Transcript Rule of Solid Solubility

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.