Chapter 6. Properties of Compounds Comparing properties of elements and compounds Compounds are formed when elements combine together in fixed proportions. The compound formed will often have properties that are very different from its constituent elements. The formation of the compound will involve an absorption or a release of energy. Compounds can be decomposed back into its constituent elements, sometimes with great difficulty. Example: When a spark is introduced into a mixture of hydrogen gas and oxygen gas an explosion occurs and droplets of liquid are formed: Hydrogen (g) + Oxygen (g) Water (I) + Energy (element) (element) (compound) Comparison of some properties - elements and a compound Property Hydrogen Oxygen Water Physical Gas at room temp - little attraction between molecules, low density. Gas at room temp - little attraction between molecules, low density Liquid at room temp - attraction between molecules evident. Density approx 1.0 g / ml. Good solvent for many substances. Chemical Burns explosively in air. Reacts vigorously with many substances. Necessary for combustion burning. Does not support combustion.
Classification of Compounds Compounds can be classified in several different ways. The following classifications rely on differences in both chemical and physical properties. 1. Organic / Inorganic Compounds Inorganic Organic Compounds composed of elements other than carbon and hydrogen. Exceptions include oxides of carbon: CO, CO 2, CO 3 2-, HCO 3 -, cyanides, CN - and strong acids, HX. Composed predominately of the elements carbon and hydrogen. Can contain small quantities of oxygen, sulfur, nitrogen, phosphorous and halogen elements. Found in living things, (plants & animals), foods, fossil fuels, fibres, plastics etc... Many organic compounds undergo charring when they are heated. They turn black, showing a carbon residue. CARE! Always heat small samples in a well ventilated area. 2. Ionic / Simple Molecular / Network Covalent Compounds Ionic Contains charged Particles Ions. Cation is a metal ion + or ammonium, NH 4 Network arranged. Covalent contain non-metals only Simple Molecular Network Covalent Small, discrete Giant, covalently groups of atoms bonded lattice
Properties of Compounds Melting Point A substance has a high melting point if it requires lots of energy (heat) to break apart the forces holding the individual particles together. Ionic compounds (eg NaCl, common salt) usually have high melting points, 900 o C +. This means that the forces holding the ions together in ionic solids are strong. Ionic bonds are strong. Simple molecular compounds (eg H 2 O water) have low melting points, 0 o C. This means that the forces holding the molecules together in simple molecular solids are weak. Intermolecular bonds are weak. Network covalent solids (eg SiO 2 silica or quartz) have very high melting points, 1500 o C +. This means that the forces holding the atoms together in network covalent solids are very strong. Covalent bonds are very strong. Solubility A substance will dissolve in a liquid if it has similar properties to that liquid. Like dissolves like... Polar inorganic liquids such as water dissolve many ionic solids, like Sodium Chloride, NaCl but not Silver Chloride, AgCl (!! Solubility rules apply). Non-polar liquids such as petrol and kerosene will dissolve some simple covalent molecular solids. Ionic compounds will NOT dissolve in organic solvents. Polar organic liquids such as alcohols, (ethanol is a good example) are miscible with a diverse range of organic and inorganic liquids, like petrol and water respectively. Ionic compounds will commonly dissolve in water: This means that the water molecules are attracted strongly to the positive and negative ions such that the energy holding the ions in the crystal lattice is overcome. Cations and anions randomly move about in aqueous solution (aq). There are still many examples of insoluble ionic compounds, and therefore there is a general guide that can be referred to when determining solubility of such compounds.
SOLUBILITY RULES Use this table as a guide to determine the solubility of ionic compounds in water SOLUBILITY RATING ION EXCEPTIONS Completely Soluble Mostly Soluble Mostly Soluble Lithium, Li + Sodium, Na + Potassium, K + + Ammonium, NH 4 - Nitrate, NO 3 Ethanoate, CH 3 COO Iodide, I - Chloride, Cl - Bromide, Br - 2- Sulfate, SO 4 None Insoluble Exceptions: Silver I, Ag + Lead II, Pb 2+ Mercury II, Hg 2+ Insoluble Exceptions: Calcium, Ca 2+ Barium, Ba 2+ Lead II, Pb 2+ Mostly Insoluble Hydroxide, OH - Carbonate, CO 3 2- Phosphate, PO 4 3- Soluble Exceptions: Lithium, Li + Sodium, Na + Potassium, K + + Ammonium, NH 4
Written Exercise: Identify each compound below as either Precipitate or Soluble according to the solubility rules. Mercury II Nitrate: Copper II Hydroxide: Magnesium Sulfate: Calcium Carbonate: Ammonium Phosphate: Lead II Ethanoate: Electrical Conductivity A compound will conduct electricity if it possesses charged particles which are free to move, ie mobile. Solutions of ionic compounds are good conductors because they contain ions which can move in the liquid. Solid ionic compounds are poor conductors because their ions are not free to move out of the solid lattice. Molten (liquid) ionic compounds are good conductors because their ions can move. Solutions of molecular compounds are poor conductors because they contain no ions. Solid molecular compounds are poor conductors because they contain no ions. Molten (liquid) molecular compounds are poor conductors because they contain no ions.