zeolite A group of hydrated aluminosilicate minerals
that occur in nature or are manufactured. They are
crystalline compounds composed of silicon, oxygen,
and some aluminum atoms. Zeolite ion-exchange units
are extensively used in water conditioning to remove
calcium and magnesium ions by replacing them with
sodium ions. Zeolites are used for separating mixtures
by selective absorption.

zero-order reaction A reaction whose rate is inde-
pendent of the concentration of reactants.

zero-point energy The energy of the lowest state of
a quantum system.

Ziegler-Natta catalyst(ZNC) A catalyst involved
in creating highly structured (stereoregular) polyolefin
polymer chains. Used to mass-produce polyethylene
and polypropylene.

zinc finger A DOMAIN, found in certain DNA-bind-
ing proteins, comprising a HELIX-loop structure in
which a zinc ion is COORDINATEDto 2–4 cysteine sul-
furs, the remaining LIGANDs being histidines. In many

proteins of this type, the domain is repeated several

times.

zone refining A technique for producing solids of

extreme purity. By moving a heater slowly along a bar

of the material to be refined, a molten region is formed

that carries impurities with it along the bar.

Zucker-Hammett hypothesis This hypothesis

states that, if in an acid-catalyzed reaction, lg k 1 (first-

order RATE CONSTANTof the reaction) is linear in Ho

(Hammett ACIDITY FUNCTION), water is not involved

in the TRANSITION STATEof the RATE-CONTROLLING

STEP. However, if lg k 1 is linear in lg[H+], then water is

involved. This has been shown to be incorrect by

Hammett.

Z-value An index of the IONIZING POWERof a sol-

vent based on the frequency of the longest-wavelength

electronic absorption maximum of 1-ethyl-4-methoxy-

carbonylpyridinium iodide in the solvent. The Z-value

is defined by

Z = 2.859 × 104 /λ

where Z is in kcal mol–1and λis in nm.

See also DIMROTH-REICHARDT ET PARAMETER;

GRUNWALD-WINSTEIN EQUATION.

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Z