Name: lattice energy patterns in ionic solids
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 54 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.3.1

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Trends in Lattice Energy: Ion Size and Charge

L'energia reticolare è associata alla formazione o alla separazione di un reticolo ionico. Tuttavia, quando si formano cloruro di sodio o ossido di magnesio, la loro energia reticolare differisce in modo significativo. Perché ogni composto ionico ha un'energia reticolare diversa, da quali fattori dipende?

Un composto ionico è costituito da una disposizione ordinata di un gran numero di ioni, attratti l'uno all'altro dalle interazioni elettrostatiche. Secondo la legge di Coulomb, l'energia potenziale di due ioni è inversamente proporzionale alla distanza fra gli ioni, che a sua volta dipende dal raggio ionico. Nella tavola periodica, il raggio ionico dei metalli alcalini e alcalino terrosi aumenta lungo la colonna.

All'aumentare della dimensione dello ione metallico, aumenta anche la distanza fra gli ioni, o lunghezza del legame. Per esempio, la lunghezza del legame del bromuro di litio e del bromuro di potassio varia tra 217 pm e 282 pm, rispettivamente. Per via della maggiore distanza internucleare, l'attrazione fra gli ioni diminuisce e diventa molto più facile separare gli ioni.

Pertanto, l'energia reticolare per separare il bromuro di litio solido è maggiore rispetto a quella del bromuro di potassio essendo rispettivamente 807 kJ mol contro 682 kJ mol. Oltre al raggio ionico, l'entità dell'energia reticolare dipende anche dalle cariche ioniche. Secondo la legge di Coulomb, l'energia potenziale degli ioni è direttamente proporzionale al prodotto delle loro cariche.

Considerate due composti ionici, fluoruro di sodio e ossido di calcio. La distanza ionica in entrambi i composti è simile, ma l'energia reticolare dell'ossido di calcio, è quasi il quadruplo del fluoruro di sodio. Sia il sodio che il fluoruro sono ioni monovalenti;il prodotto della loro carica è 1.

Tuttavia, gli ioni calcio e ossido sono bivalenti e il prodotto della loro carica è 4, che è quattro volte più grande del fluoruro di sodio. Questo è il motivo per cui è necessaria quasi il quadruplo di energia per separare l'ossido di calcio in ioni gassosi rispetto al fluoruro di sodio. Pertanto, l'entità dell'energia reticolare è direttamente proporzionale al prodotto delle cariche ioniche ed è inversamente proporzionale alla distanza fra gli ioni.

tendenze nell'energia reticolare: dimensione e carica degli ioni

Un composto ionico è stabile a causa dell'attrazione elettrostatica tra i suoi ioni positivi e negativi. L'energia reticolare di un composto è una misura della forza di questa attrazione. L'energia reticolare (ΔHlattice) di un composto ionico è definita come l'energia richiesta per separare una mole del solido negli ioni gassosi che la compogono. Per il cloruro di sodio solido ionico, l'energia reticolare è la variazione di entalpia del processo:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1058/1058_Equations-01.png" style="height: 40px;" />

Convenzioni

In questo caso viene utilizzata la convenzione secondo cui il solido ionico viene separato in ioni, il che significa che le energie reticolari saranno endotermiche (valori positivi). Un altro modo è utilizzare una convenzione equivalente ma opposta, in cui l'energia del reticolo è esotermica (valori negativi) e descritta come l'energia rilasciata quando gli ioni si combinano per formare un reticolo. Pertanto, assicurati di confermare quale definizione viene utilizzata quando cerchi le energie reticolari in un altro riferimento. In entrambi i casi, una grandezza maggiore dell'energia reticolare indica un composto ionico più stabile. Per il cloruro di sodio, ΔHreticolo = 769 kJ. Pertanto, sono necessari 769 kJ per separare una mole di NaCl solido negli ioni Na^+ e Cl^– gassosi. Quando una mole di ioni gassosi Na+ e Cl^– forma NaCl solido, vengono rilasciati 769 kJ di calore.

