Name: predicting ionic stability with lattice energy
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 44 s
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The Born-Haber Cycle

De vorming van ionische bindingen vereist een elektronenoverdracht van een metaal naar een niet-metalen atoom een proces dat vaak endotherm is. Wanneer elementair natrium en chloor echter reageren om een vast natriumchloridekristal te vormen, is dat een sterk exotherm proces. Maar waar komt de energie vandaan?Volgens de wet van Coulomb worden kationen en anionen tot elkaar aangetrokken door sterke elektrostatische krachten tot een solide reeks of rooster. De resulterende roosterstructuur wordt gestabiliseerd door het verminderen van de potentiële energie, die vrijkomt als warmte, een exotherme reactie. De totale energie die gepaard gaat met de vorming of afbraak van een kristalstructuur in zijn gasvormige bestanddelen wordt roosterenergie genoemd.In een vaste ionische verbinding interageert een groot aantal geladen deeltjes met elkaar, waardoor het moeilijk is om de exacte waarde van de roosterenergie experimenteel te bepalen. Het kan echter worden berekend met behulp van de wet van Hess in een hypothetische reeks stappen die de Born-Habercyclus wordt genoemd, die de vorming van een ionische verbinding uit de samenstellende elementen vertegenwoordigt. De Born-Habercyclus voor natriumchloridevorming beschouwt bijvoorbeeld twee alternatieve routes, de ene direct en de andere indirect.De directe route vertegenwoordigt de standaard enthalpie van vorming van NaCl uit elementair natrium en chloor. De indirecte route heeft vijf stappen. In de eerste stap wordt natrium in vaste toestand omgezet in zijn gasvormige vorm.Vervolgens dissociëren diatomische chloormoleculen in gasvormige chlooratomen. De derde en vierde stap zijn verantwoordelijk voor de elektronenoverdracht om ionen te vormen. Een elektron wordt verwijderd uit het gasvormige natrium om een natriumkation te vormen.Het elektron wordt vervolgens opgenomen door het gasvormige chloor om een chloride-anion te vormen. In de laatste stap leidt de elektrostatische aantrekking tussen de gasvormige ionen tot de vorming van de roosterstructuur. De wet van Hess stelt dat de verandering in de algehele enthalpie van een stapsgewijs proces de som is van de enthalpie-veranderingen van elke stap.Dit betekent dat de enthalpie-waarde van de directe route gelijk is aan de som van de enthalpie van de vijf stappen. Door de vergelijking voor roosterenergie op te lossen wordt een grote, negatieve waarde bepaald, wat duidt op een exotherme reactie.

de born-haber-cyclus

<h4>Haan energie </h4> 
 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 bedoeld als de energie die nodig is om één mol van de vaste stof te scheiden in zijn samengestelde gasvormige ionen. 

 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 (negatieve waarden) is en wordt beschreven als de energie die vrijkomt wanneer ionen combineren om een haan te vormen. Zorg er dus voor dat u zeker weet welke definitie wordt gebruikt wanneer u roosterenergieën in een andere referentie opzoekt. 

 In beide gevallen komt een grotere waarde voor roosterenergie op een stabielere ionische verbinding voor. 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>Bepaling van de roosterenergie van een ionische verbinding</h4> 

 Het is niet mogelijk om roosterenergieën rechtstreeks te meten. De roosterenergie kan echter worden berekend met behulp van een thermochemische cyclus. De Born-Haber-cyclus is een toepassing van de wet van Hess die de vorming van een ionische vaste stof opsplitst in een reeks individuele stappen: 

 <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;">Enthalpie van sublimatie van 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;">De helft van de bindingsenergie van 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;">Ionisatie-energie van 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;">Elektronenaffiniteit van 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;">Negatief van de roosterenergie van 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 = −ΔHrooster = ?</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Enthalpie van vorming van CsF (s), voeg stappen 1–5 toe</td>
 			<td colspan="2" style="color: black; border: 1px solid black; text-align: center;">ΔH = ΔHf° = ΔHs°+ ½ D + IE + (EA) + (−ΔHrooster) 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>Bekijk de elementen in hun meest standaard toestanden, Cs (s) en F2 (g). </li> <li>De ΔHs° vertegenwoordigt de omzetting van grote cesium in een gas (sublimatie), waardoor de ionisatie-energie de gasvormige cesiumatomen omzet in kationen. </li> <li>In de volgende stap moet rekening worden gehouden met de energie die nodig is om de F-F-binding te verbreken en fluoratomen te producent. </li> <li>Het omzetten van één mol fluoratomen in fluoride-ionen is een exotherm proces, dus deze stap geeft energie af (de exponaffiniteit) </li> <li>Er is nu één mol Cs-kationen en één mol F-anionen aanwezig. Deze ionen combineren een groot cesiumfluoride als producent. De enthalpieverandering in deze stap is de negatieve van de roosterenergie, dus het is ook een exotherme grootheid. </li> <li>De totale energie die bij deze omzetting betrokken is, is gelijk aan het experimentele bepaalde vormingsenthalpie, ΔHf°, van de verbinding uit zijn elementen. In dit geval is er sprake van een enorme verandering exotherm.</li> </ol> Roosterenergieën componenten voor ionische verbindingen zijn doorgaans veel hoger dan bindingsdissociatie-energieën gemeten voor covalente bindingen. Terwijl roosterenergieën doorgaans in het bereik van 600 – 4000 kJ/mol liggen (sommige zelfs hoger), liggen de dissociatie-energieën van covalente bindingen doorgaans tussen 150 – 400 kJ/mol voor enkelvoudige bindingen. Houd er echter rekening mee dat dit geen directe relevante waarden zijn. Voor ionische verbindingen worden roosterenergieën geassocieerd met veel interacties, waardoor kationen en anionen zich samenpakken in een uitgebreid rooster. Voor covalente bindingen is de dissociatie-energie van de binding gerelateerd aan de interactie van slechts twee atomen. <h4>Roosterenergie als functie van ionenstraal en lading </h4> De roosterenergie van een ionisch kristal neemt snel toe krachtig de ladingen van de ionen groter en de afmetingen van de ionen afnemen. Wanneer alle andere parameters constant worden gehouden, wordt een verdubbeling van de lading van zowel de kation als het anion van de roosterenergie verviervoudigd. De roosterenergie van LiF (Z+ en Z– = 1) is bijvoorbeeld 1023 kJ/mol, terwijl die van MgO (Z+ en Z– = 2) 3900 kJ/mol is (Ro = de interionische afstand bedoeld als de som van de stralen van de positieve en negatieve ionen zijn bijna hetzelfde (individueel 200 pm voor beide verbindingen). Verschillende interatomaire afstanden producent verschillende roosterenergieën. Vergelijk bijvoorbeeld de roosterenergie van MgF2 (2957 kJ/mol) met die van MgI2 (2327 kJ/mol), wat het effect op de roosterenergie aantoont van de kleinere ionengrootte van F– in vergelijking met I–. <h4>Andere toepassingen van de Born-Haber-cyclus</h4> De Born-Haber-cyclus kan ook worden gebruikt om een van de andere grootheden in de vergelijking voor roosterenergie te berekenen, op voorwaarde dat de rest bekend is. Als bijvoorbeeld de relevante sublimatie-enthalpie ΔHs°, ionisatie-energie (IE), bindingsdissociatie-enthalpie (D), roosterenergie ΔHrooster en standaardvormingsenthalpie ΔHf° bekend zijn, kan de Born-Haber-cyclus worden gebruikt om de affiniteit van een atoom.

De Born-Haber-cyclus

Lattice Energy&nbsp;

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

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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

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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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Electronegativity

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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