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Using polarizing theory:
Li+ has a smaller atomic radius than Na+. However they both have the same charge, +1. The Cl- is identical in both compounds, and has a large electron cloud surrounding it. Using the equation for static force:
F=kq1q2/r2 where k is Coulomb's constant, q1 is the charge of the Li+ or Na+ cation, q2 is the charge of an electron in the chlorine electron cloud, and r is the distance between the chlorine electron and the Li+ or Na+ nucleus.
we see that the force increases with a smaller radius r. Since the Li+ ion is smaller, the chlorine electrons are closer to the nucleus, and thus these electrons are drawn even closer to the Li+ nucleus.
All together, the electrons from the chlorine in LiCl are pulled around the Li+ ion to a much further extent than they are pulled around the Na+ in NaCl. Therefore the bond in LiCl has a much more covalent character than NaCl. Bond polarity is also reduced, since the electrons are not spending all of their time around the Cl-.
Since the LiCl molecules are less polar than NaCl, there are less electrostatic forces holding them together in a solid crystal lattice. Therefore the energy required to loosen the molecules from a crystal to a liquid is less for LiCl than NaCl.
Finally we come to the conclusion that therefore, the melting temperature of LiCl must be lower than NaCl
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Can you observe the melting point of NaCl?
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How do The dipole-dipole forces affect the melting and boiling points of substances?
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How do dipole dipole forces affect the melting and boiling points of substances?
Think relationships here... really London Dispersion Force is like an acquaintance, Dipole Dipole is like boyfriend/girlfriend and Hydrogen Bonding is like marriage. Now out of the three London dispersion is the easiest to break the bonds, Di Pole Di pole are a little harder to break up and Hydrogen bonding are the hardest to break up the bonds. Hydrogen Bonding will always have the strongest bond. London dispersion is when for a split second their is a short attraction and doesn't last long. It is very weak therefore making this non- polar. Dipole Dipole means 2 opposite sides. Positive and negative charges. This molecule is a Polar molecule. Hydrogen Bonding is a Polar molecule which attaches itself to either N,O,or F. A melting point takes a low energy to melt so the bonds are weak. the higher the boiling temperature the stronger the bonds. Think H20. Melting is comparing how hard it is to separate the molecules. Same Concept. Ex.1.) CH4 / LiCl CH4 more soluable / LiCl low soluability CH4 low melting point / LiCL higher melting point CH4 low boiling point/ LiCL Higher boiling point CH4 London Dispersion / LiCl Dipole Dipole Example2: H2O / NaCl H2O high boiling point/ NaCl lower boiling point H2O Hydrogen bonding/ NaCl Dipole Dipol
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