Name: predicting ionic stability with lattice energy
Uploaded: 2020-09-03T13:38:28.000Z
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The Born-Haber Cycle

Die Bildung von Ionenbindungen erfordert einen Elektronentransfer von einem Metall-zu einem Nichtmetallatom ein Prozess, der oft endotherm ist. Wenn jedoch elementares Natrium und Chlor reagieren, um einen festen Natriumchlorid Kristall zu bilden, ist das ein stark exothermer Prozess. Aber woher kommt die Energie?Nach dem Coulombschen Gesetz, werden Kationen und Anionen durch starke elektrostatische Kräfte voneinander angezogen, zu einer festen Anordnung bzw.Gitter. Die resultierende Gitterstruktur ist stabilisiert durch Verringerung der potentiellen Energie, die als Wärme freigesetzt wird, eine exotherme Reaktion. Die Gesamtenergie, die mit der Entstehung oder dem Abbau des kristallinen Gitters in seine gasförmigen Bestandteile entsteht, wird Gitterenergie genannt.In einer festen ionischen Verbindung ist eine große Anzahl von geladenen Teilchen, die miteinander wechselwirken, was es schwierig macht, den genauen Wert der Gitterenergie experimentell zu bestimmen. Sie kann jedoch mit dem Hess'schen Gesetz berechnet werden, in einer hypothetischen Reihe von Schritten, die als der Born-Habe-Zyklus genannt wird, der die Bildung einer ionischen Verbindung aus ihren Bestandteilen darstellt. Zum Beispiel zieht der Born-Haber-Zyklus für die Bildung von Natriumchlorid zwei alternative Wege in Betracht ein direkten und ein indirekten.Der direkte Weg stellt die Standardenthalpie der Bildung von NaCl aus elementarem Natrium und Chlor dar. Der indirekte Weg hat fünf Schritte. Im ersten Schritt wird Natrium im festen Zustand in seine gasförmige Form umgewandelt.Danach dissoziieren zweiatomigen Chlormolekülen in gasförmige Chloratome. Der dritte und vierte Schritt erklärt die Elektronübertragung in Form von Ionen. Ein Elektron wird von dem gasförmigem Natrium entfernt, um ein Natrium-Kation zu bilden.Das Elektron wird dann von dem gasförmigen Chlor zur Bildung eines Chlorid-Anions aufgenommen. Im letzten Schritt führt die elektrostatische Anziehung zwischen den gasförmigen Ionen zur Bildung der Gitterstruktur. Das Hess'sche Gesetz besagt, dass die Änderung der Gesamtenthalpie eines schrittweisen Prozesses die Summe der Enthalpieänderungen der einzelnen Schritte ist.Das bedeutet das die Enthalpiewerte des direkten Weges gleich der Summe der Enthalpien der fünf Schritte ist. Durch das Lösen der Gitterenergie-Gleichung wird ein großer, negativer Wert ermittelt, was eine exotherme Reaktion bedeutet.

