Free Solids MCQs with Answers

11. Lattice energy is defined as the energy

• A. needed to melt one mole of an ionic solid
• B. needed to vaporise one mole of a liquid
• C. released when one mole of an ionic crystal is formed from its gaseous ions
• D. absorbed when an electron is added to a gaseous atom

Explanation: Bringing gaseous ions together into an ordered lattice releases a large amount of energy, so lattice energy is strongly exothermic and is a direct measure of the strength of the ionic bonding. It cannot be measured directly and is obtained instead from a Born Haber cycle using Hess's law. Energy absorbed when an electron is added to a gaseous atom is electron affinity.

12. The lattice energy of an ionic compound increases when

• A. the charges on the ions increase and their radii decrease
• B. the charges on the ions decrease
• C. the ions become larger
• D. the compound is dissolved in water

Explanation: Electrostatic attraction is proportional to the product of the charges and inversely proportional to the distance between the ion centres, so small, highly charged ions pack closely and bind strongly. This is why magnesium oxide, with 2+ and 2- ions, has a far higher lattice energy and melting point than sodium chloride. Dissolving the compound breaks the lattice rather than strengthening it.

13. Which compound would be expected to have the highest lattice energy?

• A. NaCl
• B. KCl
• C. MgO
• D. KBr

Explanation: Magnesium oxide is built from doubly charged Mg2+ and O2- ions that are also small, so both factors in the electrostatic expression work in its favour and its lattice energy is roughly four times that of sodium chloride. Among the singly charged compounds listed, lattice energy falls as the ions get larger, so NaCl exceeds KCl, which exceeds KBr. Melting points follow the same order.

14. Solid sodium chloride does not conduct electricity, but molten sodium chloride does, because

• A. melting converts the ions into atoms
• B. the ions become free to move only when the lattice breaks down
• C. electrons are released on melting
• D. the molten salt contains water

Explanation: Conduction in an ionic compound needs mobile charge carriers, and in the solid the ions are locked in fixed lattice positions even though they are charged. Melting frees them to migrate towards the electrodes, and dissolving in water has the same effect. No electrons are released and the ions remain ions throughout, which rules out the other explanations.

15. Two different compounds that have the same crystalline form and analogous chemical formulae are said to be

• A. polymorphic
• B. allotropic
• C. isomorphic
• D. anisotropic

Explanation: Isomorphous substances such as potassium sulphate and potassium chromate crystallise in the same shape because their ions are of similar size and charge, and they can even form mixed crystals. Polymorphism is the opposite situation, one substance existing in more than one crystal form, as calcium carbonate does as calcite and aragonite. Allotropy is polymorphism restricted to elements.

16. Graphite conducts electricity while diamond does not because in graphite

• A. each carbon forms four single covalent bonds
• B. the carbon atoms are ionised
• C. the layers are held by strong covalent bonds
• D. each carbon uses only three of its four valence electrons in bonding, leaving one delocalised

Explanation: In graphite each carbon is bonded to three others in a flat hexagonal sheet, and the fourth electron is delocalised between the layers, so it can carry current along the sheets. In diamond all four electrons are locked into single bonds, leaving none free. The weak forces between graphite layers also let them slide, which is why graphite is used as a lubricant and in pencils.

17. The melting point of a solid is a measure of

• A. the mass of its particles
• B. the strength of the forces holding its particles together
• C. the size of its crystals
• D. its electrical conductivity

Explanation: Melting requires enough thermal energy to overcome the forces locking the particles in the lattice, so covalent network and ionic solids melt very high, metals vary widely and molecular solids melt low. Comparing melting points is therefore a quick way to infer the type of bonding in an unknown solid. Crystal size affects appearance but not the temperature at which melting occurs.

18. Metals are malleable and ductile because

• A. their atoms are held in rigid directional covalent bonds
• B. they contain no free electrons
• C. layers of positive ions can slide over one another while the sea of delocalised electrons holds them together
• D. their ions are held by hydrogen bonds

Explanation: Metallic bonding is non directional, so displacing one layer of cations does not break the structure because the delocalised electrons continue to bind whatever arrangement results. An ionic crystal shatters instead, because sliding brings like charges opposite each other and the layers repel. The same free electrons account for electrical and thermal conductivity.

19. A Born Haber cycle is used to determine lattice energy because it

• A. measures lattice energy directly in a calorimeter
• B. applies Hess's law to relate lattice energy to quantities that can be measured
• C. predicts the shape of the crystal
• D. measures the melting point of the ionic solid

Explanation: Lattice energy cannot be measured in an experiment, so the cycle links it to the enthalpy of atomisation, ionisation energy, electron affinity and enthalpy of formation, all of which can. Because enthalpy is a state function the total around the cycle is zero regardless of the route, which is Hess's law applied to ionic compounds. A large discrepancy between calculated and experimental values indicates covalent character.

20. Which of the following solids is held together by dispersion forces only?

• A. Solid iodine
• B. Sodium chloride
• C. Diamond
• D. Copper

Explanation: Iodine molecules are non polar, so the only attraction between them is the temporary induced dipole interaction known as the dispersion force, which is why iodine sublimes readily on gentle warming. Sodium chloride is ionic, diamond covalent network and copper metallic, all of which involve far stronger bonding. Dispersion forces still grow with molecular size, which is why iodine is a solid while chlorine is a gas.