by Henry Moseley (who arranged elements by atomic number). Below are its key advantages:

1. Arrangement Based on Atomic Number

• Elements are arranged in increasing order of atomic number (Z).
• This eliminates anomalies present in Mendeleev’s original table (where elements were arranged by atomic mass).

2. Clear Classification of Elements

• Elements are divided into blocks (s, p, d, f) based on their electron configurations.
• s-block: Groups 1-2 (alkali and alkaline earth metals).
• p-block: Groups 13-18 (includes metals, metalloids, and non-metals).
• d-block: Transition metals (Groups 3-12).
• f-block: Lanthanides and actinides (inner transition metals).

3. Predictive Power

• Gaps in the table can be filled by discovering new elements (e.g., Mendeleev predicted gallium, germanium, and scandium before their discovery).
• Properties of undiscovered elements can be predicted based on their position.

4. Periodicity of Properties

• Physical and chemical properties repeat periodically across periods and groups.
• Example:
  • Atomic radius decreases across a period (left to right).
  • Electronegativity increases across a period.
  • Ionization energy follows a periodic trend.

5. Group Similarities

• Elements in the same group have similar chemical properties due to the same valence electron configuration.
• Example:
  • Group 1 (Alkali metals): All react with water to form hydroxides and hydrogen gas.
  • Group 17 (Halogens): All exist as diatomic molecules (F₂, Cl₂, Br₂) and are highly reactive non-metals.

6. Separation of Metals, Non-Metals, and Metalloids

• Metals are on the left and center of the table.
• Non-metals are on the top right.
• Metalloids (e.g., Si, Ge, As) form a staircase between metals and non-metals.

7. Inclusion of Lanthanides and Actinides

• The f-block accommodates lanthanides (Ce-Lu) and actinides (Th-Lr), which have similar properties.
• This prevents the table from becoming too wide.

8. Explanation of Chemical Bonding and Valency

• The group number indicates the number of valence electrons, which determines bonding and valency.
• Example:
  • Group 1 elements have 1 valence electron (form +1 ions).
  • Group 17 elements have 7 valence electrons (form -1 ions).

9. Understanding Trends in Properties

• The table helps explain trends such as:
  • Electronegativity (increases left to right, decreases down a group).
  • Ionization energy (follows a periodic trend).
  • Electron affinity (varies periodically).

10. Use in Predicting Element Properties

• Newly discovered elements (e.g., Tennessine (Ts), Oganesson (Og)) can have their properties estimated based on their position in the table.

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