d and f block elements | 25 PYQ | Class 12 Chemistry | CBSE Board 2026-27| Sourabh Raina

Introduction to DNA Block Elements and Practice Questions

Overview of the Video

• The video is presented by Saurabh Raina, focusing on practicing 25 important past year questions (PYQs) related to DNA block elements over the last 10 years.

• Viewers are encouraged to practice these questions before board exams for better marks.

• A PDF containing the questions will be available on Telegram, with a link provided in the description.

Short Answer Type Questions

Transition Elements

• The first question discusses transition elements, defined as those with partially filled d-orbitals in their atoms or stable oxidation states.

• Characteristics include variable oxidation states, such as Iron's +2 and +3 states, and catalytic properties that allow them to act as catalysts in reactions.

Alloy Formation

• The second question explains why transition metals can form alloys due to their similar atomic sizes, allowing one metal to replace another in a lattice structure when melted and cooled.

Oxidation States

• It is noted that lower oxidation state oxides (like Mn2O3) are basic while higher oxidation state oxides (like Mn2O7) are acidic due to electron donation capabilities.

Variable Oxidation States of Transition Metals

Explanation of Variable States

• Transition metals exhibit variable oxidation states because they have n - 1 d orbitals with minimal energy difference, allowing both orbitals to participate in bond formation.

Soft Metals

• Zinc, cadmium, and mercury are classified as soft metals since all electrons in their d-subshell are paired, resulting in weak metallic bonds and low melting/boiling points.

Balancing Chemical Equations

Reduction-Oxidation Reactions

• The fourth question involves balancing chemical equations using reduction and oxidation half-reactions.

• For example, MnO4^- acts as a strong oxidizing agent converting I^- into I2 while being reduced itself.

Lanthanide Contraction

Definition and Effects

• Lanthanide contraction refers to the gradual decrease in atomic and ionic radii across lanthanides due to poor shielding effects from 4f electrons.

Actinide Contraction Comparison

• Actinide contraction is greater than lanthanide contraction because 5f electrons are more poorly shielded compared to 4f electrons leading to a larger effective nuclear charge increase.

Spin Only Moment Calculation

Cobalt Ion Configuration

• To calculate the spin-only moment of Co^2+, its electronic configuration (Ar 3d^7 4s^2), must be considered after removing two electrons for Co^2+.

High Third Ionization Energy of Manganese

Stability Considerations

• The third ionization enthalpy of manganese is high because removing an electron from its stable half-filled configuration (3d^5).

Non-transition Metal Classification

Reasons for Exclusion

• Zinc, cadmium, and mercury do not qualify as transition metals since they possess completely filled d-orbitals without any partially filled configurations.

D4 Species Reactivity Comparison

Reducing vs. Oxidizing Agents

Chromium(II), Cr^2+, acts as a strong reducing agent while manganese(III), Mn^3+, serves as a strong oxidizing agent due to their respective electronic configurations' stability upon losing or gaining electrons.

Instability of D1 Configuration

Electronic Behavior

• D1 configurations tend toward instability among ions because they easily lose an electron achieving noble gas configuration.

Variability in Oxidation States

Transition vs P-block Elements

• Transition metals show variability due to participation from n - 1 d ns orbitals with small energy differences; p-block elements typically show larger differences between oxidation states.

Manganese Oxoanion Formula

Calculation Methodology

• Manganese's oxoanions must reflect its group number; thus MnO4^- has an oxidation state of +7 calculated through charge balance equations involving oxygen's contributions.

Magnetic Behavior Analysis

Comparison Between Scandium & Chromium

• Scandium(III), Sc^3+, exhibits diamagnetism due to no unpaired electrons post-ionization whereas chromium(III), Cr^3+, shows paramagnetism owing three unpaired electrons remaining after ionization.

Understanding Activation Energy and Copper Ions

Lower Activation Energy Pathways

• A lower activation energy pathway allows reactions to occur more rapidly, making catalysts effective by reducing the energy required for reactants to convert into products.

Copper Ion Stability in Aqueous Solutions

• Copper(I) ions are unstable in aqueous solutions and undergo disproportionation reactions, converting into stable copper(II) ions and solid copper.

Disproportionation Reaction Explained

• In a disproportionation reaction, one species is oxidized while another is reduced; for copper(I), its oxidation state increases to +2 while forming solid copper with an oxidation state of 0.

Ionization Energy Considerations

• The formation of stable copper(II) ions requires the removal of one electron, which involves second ionization enthalpy due to the already removed electron from the initial state.

Hydration Enthalpy and Stability

• The stability of copper(II) ions in solution is attributed to their higher negative hydration enthalpy, which compensates for the energy needed during ionization.

Effects of pH on Potassium Dichromate Solutions

Acidic Medium Influence

• In acidic conditions, potassium dichromate exists in equilibrium with chromate ions; increased H+ concentration shifts equilibrium towards dichromate formation, resulting in an orange color.

