12th Chemistry One Mark Questions and Book Back Answers

About Chemistry

Hard ~40 h study

The Class 12 Chemistry textbook has three kinds of chapters. Inorganic chapters ask you to know trends and reactions: why aluminium is extracted by electrolysis, how the oxidation state of a transition metal changes, how to name a complex. Physical chapters ask you to calculate: density of a unit cell, rate constant of a first order reaction, pH of a buffer, emf of a cell. Organic chapters ask you to write reactions and explain them: preparation of phenol, aldol condensation, diazotization of aniline.

The chapters build on each other. Bonding and hybridisation from the p-block and coordination chapters come back when you explain the shape of an amine or the colour of a complex. The activation energy idea from the Arrhenius equation in kinetics explains why a catalyst in surface chemistry speeds up a reaction. Acid and base ideas from ionic equilibrium are used again for phenols, carboxylic acids and amines.

In the public exam you need both memory and method. Reactions and definitions must be written exactly, and numerical problems on unit cells, rate constants, pH and cell potential must be worked with correct units. Practising book back questions chapter by chapter is the most direct way to check both.

What you'll learn

• Explain the extraction of metals from ores using the Ellingham diagram and electrolytic methods.
• Predict the geometry and magnetic properties of coordination complexes.
• Calculate the density and packing efficiency of cubic unit cells.
• Use the Nernst equation to find the emf of an electrochemical cell.
• Calculate rate constants and half-lives for zero and first order reactions.
• Write reactions that convert one organic functional group into another.
• Distinguish DNA from RNA by their sugar and nitrogen bases.
• Classify polymers and drugs by their structure and use.

Before you start

• Atomic structure and electronic configuration of elements.
• Types of chemical bonds, hybridisation and shapes of molecules.
• Groups, periods and periodic trends from Class 11.
• Mole concept and stoichiometric calculations.

Chemistry — the syllabus in depth

What the Class 12 Chemistry Syllabus Covers

Inorganic Chemistry and Metallurgy

Metallurgy explains how metals are taken out of their ores: concentration, roasting, reduction and refining. The Ellingham diagram shows which reducing agent can reduce a given metal oxide at a given temperature. The p-block chapters cover boron, carbon, nitrogen, oxygen, halogen and noble gas families. The d-block and f-block chapter deals with oxidation states, magnetic behaviour and the lanthanide contraction. In coordination chemistry you name complexes by IUPAC rules, identify isomers, and use valence bond theory and crystal field theory to predict geometry, magnetism and colour.

Solid State and Physical Chemistry

The solid state chapter covers unit cells, packing efficiency, and defects such as Schottky and Frenkel defects. Chemical kinetics deals with rate laws, order, half-life and the Arrhenius equation. Ionic equilibrium covers acids and bases, pH, buffers, hydrolysis of salts and solubility product. Surface chemistry covers adsorption, catalysis, and colloids and emulsions you see every day, such as milk and fog.

Electrochemistry

This chapter links chemical reactions with electrical energy. It covers conductance of electrolytic solutions, Kohlrausch's law, galvanic cells, the Nernst equation, batteries and fuel cells. Corrosion of iron is explained as an electrochemical process, along with methods to prevent it.

Organic Chemistry

The organic chapters go group by group: alcohols, phenols and ethers, then aldehydes, ketones and carboxylic acids, then nitro compounds, amines, cyanides and diazonium salts. For each group you learn IUPAC names, methods of preparation, chemical properties and tests. Reaction mechanisms such as nucleophilic addition to the carbonyl group and electrophilic substitution in benzene rings explain why these reactions happen.

Biomolecules and Chemistry in Everyday Life

Biomolecules covers carbohydrates, proteins, enzymes, vitamins, lipids and nucleic acids, including the difference between DNA and RNA. The last chapter covers drugs such as antacids, antibiotics and analgesics, food additives, soaps and detergents, and polymers such as Bakelite and nylon.

