rule of solid solubility. positive deviation of the enthalpy of mixing and consequently limited...
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Rule of Solid Solubility
• Positive deviation of the enthalpy of mixing and consequently limited solid solubility may be predicted from known atomic properties.
Hume-Rothery- empirical rules to predict solid solubility
• For solid solubility exist b/t two elements, the following settlements are always true:
• Atomic Size: the atomic radii of the two elements must be with in 15 % of each other.
• Crystal Structure: the type of crystal structure must be same.
• Chemical Valence: The valence of the two elements must differ by no more than one.
• Electro negativity: It must be nearly equal if not, a compound may be formed as a result of the difference in affinity for electrons.
Atomic Size
• Extensive substitutional solid solution occurs only
• If the relative difference between the atomic diameters (radii) of the two species is less than 15%.
• If the difference > 15%, the solubility is limited.• Comparing the atomic radii of solids that form
solid solutions,
Crystal Structure Rule
• For appreciable solid solubility, the crystal structures of the elements must be identical.
• Although the crystal structure remains un changed
• However, the dimension of the unit crystal cell changes progressively with addition of solutes.
Valency Rule
• A metal will dissolve a metal of higher valency to a greater extent than one of lower valency.
• Or a solute of higher valency is more likely to dissolve in a solvent of lower valency.
• The solute and solvent atoms should typically have the same valence in order to achieve maximum solubility
• E.g copper dissolve up to 38.3 atomic percent Zinc
• But zinc dissolve only 2.8 atomic percent copper
Electronegativity
• Chemical property that describes the ability of an atom to attract electrons (or electron density) towards itself.
• An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus.
• The higher the associated electronegativity number, the more an element or compound attracts electrons towards it.
Electronegativity,
• It is not strictly an atomic property, • But rather a property of an atom in a molecule• The equivalent property of a free atom is its
electron affinity. • The opposite of electronegativity is
electropositivity: • A measure of an element's ability to donate
electrons.
Electro negativity
• Electro negativity: It must be nearly equal • If not, a compound may be formed as a result
of the difference in affinity for electrons• If electron affinity is the same for each
component, • No compound will be formed and the one
phase region will be retained
The Electro negativity
Electronegativity difference close to 0 gives maximum solubility.
• The more electropositive one element and the more electronegative the other,
• The greater is the likelihood that they will form an intermetallic compound instead of a substitutional solid solution.
• Ordered intermetallic compounds are formed if the components have very different electronegativity.
•
• The compounds limits the amount of solid solution that can occur between the elements.
• The actual extent of solid solubility depends on the stability of the intermediate phase
• The more stable the compound the less is the extent of the primary phase field.
• E.g Mg2Pb in alloy of Mg with Pb
• The principle of the fourth rule is that • The single solution becomes unstable with
regards to the compound, whose free energy near its ideal composition will be lowest.
• The phase regions (compound plus solid solution) then appear and the phase diagram no longer contains the broad region of complete solubility.
• The first three rules are based on the increase in enthalpy due to distortion of the crystal lattice,
• Disruption of crystal structure, in crude sense unsaturated bonds.
• There distortion leads to appreciable positive deviation and therefore limited mutual solid solubility.