Alkaline Medium Influence

• Conversely, in alkaline conditions (high pH), decreased H+ concentration shifts equilibrium back towards chromate ions, producing a yellow color instead.

Structural Characteristics of Chromate and Dichromate Ions

• Chromate has a tetrahedral structure with two double bonds between chromium and oxygen. Dichromate consists of two tetrahedra sharing a chromium atom, exhibiting an overall -2 charge.

Properties of Europium Ions

Strong Reducing Agent Nature

• Europium(II), being less stable than europium(III), readily loses electrons making it a strong reducing agent as it seeks to attain a more stable oxidation state (+3).

Challenges in Separating Lanthanides

• Separation difficulties arise from similar atomic radii among lanthanide elements due to lanthanide contraction leading to comparable chemical properties.

Transition Metals' Oxidation States

Manganese's Oxidation State Behavior

• Manganese exhibits higher oxidation states with oxygen (+7 vs. +4 with fluorine); this is due to oxygen's ability to form multiple bonds stabilizing higher oxidation states better than fluorine can.

Copper's Reactivity with Acids

Non-displacement of Hydrogen

• Copper does not displace hydrogen from acids because it lies below hydrogen in the electrochemical series and has a positive standard reduction potential (+0.34 V).

Transition Metal Compounds Formation

Interstitial Compounds Formation

• Transition metals form numerous interstitial compounds due to their three-dimensional crystal lattice structure allowing small atoms like hydrogen or carbon to occupy interstitial spaces effectively.

Identifying Elements with Multiple Oxidation States

Manganese's Versatile Oxidation States

• Manganese can exhibit various oxidation states ranging from +2 to +7 owing to its maximum number of unpaired electrons available for bonding interactions.

Characteristics Defining Transition Elements

Definition Criteria for Transition Elements

Copper is considered a transition element despite having fully filled d-orbitals because its most stable common oxidation state (+2: 3d9 configuration).

Paramagnetism in Potassium Permanganate

Paramagnetic vs Diamagnetic Properties

• KMnO₄ is diamagnetic as Mn⁷⁺ has no d-electrons (d³⁰). K₂MnO₄ shows paramagnetism since Mn⁶⁺ retains one unpaired d-electron (d³¹).

Titration Limitations

• Permanganate titrations cannot be performed using HCl as KMnO₄ oxidizes HCl releasing Cl₂ gas causing side reactions that lead to inaccurate results.

Complexity of Actinides Chemistry

Actinides vs Lanthanides

• Actinides display greater complexity compared to lanthanides due primarily to their larger number of possible oxidation states arising from comparable energies among f-, d-, and s-orbitals.

Cerium as an Oxidizing Agent

Cerium’s Redox Behavior

• Cerium(IV)'’’ serves as an excellent oxidizing agent transitioning easily into cerium(III), demonstrating readiness by gaining electrons during redox reactions.

Identifying Chromium Anions

Stable Anions Under Acidic Conditions

• Cr₂O₇²⁻ represents chromium’s oxoanion that remains stable under acidic conditions where it maintains its +6 oxidation state.

Lanthanide Element Identification

Lanthanide Exhibiting +4 State

• Cerium uniquely exhibits a +4 oxidation state among lanthanide elements showcasing its versatility within this group.

Copper’s Frequent Oxidation State

Commonly Observed Oxidation State

• Copper frequently displays a +1 oxidation state (Cu+) owing largely due its fully filled d-orbital configuration contributing stability.

Color Development in Ions

Color Presence Due To Unpaired Electrons

• Colored solutions arise when partially filled d-orbitals allow for electronic transitions; Fe²⁺ (d⁶), Ti³⁺ (d¹), show colors while Cu+ & Zn²⁺ remain colorless without unpaired electrons.

Preparation Methodology for Potassium Dichromate

Synthesis Steps From Chromite Ore

• Potassium dichromate synthesis begins by fusing chromite ore (FeCr₂O₄)) with Na₂CO₃ yielding sodium chromate followed by acidification producing sodium dichromate before crystallizing potassium dichromate through KCl addition.

Disproportionation Reactions Involving Manganese

Reaction Dynamics Under Acidic Conditions

• When manganese(VI)(MnO₄²-) undergoes disproportionation under acidic conditions it yields permanganates(MnO₄-) alongside manganese dioxide(MnO₂).

Thermal Decomposition Of Potassium Permanganate

Heating Effects On KMnO₄

• Upon heating KMnO₄ at elevated temperatures leads decomposition yielding K₂MnO₄ along with MnO₂ & O₂ gases released during breakdown process.

Vanadium Pentoxide Catalytic Activity

Catalyst Functionality Explained

• Vanadium pentoxide acts as catalyst via large surface area facilitating temporary unstable intermediates formation lowering activation energy pathways enhancing reaction rates significantly.