Free Lattice Energy MCQs with Answers

1. 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.

2. 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.

3. 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.

4. 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.