der born-haber-kreisprozess

<h4>Gitterenergie</h4> Eine ionische Verbindung ist aufgrund der elektrostatischen Anziehung zwischen ihren positiven und negativen Ionen stabil. Die Gitterenergie einer Verbindung ist ein Maß für die Stärke dieser Anziehung. Die Gitterenergie (ΔHGitter) einer ionischen Verbindung ist definiert als die Energie, die erforderlich ist, um ein Mol des Feststoffs in seine gasförmigen Ionen zu zerlegen. Hier wird die Konvention verwendet, das der ionische Feststoff in Ionen aufgetrennt wird und die Gitterenergien somit endotherm sind (positive Werte). Eine andere Möglichkeit besteht darin, eine äquivalente, aber entgegengesetzte Betrachtungsweise zu wählen, bei der die Gitterenergie exotherm ist (negative Werte) und als die Energie beschrieben wird, die freigesetzt wird, wenn sich Ionen zu einem Gitter verbinden. Achten Sie daher darauf, sich zu versichern, welche Definition verwendet wird, wenn Sie in einer anderen Referenz nach Gitterenergien suchen. In beiden Fällen weist ein größerer Betrag der Gitterenergie auf eine stabilere ionische Verbindung hin. Für Natriumchlorid beträgt ΔHGitter = 769 kJ. Somit sind 769 kJ erforderlich, um ein Mol festes NaCl in gasförmige Na+ und Cl–Ionen zu trennen. Wenn jeweils ein Mol gasförmiges Na+ und Cl–Ionen festes NaCl bilden, werden 769 kJ Wärme freigesetzt. <h4>Bestimmung der Gitterenergie einer ionischen Verbindung</h4> Es ist nicht möglich, Gitterenergien direkt zu messen. Die Gitterenergie kann jedoch mithilfe eines thermochemischen Zyklus berechnet werden. Der Born-Haber-Zyklus ist eine Anwendung des Hessschen Gesetzes, das die Bildung eines ionischen Feststoffs in eine Reihe einzelner Schritte unterteilt: <table style="margin-left: auto; margin-right: auto;"> <tbody style="border: 1px solid black;"> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Sublimationsenthalpie von Cs (s)</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">Cs (s) → Cs (g)</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = ΔHs° = 76.5 kJ/mol</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Die Hälfte der Bindungsenergie vonof F2</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">½ F2 (g) → F (g)</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = ½ D = 79.4 kJ/mol</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Ionization energy of Cs (g)</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">Cs (g) → Cs+ (g) + e−</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = IE = 375.7 kJ/mol</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Elektronenaffinität von F</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">F (g) + e− → F− (g)</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = EA = −328.2 kJ/mol</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Negativ der Gitterenergie von CsF (s)</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">Cs+ (g) + F− (g) → CsF (s)</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = −ΔHlattice = ?</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Bildungsenthalpie CsF (s), add steps 1–5</td> <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">ΔH = ΔHf° = ΔHs°+ ½ D + IE + (EA) + (−ΔHlattice) Cs (s) + ½ F2 (g) → CsF (s)</td> <td style="color: black; border: 1px solid black; text-align: center;">ΔH = −553.5 kJ/mol</td> </tr> </tbody> </table> <ol> <li>Betrachten Sie die Elemente in ihren häufigsten Zuständen, Cs (s) und F2 (g).</li> <li>Das ΔHs° stellt die Umwandlung von festem Cäsium in ein Gas dar (Sublimation) gefolgt von der anschließenden Umwandlung der Cäsiumatome durch die Ionisierungsenergie in Kationen</li> <li>Im nächsten Schritt muss die Energie berücksichtigt werden, die zum Aufbrechen der F–F-Bindung zur Bildung von Fluoratomen erforderlich ist.</li> <li>Die Umwandlung eines Mols Fluoratome in Fluoridionen ist ein exothermer Prozess, daher wird bei diesem Schritt Energie (durch die Elektronenaffinität) freigesetzt.</li> <li>Nun liegt ein Mol Cs-Kationen und ein Mol F-Anionen vor. Diese Ionen verbinden sich zu festem Cäsiumfluorid. Die Enthalpieänderung in diesem Schritt ist das Negative der Gitterenergie, also auch eine exotherme Größe.</li> <li>Die Gesamtenergie dieser Umwandlung entspricht der experimentell bestimmten Bildungsenthalpie ΔHf° der Verbindung aus ihren Elementen. In diesem Fall ist die Gesamtänderung exotherm.</li> </ol> Die für ionische Verbindungen berechneten Gitterenergien sind typischerweise viel höher als die für kovalente Bindungen gemessenen Bindungsdissoziationsenergien. Während die Gitterenergien typischerweise im Bereich von 600 – 4000 kJ/mol (einige sogar höher) liegen, liegen die Dissoziationsenergien kovalenter Bindungen für Einfachbindungen typischerweise zwischen 150 – 400 kJ/mol. Beachten Sie jedoch, dass es sich hierbei nicht um direkt vergleichbare Werte handelt. Bei ionischen Verbindungen sind Gitterenergien mit vielen Wechselwirkungen verbunden, da sich Kationen und Anionen in einem ausgedehnten Gitter zusammenpacken. Bei kovalenten Bindungen hängt die Dissoziationsenergie der Bindungs nur von der Wechselwirkung zweier Atome ab. <h4>Gitterenergie als Funktion von Ionenradius und Ladung</h4> Die Gitterenergie eines Ionenkristalls nimmt mit zunehmender Ladung der Ionen stark zu und die Größe der Ionen nimmt ab. Wenn alle anderen Parameter konstant gehalten werden, führt eine Verdoppelung der Ladung sowohl des Kations als auch des Anions zu einer Vervierfachung der Gitterenergie. Beispielsweise beträgt die Gitterenergie von LiF (Z+ und Z– = 1) 1023 kJ/mol, während die von MgO (Z+ und Z– = 2) 3900 kJ/mol beträgt (Ro = der interionische Abstand, definiert als die Summe der Radien der positiven und negativen Ionen ist nahezu gleich – etwa 200 pm für beide Verbindungen). Unterschiedliche interatomare Abstände erzeugen unterschiedliche Gitterenergien. Die Gitterenergie von MgF2 (2957 kJ/mol) und die von MgI2 (2327 kJ/mol) zeigen die Auswirkung der kleineren Ionengröße von F– im Vergleich zu I– und ihren Einfluss auf die Gitterenergie. <h4>Andere Anwendungen des Born-Haber-Kreisprozess</h4> Der Born-Haber-Kreisprozess kann auch zur Berechnung jeder anderen Größe in der Gleichung verwendet werden, sofern der Rest bekannt ist. Wenn beispielsweise die relevante Sublimationsenthalpie ΔHs°, die Ionisierungsenergie (IE), die Bindungsdissoziationsenthalpie (D), die Gitterenergie ΔHGitter und die Standardbildungsenthalpie ΔHf° bekannt sind, kann der Born-Haber-Kreisprozess zur Bestimmung der Elektronenaffinität eines Atoms verwendet werden. Dieser Text wurde adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/7-5-strengths-of-ionic-and-covalent-bonds">Openstax, Chemistry 2e, Section7.5: Strengths of Ionic and Covalent Bonds.</a>

