Name: lattice energy patterns in ionic solids
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 54 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

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Trends in Lattice Energy: Ion Size and Charge

A energia da malha está associada à formação ou separação de uma malha iónica. No entanto, quando o cloreto de sódio ou óxido de magnésio se formam, a sua energia de malha difere significativamente. Porque é que cada composto iónico tem uma energia de malha diferente, e de que fatores depende, um composto iónico consiste num arranjo ordenado de um grande número de íons carregados atraídos uns pelos outros por interações eletrostáticas.Segundo a lei de Coulomb, a energia potencial de dois íons é inversamente proporcional à distância entre os íons, o que, por sua vez, depende do raio iónico. Na tabela periódica, o raio iónico dos metais alcalinos e alcalino-terrosos aumenta ao longo da coluna. À medida que o tamanho do ião metálico aumenta, a distância entre os íons, ou o comprimento da ligação, também se torna maior.Por exemplo, o comprimento da ligação do brometo de lítio e do brometo de potássio difere entre 217 picômetros e 282 picômetros, respetivamente. Devido ao aumento da distância internuclear, a atração entre os íons diminui e torna-se muito mais fácil separar os íons. Assim, a energia da malha para separar o brometo de lítio sólido é maior do que para o brometo de potássio sendo 807 kilojoules/mol contra 682 kilojoules/mol, respetivamente.Para além do raio iónico, a magnitude da energia da malha também depende das cargas dos íons. De acordo com a lei de Coulomb, a energia potencial dos íons é diretamente proporcional ao produto dos seus encargos. Consideremos dois compostos iónicos de fluoreto de sódio e óxido de cálcio.A distância iónica em ambos os compostos é semelhante, mas a energia da malha do óxido de cálcio é quase quatro vezes maior do que a do fluoreto de sódio. Tanto o sódio como o flúor são íons monovalentes;o produto do seu encargo é 1. No entanto, os íons de cálcio e óxido são bivalentes e o produto do seu encargo é 4, que é quatro vezes maior do que o do fluoreto de sódio.É por isso que é preciso quase quatro vezes mais energia para separar o óxido de cálcio em íons gasosos em comparação com o fluoreto de sódio. Assim, a magnitude da energia da malha é diretamente proporcional ao produto das cargas iónicas e é inversamente proporcional à distância entre os íons.

tendências em energia reticular: tamanho e carga de iões

Um composto iónico é estável devido à atração eletrostática entre os seus iões positivos e negativos. A energia reticular de um composto é uma medida da força desta atração. A energia reticular (ΔHreticular) de um composto iónico é definida como a energia necessária para separar um mole do sólido nos seus iões gasosos constituintes. Para o sólido iónico cloreto de sódio, a energia reticular é a alteração da entalpia do processo:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1058/1058_Equations-01.png" style="height: 40px;" />

<h4>Convenções</h4>

Aqui, é usada a convenção onde o sólido iónico é separado em iões, o que significa que as energias reticulares serão endotérmicas (valores positivos). Outra forma é usar uma convenção equivalente, mas oposta, na qual a energia reticular é exotérmica (valores negativos) e descrita como a energia libertada quando os iões se combinam para formar uma rede. Assim, certifique-se de confirmar que definição é usada ao procurar energias reticulares em outra referência. Em ambos os casos, uma magnitude maior para a energia reticular indica um composto iónico mais estável. Para cloreto de sódio, ΔHreticular = 769 kJ. Portanto, requer 769 kJ para separar um mole de NaCl sólido nos iões gasosos Na+ e Cl–. Quando um mole de cada um dos iões gasosos Na+ e Cl– formam NaCl sólido, são libertados 769 kJ de calor.

<h4>A Lei de Coulomb e a Energia Reticular</h4>

A energia reticular ΔHreticular de um cristal iónico pode ser expressa pela seguinte equação (derivada da lei de Coulomb, que rege as forças entre cargas elétricas): 

ΔHreticular =  C(Z +)(Z−)/Ro 

em que C é uma constante que depende do tipo de estrutura cristalina; Z+ e Z– são as cargas dos iões, e Ro é a distância interiónica (a soma dos raios dos iões positivos e negativos). Assim, a energia reticular de um cristal iónico aumenta rapidamente à medida que as cargas dos iões aumentam e os tamanhos dos iões diminuem. Quando todos os outros parâmetros são mantidos constantes, a duplicação da carga tanto do catião como do anião quadruplica a energia reticular. 

<h4>Exemplos</h4>

<ol>
<li>A energia reticular de LiF (Z+ e Z– = 1) é de 1023 kJ/mol, enquanto que a de MgO (Z+ e Z– = 2) é de 3900 kJ/mol (Ro é quase igual — cerca de 200 pm para ambos os compostos).</li>
<li>Diferentes distâncias interatómicas produzem diferentes energias reticulares. Por exemplo, compare a energia reticular de MgF2 (2957 kJ/mol) com a de MgI2 (2327 kJ/mol) para observar o efeito na energia reticular do tamanho iónico menor de F– em comparação com I–.</li>
<li>A jóia preciosa rubi é óxido de alumínio, Al2O3, contendo vestígios de Cr3+. O composto Al2Se3 é usado na fabricação de alguns dispositivos semicondutores. Nestes dois compostos iónicos, as cargas Z+ e Z– são as mesmas, de modo que a diferença na energia reticular depende de Ro. Como o ião O2– é menor que o ião Se2–, Al2O3 tem uma distância interiónica menor que Al2Se3 e tem, portanto, maior energia reticular.</li>
<li>Outro exemplo é o óxido de zinco, ZnO, comparado com NaCl. ZnO tem uma maior energia reticular porque os valores de Z do catião e do anião no ZnO são maiores, e a distância interiónica de ZnO é menor que a do NaCl.</li>
</ol>

Este texto é adaptado de <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>

Tendências em Energia Reticular: Tamanho e Carga de Iões

Um composto iônico é estável devido à atração eletrostática entre seus íons positivos e negativos. A energia da rede de um composto é uma medida ...

Education

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Chemical Bonding: Basic Concepts

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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.

Lattice Energy Patterns in Ionic Solids

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Chemical Bonding and Bond Properties

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Ionic Compounds and Crystal Lattices

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trends in lattice energy: ion size and charge

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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Shared Electrons in Covalent Molecules

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lewis structures of molecular compounds and polyatomic ions

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Drawing Lewis Structures with Valence Electron Counts

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Resonance Forms in Lewis Structures

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Lewis Structures with Formal Charge

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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.

Lattice Energy Patterns in Ionic Solids | JoVE K12

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Lattice Energy Patterns in Ionic Solids

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