Q.Explain how zeolites are used to soften hard water, and how a shape-selective zeolite catalyst such as ZSM-5 is used in the petrochemical industry.

[!TLDR] Zeolites soften hard water by exchanging their Na+ for dissolved Ca2+/Mg2+, and shape-selective zeolites like ZSM-5 catalyse reactions (e.g. alcohol-to-gasoline conversion) using their uniform, molecule-sized pores. [!ANSWER] Sodium-form zeolites soften hard water by exchanging framework Na+ for the Ca2+/Mg2+ ions responsible for hardness; ZSM-5 is a shape-selective zeol …

Zeolites' two major industrial applications both stem from the two structural features described previously: their exchangeable balancing cations, and their uniform, molecule-sized internal pore structure. For water softening, a bed of sodium-form zeolite (historically sold as 'permutit') is used: as hard water — water containing dissolved $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, which are responsible for water 'hardness' (poor lathering with soap, scale formation) — is passed through the zeolite bed, the doubly-charged $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ ions are preferentially taken up by the zeolite framework, displacing and releasing the framework's own $\text{Na}^{+}$ ions into the water in exchange. The result is water from which the hardness-causing ions have been removed and replaced by harmless sodium ions; the same underlying ion-exchange principle is used in some laundry detergent formulations, which include zeolites to reduce the effective hardness of the wash water. For shape-selective catalysis, zeolites' rigid framework provides both a regular, uniform network of molecule-sized pores and acidic catalytic sites generated at the aluminium centres within the framework; only molecules of the appropriate size and shape can diffuse into (or leave) these internal pores to reach the active sites, giving the catalyst high selectivity for particular reaction prod …

Explain the ion-exchange mechanism for water softening (Na+ swapped for Ca2+/Mg2+), then explain shape-selective catalysis via pore-size …

Describing water softening as removing hardness ions by simple filtration/adsorption rather than by genuine cation exchange; omitting the specific role …