Name: predicting ionic stability with lattice energy
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 44 s
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North Carolina K-12 Science Standard Course of Study

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Chemistry

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PS.Chm.3.1

The Born-Haber Cycle

离子键的形成需要电子 从一个金属原子转移到一个非金属原子 这一过程 通常是吸热的。然而，当元素钠和氯 反应形成固体氯化钠晶体时，这是一个高度放热的过程。但是能量从何而来？根据库仑定律，阳离子和阴离子 在强大的静电作用下相互吸引，形成一个固体阵列，或称晶格。由此产生的晶格结构 是通过降低势能来稳定的，这种势能 以热量的形式释放出来，使反应成为一个放热反应。与晶格的形成或分解成 其气态成分有关的总能量 称为晶格能。在固体离子化合物中，大量的 带电粒子相互作用，这使得实验上很难确定晶格能的 精确值。然而，它可以用赫斯定律 在一系列被称为 Born-Haber 循环的假设步骤中 进行计算，这个循环代表了由其组成元素 形成离子化合物的过程。例如，氯化钠形成的 Born-Haber 循环 考虑了两条替代路线，一条是直接的，另一条是间接的。直接路线表示由 元素钠和氯生成氯化钠的标准焓。间接路线有五个步骤。在第一步中，固态钠被转化成气态。被转化成它的气态。接下来，双原子氯分子 分解成气态氯原子。第三和第四步解释了电子转移 形成离子的过程。从气态钠中除去一个电子，形成一个钠离子。然后电子被气态氯吸收，形成一个氯离子。最后一步，气体离子间的 静电吸引导致晶格 结构的形成。赫斯定律指出，一个分步过程的 总焓的变化是每一步焓变化 的总和。这就意味着直接路线的焓值 等于五个步骤的焓之和。通过求解晶格能方程 确定了一个较大的负值，这意味着反应是一个放热反应。

玻恩-哈伯循环

<h4> 晶格能 </ h4>
 
 离子化合物由于其正离子和负离子之间的静电吸引而稳定。化合物的晶格能量是该吸引力强度的量度。离子化合物的晶格能（Δ H lattice ）定义为将一摩尔固体分离成其气态离子所需的能量。</ n >
 
 在这里，使用约定将离子固体分离为离子，这意味着晶格能量将是吸热的（正值）。另一种方法是使用等效但相反的约定，其中晶格能量是放热的（负值），并且描述为当离子结合形成晶格时释放的能量。因此，请确保确认在其他参考文献中查找晶格能量时使用的定义。
 
 在两种情况下，较大的晶格能量表明离子化合物更稳定。对于氯化钠，Δ H 晶格 = 769 kJ。因此，将一摩尔固体NaCl分离成气态Na + 和Cl n 离子需要769 kJ。当气态Na + 和Cl n 离子每摩尔形成固体NaCl时，会释放769 kJ热量。
 
 <h4>离子化合物晶格能的测定</ h4>
 
 不可能直接测量晶格能量。但是，可以使用热化学循环来计算晶格能量。玻恩-哈伯循环是赫斯定律的应用，它将离子固体的形成分解为一系列单独的步骤：
 
 <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;"> Cs的升华焓 (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;"> F2的键合能量的一半</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;"> Cs的电离能 (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;"> F的电子亲和力</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;"> CsF的晶格能量为负 (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 = −ΔH晶格= ?</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;"> CsF的形成焓 (s), 添加步骤 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>考虑处于最常见状态的元素Cs（ s ）和F 2 （ g ）。
 <li>＆Delta; H s ＆deg;代表固态铯转化为气体（升华），然后电离能将气态铯原子转化为阳离子。</ li>
 
 <li>下一步，需要考虑打破F–F键以产生氟原子所需的能量。</ li>
 <li>将一摩尔氟原子转化为氟离子是一个放热过程，因此此步骤释放出能量（电子亲和力）</ li>
 现在，存在一摩尔的Cs阳离子和一摩尔的F阴离子。这些离子结合产生固体氟化铯。此步骤中的焓变是晶格能量的负值，因此它也是一个放热量。</ li>
 参与该转化的总能量等于从化合物的元素确定的实验确定的形成焓Δ H f °。在这种情况下，总体变化是放热的。</ li>
 </ ol>
 
 对于离子化合物计算的晶格能通常比对共价键测得的键解离能高得多。而晶格能量通常在600的范围内。在4000 kJ / mol（甚至更高）时，共价键解离能通常在150至100之间。单键为400 kJ / mol。但是请记住，这些不是直接可比较的值。对于离子化合物，由于阳离子和阴离子在扩展的晶格中堆积在一起，因此晶格能量与许多相互作用相关。对于共价键，键解离能与两个原子的相互作用有关。
 
 <h4>晶格能随离子半径和电荷的变化</ h4>
 
 离子晶体的晶格能随着离子电荷的增加而迅速增加，并且离子的尺寸减小。当所有其他参数保持恒定时，将阳离子和阴离子的电荷加倍会使晶格能翻两番。例如，LiF的晶格能量（ Z + 和 Z ＆s = 1）为1023 kJ / mol，而MgO（ Z + 和 Z ＆ndash; == 2）的MgO为3900 kJ / mol（ R o =离子间距离，定义为正离子和负离子的半径之和，两种化合物几乎相同-大约200 pm）。
 
 不同的原子间距离产生不同的晶格能量。例如，将MgF 2 （2957 kJ / mol）的晶格能与MgI 2 （2327 kJ / mol）的晶格能进行比较，证明了对MgF 2 的晶格能的影响。与I –相比，F –的离子尺寸更小。
 
 <h4> 玻恩-哈伯循环的其他应用</ h4>
 
 玻恩-哈伯循环也可用于计算晶格能量方程中的任何其他量，前提是已知余数。例如，如果升华＆nbsp;Δ H s s的相关焓，电离能（ IE ），则键解离焓（ D ），晶格能Δ H 晶格和标准形成焓Δ H f ＆deg;众所周知，玻恩-哈伯循环可用于确定原子的电子亲和力。
 
 此文本改编自<a href="https://openstax.org/books/chemistry-2e/pages/7-5-strengths-of-ionic-and-covalent-bonds"> Openstax ，化学2e，第7.5节：离子键和共价键的强度。</a>

晶格能

离子化合物因其正负离子之间的静电吸引力而保持稳定。该化合物的晶格能是指……

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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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types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

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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Ionic Compounds and Crystal Lattices

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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...

covalent bonding and lewis structures

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lewis structures of molecular compounds and polyatomic ions

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key terms and concepts

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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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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Lattice Energy&#xA0;

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

Trends in Lattice Energy: Ion Size and Charge

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 ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

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:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

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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