All Free Chemistry MCQs with Answers

91. Molecular crystals such as solid iodine have low melting points because

• A. the covalent bonds within the molecules are weak
• B. they contain no bonds at all
• C. only weak intermolecular forces have to be overcome on melting
• D. their molecules are very heavy

Explanation: Melting separates molecules from one another, and in a molecular crystal they are held only by dispersion forces or dipole interactions, which are far weaker than ionic or covalent bonds. The strong covalent bonds inside each iodine molecule are not broken at all when the solid melts, which is the distinction the question turns on. This is also why such solids are soft and often volatile.

92. Which of the following is a covalent network solid?

• A. Ice
• B. Sodium chloride
• C. Solid carbon dioxide
• D. Diamond

Explanation: In diamond every carbon is covalently bonded to four others in a continuous three dimensional network, so melting it means breaking covalent bonds and the melting point is above 3500 degrees Celsius. Ice and dry ice are molecular solids held by hydrogen bonds and dispersion forces respectively, and sodium chloride is ionic. Silicon dioxide and silicon carbide are the other standard network examples.

93. The smallest repeating portion of a crystal lattice that shows the full symmetry of the crystal is called the

• A. unit cell
• B. crystal face
• C. lattice point
• D. domain

Explanation: Stacking the unit cell repeatedly in three dimensions generates the entire crystal, so knowing its dimensions and contents describes the whole structure. A lattice point is a single position within that cell rather than the repeating block itself. The seven crystal systems are classified by the shapes and angles the unit cell can take.

94. The number of crystal systems into which all crystals are classified is

• A. four
• B. seven
• C. fourteen
• D. twenty

Explanation: There are seven crystal systems, defined by the relative lengths of the unit cell axes and the angles between them, the cubic system being the most symmetrical. These give rise to fourteen Bravais lattices when the different centring arrangements are counted, which is why fourteen is offered as a distractor. Sodium chloride belongs to the cubic system.

95. 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.

96. 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.

97. 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.

98. 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.

99. 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.

100. 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.