Legge di Coulomb ed energia del reticolo

L’energia reticolare ΔHreticolo di un cristallo ionico può essere espressa dalla seguente equazione (derivata dalla legge di Coulomb, che governa le forze tra le cariche elettriche):

ΔHreticolo = C(Z^+)(Z^−)/R_o

in cui C è una costante che dipende dal tipo di struttura cristallina; Z+ e Z– sono le cariche degli ioni e Ro è la distanza interionica (la somma dei raggi degli ioni positivi e negativi). Pertanto, l'energia reticolare di un cristallo ionico aumenta rapidamente all'aumentare delle cariche degli ioni e alla diminuzione delle dimensioni degli ioni. Quando tutti gli altri parametri sono mantenuti costanti, raddoppiando la carica sia del catione che dell'anione si quadruplica l'energia reticolare.

Esempi

<ol>
	<li>L'energia reticolare di LiF (Z^+ e Z^– = 1) è 1023 kJ/mol, mentre quella di MgO (Z^+ e Z^– = 2) è 3900 kJ/mol (R_o è quasi la stessa — circa 200 pm per entrambi i composti) .</li>
	<li>Differenti distanze interatomiche producono differenti energie reticolari. Ad esempio, confrontare l'energia reticolare di MgF_2 (2957 kJ/mol) con quella di MgI_2 (2327 kJ/mol) per osservare l'effetto sull'energia reticolare della dimensione ionica più piccola di F– rispetto a I–.</li>
	<li>La preziosa gemma rubino è ossido di alluminio, Al2O3, contenente tracce di Cr^3^+. Il composto Al_2Se_3 viene utilizzato nella fabbricazione di alcuni dispositivi a semiconduttore. In questi due composti ionici, le cariche Z^+ e Z– sono le stesse, quindi la differenza nell'energia reticolare dipende da R_o. Poiché lo ione O_2– è più piccolo dello ione Se2–, Al2O3 ha una distanza interionica più breve di Al_2Se_3 e ha, quindi, un’energia reticolare maggiore.</li>
	<li>Un altro esempio è l'ossido di zinco, ZnO, rispetto a NaCl. ZnO ha un'energia reticolare maggiore perché i valori Z sia del catione che dell'anione in ZnO sono maggiori e la distanza interionica di ZnO è inferiore a quella di NaCl.</li>
</ol>



Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-5-strengths-of-ionic-and-covalent-bonds">Openstax, Chemistry 2e, Section 7.5: Strengths of Ionic and Covalent Bonds.</a>

Tendenze nell'energia reticolare: dimensione e carica degli ioni

Un composto ionico è stabile a causa dell'attrazione elettrostatica tra i suoi ioni positivi e negativi. L'energia reticolare di un composto è una misura ...

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lattice energy patterns in ionic solids

Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound.

For ionic crystals, lattice energy follows a pattern based on charge and ion size. The relationship can be written as ΔH lattice = C(Z+)(Z−)/Ro. In this equation, C depends on the crystal structure, Z+ and Z− are the ion charges, and Ro is the distance between the ions. Because of this, lattice energy rises quickly when ion charges increase and when ion sizes decrease. If both ion charges double, the lattice energy becomes four times larger.

Examples show these trends clearly. LiF, with ion charges of 1 and 1, has a lattice energy of 1023 kJ/mol, while MgO, with charges of 2 and 2, has a lattice energy of 3900 kJ/mol. Their interionic distances are nearly the same, at about 200 pm. Ion size also matters. MgF2 has a lattice energy of 2957 kJ/mol, while MgI2 has a lower value of 2327 kJ/mol because F− is smaller than I−.

The same idea explains differences in other compounds. Ruby is aluminum oxide, Al2O3, and it contains traces of Cr3+. Al2Se3 is used in some semiconductor devices. These compounds have the same ion charges, so the difference in lattice energy depends on Ro. O2− is smaller than Se2−, so Al2O3 has the shorter interionic distance and the larger lattice energy. ZnO also has a larger lattice energy than NaCl because the ion charges are greater and the interionic distance is smaller.

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Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound. For ionic crystals, lattice energy follows a pattern based on charge and ion size. The...

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Learn how ion charge and ion size affect lattice energy in ionic solids, with examples like NaCl, LiF, MgO, MgF2, MgI2, Al2O3, and ZnO.

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Lattice Energy Patterns in Ionic Solids

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