Ionic Compounds and Ionic Bonding

Transcription

1 Ionic Compounds and Ionic Bonding Definitions Review: Crystal Lattice - 3D continuous repeating pattern of positive and negative ions in an ionic solid Formula Unit- smallest possible neutral unit of an ionic crystal (balance of charges) Empirical Formula- lowest whole number ratio between atoms or ions in a compound IONIC COMPOUNDS -are made up of crystals with alternating positive and negative ions attracted to each other by strong ionic bonds. This helps to explain the following properties of ionic compounds: 1. Ionic compounds are hard crystalline solids with high melting points and boiling points 2. They are brittle and break easily. The crystals will rather shatter than bend. 3. When they dissolve, the ions will separate and are free in solution. Thus they can conduct electricity if in solution or are melted.

2 METALLIC BONDING Characteristics of metals can be explained by examining metallic bonding. Metals have such low electronegativity that the valence electrons move freely from the valence level of one atom to the valence level of other atoms, forming a sea of continuously moving electrons around a group of cations. Metallic Bonding: forces of attraction where metal ions simultaneously attract freemoving valence electrons within metals. This theory of metallic bonding is supported by the properties of metals: 1. Metals are malleable and ductile. The moving electrons don t result in the repulsion of ions, like when you try to bend ionic compounds. 2. Metals conduct electricity because their electrons are always moving 3. Metals have a wide range of melting and boiling points. The more valence electrons moving around, the stronger the metal and higher its boiling point. Elements with fewer valence electrons like Group IA, would be softer and more easily melted.

3 4. Metals can form alloys. Alloys are solid solutions of 2 or more metals. The valence electrons can move within the other metals as well. Compound Properties Examples Molecular Solid, liquid or gas at STP; low mp/bp, does not conduct electricity in solution; simple compounds are insoluble in water SO 2, NH 3 Metallic Ductile; malleable; good conductors of heat/electricity; shiny when freshly cut or polished CuZn (brass); all metallic alloys (ex: steel) Ionic Crystalline solid at STP; high mp/bp; conducts electricity in solution; usually soluble in water NaCl. CuSO 4

4 Solutions and Intermolecular Forces Solutions are mixtures that form by physically mixing at the particle level and do not involve a chemical change. The phrase like dissolves like can be used to describe solubility, since polar substances and ionic compounds generally dissolve in polar solvents. Nonpolar substances generally dissolve in nonpolar substances. Polar molecular substances dissolve in polar molecular solvents because of the attraction between the oppositely charged ends of the molecular dipoles. While there is very little dipole-dipole electrostatic attractive forces between a nonpolar solute for a nonpolar solvent, there are very small electrostatic attractions within the solute and within the solvent. The Kinetic Molecular Theory states that the particles of both the solute and solvent are in constant motion, therefore with no strong forces of attraction, the nonpolar solute will dissolve in the nonpolar solvent. Ionic compounds have permanent positive and negative charged ions. Since polar molecular solvents have a permanent molecular dipole the positive ions attract the partially negative end of the dipole and vice versa. This is referred to as Ion-Dipole Attraction. The degree of dissolving is dependent upon the forces of attraction in the dissolving process. So for different substances, solubility occurs to varying degrees. Which of the following substances are expected to dissolve in water? CCl 4 Mg(NO 3 ) 2 CH 3 OH S 8 CH 2 Cl 2 NH 3

5 Strengths of Predictions and making Predictions Based on the relative strengths of intermolecular and intramolecular forces we can make predictions about the properties of substances. Keep in mind there are many exceptions, but we can draw some generalizations. Generalizations from strongest to weakest: 1. Substances with network covalent bonding (network solids) have the highest melting and boiling points and are the hardest common substances (diamond, silicon carbide, silicon dioxide) 2. Ionic compounds such as NaCl 3. Metallic compounds 4. Regular molecular compounds (not network covalent) are the weakest, but there are differences based on what intermolecular forces are present: a. Molecules with hydrogen bonding are the strongest b. Molecules that have dipole-dipole ( no hydrogen bonding ) are next c. Molecules that only have London Dispersion Forces are the weakest i. More electrons = More LDF ii. If molecules are isoelectronic, then the bigger molecule will have more LDF

6 Ex: Rank SiO 2, CCl 4 and Na 2 S in order of increasing boiling point. CCl 4 (lowest b/c molecular) Na 2 S (ionic) SiO 2 (network covalent) Ex: Rank NaBr, C 2 H 5 OH, SiC, C 3 H 8, C 2 H 5 F in order of increasing boiling point. 5: C 3 H 8 ( only has LDF) 4: C 2 H 5 F (has LDF and dipole-dipole) 3: C 2 H 5 OH ( has HB, dipole-dipole, LDF) 2: NaBr (ionic) 1: SiC (network covalent)