domingo, noviembre 18, 2007

Nomenclatura de compuestos inorgánicos...

TEMAS: Estructuras de Lewis - Balanceo de ecuaciones químicas - Nomenclatura de compuestos inorgánicos - Números de oxidación

Ejercicio
-Nombrar los siguientes compuestos e indicar a que clase de compuestos pertenecen. Escribir la ecuación de formación de cada uno y su estructura de Lewis.

NaCl

H2O

KF

Na2O

K2O

MgO

CaO

Al2O3

sábado, noviembre 17, 2007

Comparison of Properties of Ionic and Covalent Compounds



A continuación las referencias que dan dos páginas sobre química en ingles a la cuestión de la comparación entre las propiedades de los compuestos covalentes e iónicos.


Because of the nature of
ionic and covalent bonds, the materials produced by those bonds tend to have quite different macroscopic properties. The atoms of covalent materials are bound tightly to each other in stable molecules, but those molecules are generally not very strongly attracted to other molecules in the material. On the other hand, the atoms (ions) in ionic materials show strong attractions to other ions in their vicinity. This generally leads to low melting points for covalent solids, and high melting points for ionic solids. For example, the molecule carbon tetrachloride is a non-polar covalent molecule, CCl4. It's melting point is -23°C. By contrast, the ionic solid NaCl has a melting point of 800°C.

Ionic Compounds

  1. Crystalline solids (made of ions)
  2. High melting and boiling points
  3. Conduct electricity when melted
  4. Many soluble in water but not in nonpolar liquid

Covalent Compounds

  1. Gases, liquids, or solids (made of molecules)
  2. Low melting and boiling points
  3. Poor electrical conductors in all phases
  4. Many soluble in nonpolar liquids but not in water

You can anticipate some things about bonds from the positions of the constituents in the periodic table. Elements from opposite ends of the periodic table will generally form ionic bonds. They will have large differences in electronegativity and will usually form positive and negative ions. The elements with the largest electronegativities are in the upper right of the periodic table, and the elements with the smallest electronegativities are on the bottom left. If these extremes are combined, such as in RbF, the dissociation energy is large.


FUENTE: http://hyperphysics.phy-astr.gsu.edu/Hbase/chemical/bond2.html

· Electrical conductivity of the compound in aqueous solution. Ionic compounds conduct electricity when dissolved in water, because the dissociated ions can carry charge through the solution. Molecular compounds don't dissociate into ions and so don't conduct electricity in solution.

· Electrical conductivity of the compound in liquid form. Ionic compounds conduct electricity well when melted; metallic solids do as well. Covalent molecular compounds do not, because they usually don't transfer electrons unless they react.

· Hardness. Molecular solids are usually much softer than ionic materials. Ionic crystals are harder but often quite brittle. Squeezing an ionic crystal can force ions of like charge in the lattice to slide into alignment; the resulting electrostatic repulsion splits the crystal.

· Melting points and boiling points. In an ionic compound, the forces of attraction between positive and negative ions are strong and high temperatures are required to overcome them. The melting and boiling points of ionic compounds are usually very high. A smaller amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do. In fact, many compounds in this class are liquids or gases at room temperature.

FUENTE: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/properties-ionic-vs-covalent.shtml