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The questions your board exam loves to ask
800 most-asked Class 11 & 12 (+1 / +2) questions across Physics, Chemistry, Maths and Biology — each with a model answer and the exact marking-scheme points examiners reward. Revise smart, walk in calm.
ChemistryClass 112 marksmedium
Chemical Bonding and Molecular Structure
Why does water (H2O) have a much higher boiling point than hydrogen sulphide (H2S)?
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Oxygen is much more electronegative and smaller than sulphur, so water molecules form strong intermolecular hydrogen bonds, whereas H2S molecules are held only by weak van der Waals (dipole) forces. Extra energy is needed to break the hydrogen bonds in water, so water has a much higher boiling point than H2S even though H2S has a higher molar mass.
Marking-scheme points
- ✓Water forms strong intermolecular H-bonds (O is small, electronegative)
- ✓H2S has only weak van der Waals forces
- ✓Breaking H-bonds needs more energy -> higher boiling point
ChemistryClass 112 marksmedium
Chemical Bonding and Molecular Structure
Distinguish between a sigma bond and a pi bond. Which is stronger and why?
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A sigma bond is formed by the head-on (axial) overlap of orbitals along the internuclear axis, while a pi bond is formed by the sidewise (lateral) overlap of parallel p orbitals. A sigma bond is stronger because axial overlap is more effective and greater, giving a larger region of electron density between the nuclei; pi bonds have smaller lateral overlap and are weaker and more reactive.
Marking-scheme points
- ✓Sigma: head-on/axial overlap; pi: sidewise overlap of p orbitals
- ✓Sigma has greater, more effective overlap
- ✓Sigma bond is stronger than pi bond
ChemistryClass 112 markseasy
States of Matter
State Boyle's law and Charles's law with their mathematical expressions.
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Boyle's law: at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure, i.e. V is proportional to 1/P, so PV = constant. Charles's law: at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature, i.e. V is proportional to T, so V/T = constant.
PV = constant (Boyle); V/T = constant (Charles)
Marking-scheme points
- ✓Boyle: V proportional to 1/P at constant T -> PV = constant
- ✓Charles: V proportional to T at constant P -> V/T = constant
- ✓Temperature must be in Kelvin
ChemistryClass 112 markseasy
States of Matter
Write the ideal gas equation and give the value of the gas constant R in two units.
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States of Matter
Write the van der Waals equation for n moles of a real gas and explain the significance of the constants a and b.
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Thermodynamics
Define system and surroundings. Name the three types of thermodynamic systems.
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Thermodynamics
State the first law of thermodynamics and give its mathematical expression with sign convention.
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Thermodynamics
Define enthalpy. Derive the relation between delta H and delta U for a reaction involving gases.
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Thermodynamics
State Hess's law of constant heat summation and mention one of its applications.
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Thermodynamics
What is entropy? Predict the sign of delta S when ice melts into water.
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Thermodynamics
Define standard enthalpy of formation and standard enthalpy of combustion.
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Equilibrium
What is a reversible reaction? State two characteristics of chemical equilibrium.
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Equilibrium
State Le Chatelier's principle. What is the effect of increasing pressure on the equilibrium N2(g) + 3H2(g) <=> 2NH3(g)?
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Equilibrium
Define an acid and a base according to the Bronsted-Lowry concept. What is a conjugate acid-base pair?
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Equilibrium
Calculate the pH of a 0.001 M HCl solution.
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Equilibrium
What is the ionic product of water (Kw)? State its value at 298 K and its relation with pH.
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Equilibrium
What is the common ion effect? Explain with a suitable example.
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Redox Reactions
Define oxidation and reduction in terms of electron transfer and oxidation number.
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Redox Reactions
Calculate the oxidation number of manganese in KMnO4 and of chromium in K2Cr2O7.
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Redox Reactions
Define an oxidising agent and a reducing agent with one example each.
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