The magnitude of enthalpy for formation of alkali metal...

The magnitude of enthalpy for formation of alkali metal halides decreases in the order
(a) fluoride $>$ chloride $>$ iodide $>$ bromide
(b) iodide $>$ bromide $>$ chloride $>$ fluoride
(c) fluoride $>$ chloride $>$ bromide $>$ iodide
(d) bromide $>$ iodide $>$ fluoride $>$ chloride

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Key Concepts

Enthalpy of Formation

This is the heat change associated with forming a compound from its constituent elements in their standard states. It combines contributions from various energetic processes, including bond breaking in the reactants and bond making in the products, and is a key thermodynamic quantity used to understand the stability of compounds.

Lattice Energy

Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is a major contributor to the enthalpy of formation of ionic compounds, with higher lattice energies typically resulting in more exothermic formation enthalpies. The magnitude of lattice energy depends on the sizes and charges of the ions involved.

Ionic Radii and Periodic Trends

The size of the ions, which follows periodic trends, plays a crucial role in determining lattice energy and overall stability of the ionic compound. In alkali metal halides, as the halide ion becomes larger (from fluoride to iodide), the lattice energy decreases due to increased interionic distance, thus directly influencing the magnitude of the formation enthalpy.

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