How to study Chemistry

Treat inorganic, physical and organic chapters differently. For inorganic chapters, learn the trends and the named reactions, and practise naming coordination compounds until you can do it without the rules in front of you. For physical chapters, solve numerical problems regularly, writing the formula, substitution and units every time.

In organic chemistry, group reactions by type instead of learning each one alone. Focus on:

• Writing the intermediates in mechanisms and explaining them with inductive and resonance effects.
• Practising IUPAC names for compounds with more than one functional group.
• Revising biomolecule structures and the table of vitamins and their deficiency diseases.

Keep separate time for unit cell calculations and pH problems, since students lose marks there through small arithmetic slips. In the last weeks, revise the solved problems in the textbook and the book back questions for every chapter.

Common mistakes to avoid

• Mixing up Schottky and Frenkel defects. In a Schottky defect equal numbers of cations and anions are missing; in a Frenkel defect an ion leaves its site and sits in an interstitial position.
• Finding the wrong oxidation number of the central metal because the charges of the ligands are not added correctly.
• Using the wrong sign convention when calculating cell emf, for example subtracting the cathode potential from the anode potential.
• Swapping the dispersed phase and the dispersion medium when classifying colloids.
• Applying aldol condensation to aldehydes that have no alpha-hydrogen, such as formaldehyde or benzaldehyde.

Frequently asked questions

How do Ellingham diagrams help in metallurgy?

An Ellingham diagram plots the standard Gibbs free energy change for the formation of metal oxides against temperature. A metal or reducing agent whose line lies lower can reduce the oxide of a metal whose line lies above it at that temperature. This tells you, for example, above what temperature carbon can reduce a particular oxide, so you can choose a reducing agent that makes the reduction thermodynamically feasible.

What is the difference between crystalline and amorphous solids?

In a crystalline solid the particles are arranged in a regular, repeating three-dimensional pattern, so it has a sharp melting point and cleaves along definite planes. An amorphous solid has only short range order, like a liquid, and softens over a range of temperatures. Crystals are true solids; amorphous solids such as glass are called pseudo solids or supercooled liquids.

Why is the Nernst equation important in electrochemistry?

Standard electrode potentials apply only at 1 M concentration, 1 bar pressure and 298 K. The Nernst equation gives the electrode or cell potential when concentrations are different, by relating it to the reaction quotient, the temperature and the number of electrons transferred. It explains, for example, why the voltage of a cell falls as the reactants are used up.

How does Crystal Field Theory explain the color of complexes?

When ligands approach the central metal ion, its five d-orbitals split into sets of different energy. An electron in a lower set can absorb visible light of a particular wavelength and move to a higher set (a d-d transition). The complex shows the complementary colour of the light it absorbed. Ions such as Sc3+ and Zn2+, with no d-d transition possible, are colourless.

What are essential amino acids in human nutrition?

Essential amino acids are those the human body cannot make, so they must come from food. There are ten of them, including lysine, leucine, valine and phenylalanine. The mnemonic PVT TIM HALL helps you recall all ten. Non-essential amino acids, such as glycine and alanine, are synthesised by the body.

How do soaps clean oily stains from fabric?

A soap molecule has a long hydrocarbon tail that dissolves in oil and a carboxylate head that is attracted to water. In water the molecules gather around a grease drop with their tails inside the grease and heads facing outward, forming a micelle. The grease is held as an emulsion in water and is washed away when the cloth is rinsed.

What is the significance of the half-life in chemical kinetics?

Half-life is the time taken for the concentration of a reactant to fall to half its initial value. For a first order reaction it equals 0.693/k and does not depend on the starting concentration. For a zero order reaction it is proportional to the initial concentration. Radioactive decay is first order, which is why a constant half-life can be used in radiocarbon dating.

What is the role of DNA fingerprinting in forensic science?

Every person, except identical twins, has a unique pattern of repeated base sequences in their DNA. DNA fingerprinting compares these patterns. A sample from a crime scene, such as blood or hair, can be matched with a suspect's DNA to confirm identity. The same technique is used to settle paternity disputes and identify bodies.