Chemistry Final Exams Question Bank and answers

Chemistry Revision: Set 1

1. Define Oxidation Number and Reduction

• Oxidation Number: It is the formal charge that an atom appears to have in a chemical species when all shared electrons are counted as belonging to the more electronegative atom. It is a theoretical tool used to track electron flow.
• Reduction: In terms of electron transfer, reduction is the gain of electrons by an atom or ion. In terms of oxidation number, it is the decrease in the oxidation state of an element.

Reaction: Cu²⁺ + 2e^- → Cu

Example: Copper is reduced as its oxidation number changes from +2 to 0.

2. State and Explain Mendeleev’s Periodic Law

Statement: The physical and chemical properties of elements are a periodic function of their atomic masses.

• Arrangement: Mendeleev arranged the then-known 63 elements in increasing order of atomic mass.
• Periodicity: He observed that elements with similar properties appeared at regular intervals (periods).
• Gaps: He boldly left gaps for yet-to-be-discovered elements (e.g., Eka-Boron, Eka-Aluminium) and predicted their properties accurately.
• Correction: He used the periodic position to correct the previously accepted atomic masses of certain elements like Beryllium.

3. Write a Short Note on Dipole Moment and Types of Interaction

Dipole Moment (μ): It is the product of the magnitude of the charge (q) and the distance (d) between the centers of positive and negative charges. It is a vector quantity. It indicates the degree of polarity in a molecule.

Formula: μ = q × d

Unit: Debye (D).

Types of Interactions:

• Dipole-Dipole: Between permanent polar molecules (e.g., HCl).
• Ion-Dipole: Between an ion and a polar molecule (e.g., Na⁺ in water).
• London Dispersion Forces: Weakest; occurs in non-polar molecules due to temporary dipoles (e.g., CH₄, He).
• Hydrogen Bonding: A strong type of dipole-dipole interaction involving Hydrogen bonded to N, O, or F.

4. Explain Nuclear Chemistry in Detail

Definition: The sub-field of chemistry dealing with changes in the nucleus of atoms, radioactivity, and nuclear processes.

• Radioactivity: Spontaneous emission of particles/energy (α, β, γ) from unstable nuclei.
• Nuclear Transmutation: The conversion of one chemical element or isotope into another through a nuclear reaction.
• Fission vs. Fusion: Fission involves splitting a heavy nucleus (e.g., U-235), while fusion is the combining of light nuclei (e.g., Hydrogen isotopes in the sun). Both release massive amounts of energy according to E=mc².

5. Write Short Notes on Alpha (α) and Beta (β) Decay

Alpha (α) Decay:

• The nucleus emits an alpha particle (a Helium nucleus, ⁴₂He).
• Result: Atomic number (Z) decreases by 2; Mass number (A) decreases by 4.

General Equation:
^A_ZX → ^A-4_Z-2Y + ⁴₂He (α)

Beta (β) Decay:

• A neutron in the nucleus converts into a proton and emits an electron (beta particle, ⁰_-1e).
• Result: Atomic number (Z) increases by 1; Mass number (A) remains unchanged.

General Equation:
^A_ZX → ^A_Z+1Y + ⁰_-1e (β)

6. Explain Common Name and IUPAC Name

• Common (Trivial) Name: Names based on the source, discoverer, or a specific property of the substance. They lack a systematic structure and must be memorized individually.
  Example: Acetone (derived from acetic acid) or Morphine (from Morpheus, the god of dreams).
• IUPAC Name: A systematic method of naming chemical compounds as per the International Union of Pure and Applied Chemistry.
  Rules: It uses a Root word (number of carbons), Suffix (functional group), and Prefix (substituents).
  Example: The common name Isopropyl alcohol is systematically named Propan-2-ol.
• Significance: IUPAC naming ensures that every distinct compound has a unique name, preventing global confusion in research and industry.

7. Explain the Anomalous Behaviour of Oxygen and Copper

Oxygen (Group 16): Unlike other members (S, Se, Te), Oxygen is a gas and diatomic (O₂).

• Small Size & High Electronegativity: Leads to strong Hydrogen bonding, which is why H₂O is liquid while H₂S is a gas.
• Absence of d-orbitals: Oxygen cannot expand its octet; its covalency is limited to 2, whereas others can show +4 or +6.

Copper (Cu): Shows an anomalous electronic configuration.

