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"The laws of thermodynamics are not just rules of nature; they are the foundation upon which our understanding of energy and matter rests."
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INORGANIC THERMODYNAMICS STUDY NOTESLearning Outcomes
1. A student should be able to define inorganic thermodynamics
2. A student should be able to explain and apply the theorical steps
involved in The Born-Haber for the formation of Ionic compounds
and
3. Solubility of ionic compounds
What is inorganic thermodynamics?
Inorganic thermodynamics is a branch of thermodynamics that focuses on
the thermal properties and energy changes associated with inorganic
compounds and reactions. It deals with understanding how heat and work
interact with inorganic substances, which include metals, minerals, and
coordination compounds.
• For ionic bonds to develop, there must be a reduction in the energy
of the particles to form ionic compounds.
• The reduction in energy is associated with an increased stability of
the compound.
• The enthalpy change during a reaction between a metal and a non‐
metal (to make a salt) is quite valuable.
• From the various enthalpy values involved in a simple reaction to
produce a simple salt composed of a mono‐valent cation and a
mono‐valent anion, we may determine an entity known as the
lattice energy.
Lattice
energy
• The lattice energy is defined as the energy required to separate the
ions from a crystal lattice to infinite distances.
, • The flip side of lattice energy is that it’s a measure of the cohesion
of the ions in a crystal lattice and is “the energy released when the
crystal is formed by bringing together the separate ions”.
• The most stable ionic compounds form when lattice energies are
very large or when elements with low ionization energies (metals)
react with elements with high electron affinities (non‐metals).
Lattice energies also describe the ion‐ion interaction strengths: as the
charge of the ions increases, the lattice energy decreases
• As the size of the ions increases, the lattice energy decreases. Ionic
compounds with smaller lattice energies tend to be more soluble in
water.
• Ionic compounds with higher lattice energies require more thermal
energy to boil and melt.
• Lattice energies are not easily empirically determined as it’s difficult
to isolate gaseous ions: rather, lattice energies are calculated using
Born‐Haber cycle methodology, which is an application of Hess’ Law.
• Hess's Law states that regardless of the multiple stages or steps of
a reaction, the total enthalpy change for the reaction is the sum of
all changes. This law is a manifestation that enthalpy is a state
function.
Lattice Energy Cycle:
1. Start with the Ionic Solid:
Begin with the ionic solid, which is represented as MXs\
text{MX}_{s}MXs (where M is a metal and X is a non-metal).
2. Break the Ionic Solid into Gaseous Ions:
Sublimation of the Metal (M):
o Ms→Mg\text{M}_{s} \rightarrow \text{M}_{g}Ms→Mg
o This step involves the enthalpy change of sublimation, ΔHsubl\
Delta H_{\text{subl}}ΔHsubl.
Dissociation of the Non-Metal (X):
o X2(g)→2Xg\text{X}_{2(g)} \rightarrow 2\text{X}_{g}X2(g)
→2Xg
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