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# 化学代写|无机化学代考Inorganic Chemistry代写|CHEM5150 The energetics of ionic bonding

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## 化学代写|无机化学代考Inorganic Chemistry代写|The energetics of ionic bonding

A compound tends to adopt the crystal structure that corresponds to the lowest Gibbs energy. Therefore, if for the process
$$\mathrm{M}^{+}(\mathrm{g})+\mathrm{X}^{-}(\mathrm{g}) \rightarrow \mathrm{MX}(\mathrm{s})$$
the change in standard reaction Gibbs energy, $\Delta_{\mathrm{r}} G^{\ominus}$, is more negative for the formation of a structure A rather than B, then the transition from B to A is spontaneous under the prevailing conditions, and we can expect the solid to be found with structure A.

The process of solid formation from the gas of ions is so exothermic that at and near room temperature the contribution of the entropy to the change in Gibbs energy (as in $\left.\Delta G^{\ominus}=\Delta H^{\ominus}-T \Delta S^{\ominus}\right)$ may be neglected; this neglect is rigorously true at $T=0$. Hence, discussions of the thermodynamic properties of solids normally focus, initially at least, on changes in enthalpy. That being so, we look for the structure that is formed most exothermically and identify it as the thermodynamically most stable form. Some typical values of lattice enthalpies are given in Table $3.7$ for a number of simple ionic compounds.

## 化学代写|无机化学代考Inorganic Chemistry代写|Lattice enthalpy and the Born–Haber cycle

Key points: Lattice enthalpies are determined from enthalpy data by using a Born-Haber cycle; the most stable crystal structure of the compound is commonly the structure with the greatest lattice enthalpy under the prevailing conditions.

The lattice enthalpy, $\Delta_{\mathrm{L}} H^{\ominus}$, is the standard molar enthalpy change accompanying the formation of a gas of ions from the solid:
$$\operatorname{MX}(\mathrm{s}) \rightarrow \mathrm{M}^{+}(\mathrm{g})+\mathrm{X}^{-}(\mathrm{g}) \quad \Delta_{\mathrm{L}} H^{\ominus}$$
A note on good practice The definition of lattice enthalpy as an endothermic (positive) term corresponding to the break-up of the lattice is correct but contrary to many school and college texts, where it is defined with respect to lattice formation (and listed as a negative quantity).

A note on good practice The terms ‘lattice enthalpy’ and ‘lattice energy’ are often used interchangeably, though because of variations in thermodynamic functions that define these quantities under standard conditions (such as the work, P $\Delta \mathrm{V}$, involved in forming the gaseous ions) they differ by a few $\mathrm{kJ} \mathrm{mol}^{-1}$. This difference is, however, negligible compared with errors in experimental and theoretical determined values, hence the accepted use of either term.

## 化学代写|无机化学代考|norganic Chemistry代写|The energetics of ionic bonding

$$\mathrm{M}^{+}(\mathrm{g})+\mathrm{X}^{-}(\mathrm{g}) \rightarrow \mathrm{MX}(\mathrm{s})$$

## 化学代写|无机化学代考Inorganic Chemistry代写|Lattice enthalpy and the Born-Haber cycle

$$\mathrm{MX}(\mathrm{s}) \rightarrow \mathrm{M}^{+}(\mathrm{g})+\mathrm{X}^{-}(\mathrm{g}) \quad \Delta_{\mathrm{L}} H^{\ominus}$$

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