Configuration: [Ar] 3d¹⁰ 4s¹

• Reason: The completely filled 3d¹⁰ subshell provides extra stability due to symmetry and high exchange energy.
• Variable Valency: It shows +1 (Cu⁺) and +2 (Cu²⁺) oxidation states. Cu²⁺ is generally more stable in aqueous solution due to its high hydration enthalpy.

8. Define Isotopes and Isotones

• Isotopes: Atoms of the same element having the same Atomic Number (Z) but different Mass Numbers (A). They have the same number of protons but a different number of neutrons.
  Example: Protium (¹₁H), Deuterium (²₁H), and Tritium (³₁H).
• Isotones: Atoms of different elements which contain the same number of neutrons. Both their Atomic Number (Z) and Mass Number (A) are different, but the value of (A - Z) is constant.
  Example: ¹⁴₆C and ¹⁶₈O. Both have 8 neutrons (14-6=8 and 16-8=8).

9. Complete the Following Reaction (Conceptual Explanation)

To "Complete a Reaction" in an exam, students must follow three strict steps:

• Identify the Reactants: Determine the functional groups or oxidation states involved.
• Apply Stoichiometry: Ensure the Law of Conservation of Mass is satisfied by balancing atoms on both sides.
• State Conditions: Mention catalysts, temperature, or pressure if required.

Example: 2H₂(g) + O₂(g) → 2H₂O(l)
(Condition: Electric Spark)

Note: Students must always include the physical states (s, l, g, aq) for full credit in 11th-grade board patterns.

10. Explain with Example: Oxidizing and Reducing Agents

• Oxidizing Agent (Oxidant): A substance that gains electrons and gets reduced itself. It increases the oxidation number of another substance.
• Reducing Agent (Reductant): A substance that loses electrons and gets oxidized itself. It decreases the oxidation number of another substance.

Example: CuO + H₂ → Cu + H₂O

• CuO: Oxidizing agent (accepts electrons/loses Oxygen).
• H₂: Reducing agent (donates electrons/gains Oxygen).

11. Explain Magnetic Quantum Number

• Symbol (m_l): It describes the spatial orientation of the orbital in three-dimensional space relative to a coordinate system.
• Values: For a given azimuthal quantum number (l), m_l can have (2l + 1) values, ranging from -l to +l, including zero.
• Significance:
  • For s-orbital (l=0): m_l = 0 (only 1 orientation).
  • For p-orbital (l=1): m_l = -1, 0, +1 (3 orientations: p_x, p_y, p_z).
  • For d-orbital (l=2): m_l = -2, -1, 0, +1, +2 (5 orientations).
• Physical Meaning: It explains the splitting of spectral lines in a magnetic field (Zeeman Effect).

12. Characteristics of s, p, and d Block Elements

In 11th-grade exams, a "brief" answer won't suffice for a 4-mark or 5-mark question. Students must categorize the characteristics clearly:

A. s-Block Elements (Groups 1 & 2)

• Electronic Configuration: General valence shell configuration is ns¹ (Alkali metals) or ns² (Alkaline earth metals).
• Physical State: They are soft, silvery-white metals with low melting and boiling points.
• Chemical Reactivity: Highly reactive due to low ionization enthalpy; they lose electrons easily to form unipositive (M⁺) or dipositive (M²⁺) ions.
• Compounds: Mostly form Ionic compounds. They impart characteristic colors to a flame (Flame Test).

B. p-Block Elements (Groups 13 to 18)

• Electronic Configuration: General valence shell configuration is ns² np^1-6.
• Diversity: This is the only block containing metals, non-metals, and metalloids.
• Oxidation States: Show variable oxidation states, often differing by two units (due to the Inert Pair Effect in heavier elements).
• Nature of Oxides: Generally form acidic oxides (non-metals) or amphoteric oxides.
• Group 18: Includes Noble Gases with completely filled ns² np⁶ shells, making them chemically inert.

C. d-Block Elements (Groups 3 to 12 - Transition Elements)

• Electronic Configuration: General configuration is (n-1)d^1-10 ns^1-2.
• Metallic Character: They are all hard, high-melting-point metals (except Mercury, which is liquid).
• Variable Oxidation States: Unlike s-block, they show multiple oxidation states (e.g., Fe as +2 and +3, Mn from +2 to +7).
• Color & Catalysis: Most ions are colored (due to d-d transitions) and show paramagnetic behavior (due to unpaired electrons). They are widely used as catalysts (e.g., Ni, Pt, V₂O₅).
• Complexes: They have a strong tendency to form Complex (Coordination) Compounds.