Der Born-Haber-Kreisprozess

Gitterenergie 

Eine ionische Verbindung ist aufgrund der elektrostatischen Anziehungskraft zwischen ihren positiven und negativen Ionen stabil. Die Gitterenergie eines ...

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predicting ionic stability with lattice energy

Lattice energy helps explain why ionic compounds are stable. It measures the strength of attraction between positive and negative ions in a crystal. In one common definition, lattice energy is the energy required to separate one mole of an ionic solid into gaseous ions, so it has a positive value. Another definition uses the opposite sign and describes the energy released when gaseous ions form a lattice. When checking data, it is important to confirm which convention is being used. In either case, a larger lattice energy magnitude means a more stable ionic compound.

Sodium chloride gives a simple example. Its lattice energy is 769 kJ/mol, which means 769 kJ is needed to pull one mole of solid NaCl apart into Na+ and Cl- ions in the gas phase. The same amount of heat is released when those ions come together to form solid NaCl. This value shows how strongly the ions attract each other in the crystal.

Lattice energy cannot be measured directly. Instead, it is found with a thermochemical cycle. The Born-Haber cycle uses Hess’s law to break the formation of an ionic solid into separate steps. For cesium fluoride, these steps include sublimation of Cs(s) to Cs(g), half of the F2 bond energy to make F(g), ionization of Cs(g) to Cs+(g) plus an electron, electron affinity of F(g) to make F-(g), and formation of solid CsF from the gaseous ions. The enthalpy change for forming the lattice is the negative of the lattice energy.

For CsF, the overall reaction Cs(s) + 1/2 F2(g) -&gt; CsF(s) has a standard enthalpy of formation of -553.5 kJ/mol. The cycle combines the sublimation enthalpy of cesium, 76.5 kJ/mol, one-half of the F2 bond energy, 79.4 kJ/mol, the ionization energy of cesium, 375.7 kJ/mol, and the electron affinity of fluorine, -328.2 kJ/mol, with the lattice term. Because the total is exothermic, energy is released overall when the solid forms from the elements in their most common states, Cs(s) and F2(g).

Lattice energies for ionic compounds are usually much larger than bond dissociation energies for covalent bonds. Ionic lattice energies often fall between 600 and 4000 kJ/mol, and some are even higher. Single covalent bond dissociation energies are usually about 150 to 400 kJ/mol. These values are not directly comparable because lattice energy reflects many attractions in an extended crystal, while a covalent bond energy describes the interaction of just two atoms.

Ion charge and ion size strongly affect lattice energy. When all other factors stay the same, larger charges give much larger lattice energies, and smaller ions also increase lattice energy. Doubling the charge on both ions can quadruple the lattice energy. For example, LiF has a lattice energy of 1023 kJ/mol, while MgO is 3900 kJ/mol, even though their interionic distances are nearly the same at about 200 pm. Size also matters in compounds such as MgF2 and MgI2, where the smaller F- ion leads to a higher lattice energy than the larger I- ion.

The Born-Haber cycle can also solve for other unknowns in the lattice energy equation. If the other terms are known, the cycle can be used to find a missing quantity such as electron affinity. This makes the method useful for linking measured enthalpy values to the energy changes inside ionic compounds.

Predicting Ionic Stability with Lattice Energy

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the born-haber cycle

Lattice energy helps explain why ionic compounds are stable. It measures the strength of attraction between positive and negative ions in a crystal. In one common definition, lattice energy is the energy required to separate one mole of an ionic solid into gaseous ions, so it has a positive value. Another definition uses the opposite sign and describes the energy released when gaseous ions form a lattice. When checking data, it is important to confirm which convention is being used. In either...

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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 lattice energy, ion charge, ion size, and the Born-Haber cycle help explain ionic stability and calculate values for compounds like CsF.

Predicting Ionic Stability with Lattice Energy | JoVE K12

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Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

Analyze the structure of atoms and isotopes.

Predicting Ionic Stability with Lattice Energy

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