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

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Trends in Lattice Energy: Ion Size and Charge

Roosterenergie wordt geassocieerd met de vorming of scheiding van een ionrooster. Maar wanneer natriumchloride of magnesiumoxide wordt gevormd, verschilt hun roosterenergie aanzienlijk. Waarom heeft elke ionische verbinding een andere roosterenergie en van welke factoren is deze afhankelijk?Een ionische verbinding bestaat uit een geordende opstelling van een groot aantal geladen ionen die door elektrostatische interacties naar elkaar worden aangetrokken. Volgens de wet van Coulomb is de potentiële energie van twee ionen omgekeerd evenredig met de afstand tussen de ionen, die op zijn beurt afhangt van de ionenstraal. In het periodiek systeem neemt de ionenstraal van de alkali-en aardalkalimetalen toe verder naar beneden in de kolom.Naarmate de grootte van het metaalion toeneemt, wordt de afstand tussen de ionen of de bindingslengte ook groter. De bindingslengte van lithiumbromide en kaliumbromide verschilt bijvoorbeeld tussen respectievelijk 217 picometer en 282 picometer. Door de grotere internucleaire afstand neemt de aantrekkingskracht tussen de ionen af en wordt het veel gemakkelijker om de ionen te scheiden.De roosterenergie om vast lithiumbromide te scheiden is dus groter dan voor kaliumbromide, respectievelijk 807 kJ/mol versus 682 kJ/mol. Naast de ionenstraal hangt de grootte van de roosterenergie ook af van de ionenladingen. Volgens de wet van Coulomb is de potentiële energie van ionen recht evenredig met het product van hun ladingen.Denk aan twee ionische verbindingen natriumfluoride en calciumoxide. De ionische afstand in beide verbindingen is vergelijkbaar, maar de roosterenergie van calciumoxide is bijna vier keer groter dan die van natriumfluoride. Zowel natrium als fluoride zijn monovalente ionen;het product van hun lading is 1.Calcium-en oxide-ionen zijn echter divalent en het product van hun lading is 4, wat vier keer groter is dan natriumfluoride. Daarom kost het bijna vier keer zoveel energie om het calciumoxide in gasvormige ionen te scheiden in vergelijking met natriumfluoride. De grootte van de roosterenergie is dus recht evenredig met het product van de ionenladingen en omgekeerd evenredig met de afstand tussen de ionen.

trends in roosterenergie: ionengrootte en lading

Een ionische verbinding is stabiel vanwege de elektrostatische aantrekkingskracht tussen de positieve en negatieve ionen. De roosterenergie van een verbinding is een maatstaf voor de sterkte van deze aantrekkingskracht. De roosterenergie (ΔHrooster) van een ionische verbinding wordt gedefinieerd als de energie die nodig is om één mol van de vaste stof te scheiden in zijn samenstellende gasvormige ionen. Voor het ionische vaste natriumchloride is de roosterenergie de enthalpieverandering van het proces: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1058/1058_Equations-01.png" style="height: 40px;"> <h4>Conventies</h4> Hier wordt de conventie gebruikt waarbij de ionische vaste stof wordt gescheiden in ionen, wat betekent dat de roosterenergieën endotherm zullen zijn (positieve waarden). Een andere manier is om een gelijkwaardige maar tegengestelde conventie te gebruiken, waarbij de roosterenergie exotherm is (negatieve waarden) en wordt beschreven als de energie die vrijkomt wanneer ionen combineren om een rooster te vormen. Zorg er dus voor dat u bevestigt welke definitie wordt gebruikt wanneer u roosterenergieën in een andere referentie opzoekt. In beide gevallen duidt een grotere waarde voor roosterenergie op een stabielere ionische verbinding. Voor natriumchloride is ΔHrooster = 769 kJ. Er is dus 769 kJ nodig om één mol vast NaCl te scheiden in gasvormige Na+- en Cl–-ionen. Wanneer één mol gasvormige Na+- en Cl–-ionen vast NaCl vormt, komt er 769 kJ aan warmte vrij. <h4>De wet van Coulomb en roosterenergie</h4> De roosterenergie ΔHrooster van een ionisch kristal kan worden uitgedrukt door de volgende vergelijking (afgeleid van de wet van Coulomb, die de krachten tussen elektrische ladingen regelt): ΔHrooster = C(Z +)(Z−)/Ro waarin C een constante is die afhangt van het type kristalstructuur; Z+ en Z– zijn de ladingen op de ionen, en Ro is de interionische afstand (de som van de stralen van de positieve en negatieve ionen). De roosterenergie van een ionisch kristal neemt dus snel toe naarmate de ladingen van de ionen toenemen en de afmetingen van de ionen afnemen. Wanneer alle andere parameters constant worden gehouden, verviervoudigt een verdubbeling van de lading van zowel het kation als het anion de roosterenergie. <h4>Voorbeelden</h4> <ol> <li>De roosterenergie van LiF (Z+ en Z– = 1) is 1023 kJ/mol, terwijl die van MgO (Z+ en Z– = 2) 3900 kJ/mol is (Ro is bijna hetzelfde - ongeveer 200 pm voor beide verbindingen).</li> <li>Verschillende interatomaire afstanden produceren verschillende roosterenergieën. Vergelijk bijvoorbeeld de roosterenergie van MgF2 (2957 kJ/mol) met die van MgI2 (2327 kJ/mol) om het effect op de roosterenergie van de kleinere ionengrootte van F– in vergelijking met I– te observeren.</li> <li>De kostbare edelsteen robijn is aluminiumoxide, Al2O3, dat sporen van Cr3+ bevat. De verbinding Al2Se3 wordt gebruikt bij de vervaardiging van sommige halfgeleiderapparaten. In deze twee ionische verbindingen zijn de ladingen Z+ en Z– hetzelfde, dus het verschil in roosterenergie hangt af van Ro. Omdat het O2–ion kleiner is dan het Se2–ion, heeft het Al2O3 een kortere interionische afstand dan Al2Se3 en heeft het daarom een grotere roosterenergie.</li> <li>Een ander voorbeeld is zinkoxide, ZnO, vergeleken met NaCl. ZnO heeft een grotere roosterenergie omdat de Z-waarden van zowel het kation als het anion in ZnO groter zijn, en de interionische afstand van ZnO kleiner is dan die van NaCl.</li> </ol> Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/7-5-strengths-of-ionic-and-covalent-bonds">Openstax, Chemistry 2e, Sectie 7.5: Sterkten van ionische en covalente bindingen.</a>

Trends in roosterenergie: ionengrootte en lading

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

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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 form when positive and negative ions attract each other. A cation is a positive ion, and it forms when an atom loses one or more valence electrons. An anion is a negative ion, and it forms when an atom gains one or more valence electrons. Compounds made of these ions are called ionic compounds, or salts, and the force between them is an ionic bond. The properties of ionic solids help show how strong this attraction is. They usually form rigid, brittle crystals and have high...

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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Lewis Structures with Formal Charge

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Octet Rule Exceptions in Lewis Structures

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Electron sea model explains how metallic bonding holds metals together. In this model, most metal atoms do not have enough valence electrons to form ionic or covalent bonds in the usual way. Their valence electrons are held loosely because metals have low electronegativity, and their ionization energy is low, so electrons are removed easily. When a metal atom loses these outer electrons, it becomes a positively charged metal ion. The freed electrons form a delocalized electron cloud around...

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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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	...

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Analyze the structure of atoms and isotopes.

Lattice Energy Patterns in Ionic Solids

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