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4.5 Physical Properties of Giant Ionic Compounds

4.5 Physical Properties of Giant Ionic Compounds. Giant ionic solids form as the result of electron transfer from a metal to a non-metal atom leading to an electrostatic attraction between the positive and negative ions – the IONIC BOND. Intramolecular

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4.5 Physical Properties of Giant Ionic Compounds

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  1. 4.5 Physical Properties of Giant Ionic Compounds

  2. Giant ionic solids form as the result of electron transfer from a metal to a non-metal atom leading to an electrostatic attraction between the positive and negative ions – the IONIC BOND. Intramolecular Electrostatic attraction between the oppositely charged ions are very STRONG

  3. High Melting and Boiling point • The strong intramolecular electrostatic attractions between the positive and negative ions, means that a large amount of heat energy is needs to be absorbed to overcome (break) the electrostatic attraction. Physical properties of giant ionic compounds

  4. Electrical Conductivity • To conduct electricity an ionic compound must have mobile ions. • Solid ionic compounds don’t conduct electricity, because there are no free ions to carry a current, they are locked into the lattice. • When an ionic compound is heated above its melting point it becomes molten (pure liquid). The ions are separated from the lattice and are free to move and conduct electricity. NaCl(s) + heat  NaCl(l) • Ionic compounds will be soluble in water if the polar water molecule and can form a dipole-ion interaction that is strong enough to overcome the intramolecular electrostatic attraction between the ions in the ionic lattice. When this happens the ions are separated from its lattice and are free to move through the solution and conduct a current. NaCl(s) + H2O (l) Na+ (aq) + Cl- (aq)

  5. Solubility • Ionic solids will be soluble in water if the attraction between water and the ions is strong enough to overcome the electrostatic attraction between the ions in the lattice. Volatility • Ionic compounds are not volatile. This means they don’t turn into a vapor (gas) at low temperatures because the strong intramolecular electrostatic attractions between the ions are too strong to be broken at low temperatures.

  6. Brittle • When a stress is applied to an ionic solid it shifts the ion layers slightly. • Ions of the same charge (+ and + or – and -) are brought side-by-side and repel causing the crystal to shatter!

  7. Comparing the physical properties of melting point and boiling point

  8. Differences in m.pt and b.pt of NaCl, MgO and RbI are the result of: 1. The charge on the ions. The greater the charge on the ions the stronger the electrostatic attraction between them. The 2+ and 2- ions in MgO attract each other more strongly than the 1+ and 1- ions in NaCl and RbI, giving MgO a higher mpt and bpt. Mg2+ O2- Na+ Cl-

  9. 2. Radius of the ions • Smaller ions can get closer together and so the electrostatic attraction between them will be greater, decreasing the radius. ionic radius of Rb+> Na+ > Mg2+ ionic radius of I- > Cl- > O2- • The electrostatic attraction between bigger ions is weaker, thus less heat energy needs to be absorbed to separate them, decreasing the mpt and bpt mpt RbI  NaCl  MgO

  10. Effects of impurities on the mpt and bpt of ionic solids • Impurities in an ionic solid prevent the ordering necessary to form a strong crystal lattice. This weakens the electrostatic attraction between the ions lowering the melting point of the solid. • Therefore ionic solids with impurities have lower melting points than in the pure substance. This is why salt is placed on icy roads in the winter. Pure water melts at 0C but water (ice) with salt added as an impurity freezes at about -20C causing the ice to melt, even when the air temperature is below freezing (0C).

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