Why does melting point decrease for impure solid?

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SUMMARY

The melting point of impure solids, such as crude acetaminophen compared to pure acetaminophen, decreases due to increased entropy and weakened intermolecular forces. The presence of impurities disrupts the crystal lattice structure, resulting in a less stable arrangement of molecules. This instability requires lower energy to transition from solid to liquid, thus lowering the melting point. The driving force for melting is diminished as the entropy of the solid increases, necessitating higher temperatures for melting to occur.

PREREQUISITES
  • Understanding of thermodynamics, specifically entropy (ΔS)
  • Knowledge of intermolecular forces and crystal lattice structures
  • Familiarity with the properties of pure and impure substances
  • Basic concepts of phase transitions in solids and liquids
NEXT STEPS
  • Research the impact of impurities on melting points in organic compounds
  • Study the principles of thermodynamics related to entropy and phase changes
  • Explore the effects of intermolecular forces on the physical properties of substances
  • Learn about crystallography and how crystal lattice structures influence melting behavior
USEFUL FOR

Chemistry students, laboratory technicians, and researchers interested in the physical properties of organic compounds and the effects of impurities on melting points.

erjkism
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Okay, i am doing a lab involving crude acetaminophen and pure acetaminophen. The crude acetaminophen is supposed to have a lower melting point than the pure acetaminophen...

how does that work and why? does it have to do with thermodynamics or what??
 
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The substance its mixed with may have a lower melting point; that's my guess anyway.
 
Mixtures have higher entropy than pure substances. Because the transition from solid to liquid is mostly driven by the increase in entropy (ΔS = Sliquid - Ssolid), increasing Ssolid lowers the overall ΔS, lowering the driving force for melting. Since melting is less favorable, you need higher temperatures to accomplish it.
 
ahhh... i see

thanks man
 
You may also think in terms of intermolecular forces. In crystal molecules are packed in such a way that their interactions are strongest. When you add some other compound it stretches the crystal lattice, molecules are not in optimal positions and their interactions are weaker, thus less energy is required to break them apart.
 

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