All Free Chemistry MCQs with Answers

371. The hybridisation of the carbon atom in methane is

• A. sp
• B. sp2
• C. sp3
• D. sp3d

Explanation: One 2s and three 2p orbitals mix to give four equivalent sp3 hybrids directed to the corners of a tetrahedron, which explains why all four C-H bonds are identical and the angle is 109.5 degrees. Without hybridisation the s and p orbitals would give bonds of different strengths and 90 degree angles, which contradicts experiment. Counting four electron regions is the quickest route to sp3.

372. The percentage of s character in an sp2 hybrid orbital is

• A. 25 per cent
• B. 33.3 per cent
• C. 50 per cent
• D. 100 per cent

Explanation: An sp2 hybrid is formed from one s and two p orbitals, so the s contribution is one in three. The figures for sp3 and sp are 25 and 50 per cent respectively, and greater s character holds electrons closer to the nucleus, which is why sp hybridised carbon in ethyne makes the attached hydrogen weakly acidic. Bond length also shortens as s character rises.

373. The hybridisation of the nitrogen atom in ammonia is

• A. sp3, with the lone pair occupying one of the four hybrid orbitals
• B. sp2
• C. sp
• D. unhybridised

Explanation: Nitrogen has four regions of electron density, three bonds and one lone pair, so four sp3 hybrids are needed even though only three of them are used for bonding. This is why the observed angle of 107 degrees is close to the tetrahedral value rather than to 120 degrees. Lone pairs must always be counted when deciding hybridisation.

374. A molecule is polar only if it contains polar bonds and

• A. has an even number of atoms
• B. contains at least one lone pair
• C. the individual bond dipoles do not cancel because of the molecular shape
• D. contains hydrogen

Explanation: Carbon dioxide has two strongly polar bonds but is linear, so the two dipoles point in opposite directions and cancel, leaving a non polar molecule with zero dipole moment. Water has similar bonds but is bent, so its dipoles add and the molecule is strongly polar. Shape therefore decides polarity just as much as electronegativity difference does.

375. Which of the following molecules has a zero dipole moment?

• A. H2O
• B. NH3
• C. HCl
• D. CCl4

Explanation: Tetrachloromethane has four polar carbon to chlorine bonds arranged symmetrically about the central carbon, so the four dipoles cancel exactly and the molecule is non polar overall. Water and ammonia are bent and pyramidal respectively, so their dipoles reinforce, and hydrogen chloride is a simple polar diatomic. Symmetry is the property to look for in this type of question.

376. Dipole moment is measured in

• A. debye units
• B. joules
• C. angstroms
• D. moles

Explanation: Dipole moment is the product of the magnitude of the separated charge and the distance between the centres of charge, and it is quoted in debye, with water at about 1.85 D. A larger value means a more polar molecule and therefore stronger dipole to dipole attraction and a higher boiling point. Angstroms measure length and joules measure energy, so neither can express this quantity.

377. The percentage ionic character of a covalent bond increases as

• A. the two atoms become more similar
• B. the electronegativity difference between the two atoms increases
• C. the bond becomes shorter
• D. the temperature rises

Explanation: The greater the electronegativity difference, the more unequally the pair is shared, until at a difference of roughly 1.7 the bond is regarded as more ionic than covalent. Hydrogen fluoride is highly polar for this reason while hydrogen iodide is nearly non polar. This makes the covalent and ionic extremes two ends of a continuous scale rather than separate categories.

378. Bond energy is defined as the energy

• A. released when a bond is formed between two ions
• B. required to melt one mole of a covalent solid
• C. required to break one mole of a particular bond in the gaseous state
• D. stored in the nucleus of an atom

Explanation: Because bond breaking is always endothermic, bond energy values are positive, and the same amount is released when the bond re-forms. The gaseous state is specified so that intermolecular forces do not contribute to the measurement. Values quoted in data books are averages taken across many compounds, which is why calculations using them give approximate results.

379. Among the carbon to carbon bonds, the order of bond energy is

• A. single greater than double greater than triple
• B. all three are equal
• C. double greater than triple greater than single
• D. triple greater than double greater than single

Explanation: More shared pairs mean a greater attraction between the nuclei and the bonding electrons, so the triple bond is the strongest and also the shortest at 0.120 nm, against 0.154 nm for a single bond. Bond energy and bond length always move in opposite directions. Note that a triple bond is not three times as strong as a single one, because the two pi bonds are weaker than the sigma bond.

380. A reaction is exothermic overall when

• A. the energy released in forming new bonds exceeds the energy absorbed in breaking old ones
• B. the energy absorbed in breaking bonds exceeds the energy released in forming them
• C. no bonds are broken
• D. the products are gases

Explanation: Breaking bonds always costs energy and forming them always releases it, so the sign of the overall enthalpy change is decided by which total is larger. This is the basis of calculating an approximate enthalpy change from tabulated bond energies. Combustion is strongly exothermic because the very strong bonds in carbon dioxide and water release more than was spent breaking the fuel apart.