13. Explain in Brief Rutherford Alpha Particle Scattering Experiment

The Experiment: Rutherford bombarded a thin gold foil (approx. 100 nm thick) with high-energy α-particles (He²⁺).

• Observations:
  1. Most α-particles passed straight through the foil.
  2. A small fraction was deflected by small angles.
  3. A very few (1 in 20,000) bounced back (180° deflection).
• Conclusions:
  • Most of the space in an atom is empty.
  • The entire positive charge and mass are concentrated in a very small, dense region called the nucleus.
  • Electrons revolve around the nucleus in circular paths.

15. Explain Variation of Electron Gain Enthalpy Across Group and Period

Definition: It is the enthalpy change when an electron is added to an isolated gaseous atom.

• Variation Across a Period: It becomes more negative from left to right.
  Reason: Atomic size decreases and nuclear charge increases, making it easier for the nucleus to attract an incoming electron.
• Variation Down a Group: It becomes less negative (or more positive) moving down.
  Reason: Atomic size increases; the distance between the nucleus and the added electron increases, weakening the attraction.
• Exceptions: Chlorine has a more negative electron gain enthalpy than Fluorine because of the high inter-electronic repulsion in the small 2p subshell of Fluorine.

16. Explain Properties of Gases

• Measurable Properties: Gases are defined by Pressure (P), Volume (V), and Temperature (T). They are highly compressible due to large intermolecular spaces.
• Physical Characteristics: Gases exert pressure equally in all directions on the walls of the container. They possess very low density compared to solids and liquids.
• Diffusion: Gases mix spontaneously and rapidly regardless of gravity to form a homogeneous mixture.
• Intermolecular Forces: In ideal conditions, intermolecular forces of attraction are considered negligible, allowing particles to move freely.

17. Explain Half-Life and Decay Constant

• Decay Constant (λ): It is the fraction of the total number of atoms that disintegrate per unit time. It is a measure of the rate of radioactive disintegration.
• Half-Life (t_1/2): It is the time required for the number of radioactive nuclei to reduce to exactly half of its initial value.
• Relationship: For a first-order radioactive process, the relationship is constant:

t_1/2 = 0.693 / λ

Significance: Half-life is independent of the initial concentration of the substance and external factors like temperature or pressure.

19. What is Alkylation Reaction?

Definition: The introduction of an alkyl group (-R) into a molecule, typically an aromatic ring. This is commonly known as Friedel-Crafts Alkylation.

• Process: Benzene reacts with an alkyl halide in the presence of an anhydrous AlCl₃ catalyst.
• Mechanism: It is an Electrophilic Substitution Reaction where a carbocation acts as the electrophile.

C₆H₆ + CH₃Cl → C₆H₅CH₃ (Toluene) + HCl

20. Explain Acylation Reaction

Definition: The introduction of an acyl group (R-CO-) into a molecule. This is known as Friedel-Crafts Acylation.

• Process: Benzene reacts with an acyl halide or acid anhydride using anhydrous AlCl₃.
• Product: Produces aromatic ketones. It is preferred over alkylation as it avoids carbocation rearrangement.

C₆H₆ + CH₃COCl → C₆H₅COCH₃ (Acetophenone) + HCl

21. Write Short Note on N/Z Ratio Effect

• Definition: The N/Z ratio (Neutron to Proton ratio) is the fundamental factor determining nuclear stability.
• Stability Belt: Light elements are stable at a ratio of 1.0. As Z increases, the ratio must increase to ~1.5 to maintain stability against proton-proton repulsion.
• Radioactive Decay: Nuclei with a ratio outside the "Stability Belt" undergo decay (β emission or positron emission) to bring the ratio back to a stable value.

22. Explain Nuclear Chain Reaction in Detail

Definition: A self-sustaining process where neutrons released in one fission event trigger at least one subsequent fission in neighboring nuclei.

• Process: A slow neutron hits a ²³⁵U nucleus, splitting it and releasing energy plus 2–3 new neutrons.
• Types:
  • Controlled: Neutrons are absorbed (using Cadmium rods) to maintain a steady rate (Nuclear Reactors).
  • Uncontrolled: Reaction accelerates exponentially (Atomic Bomb).
• Critical Mass: The minimum mass of fissionable material required to sustain the chain reaction.