Metals and Non-metals. Comparison of physical properties of metals and non metals

Transcription

1 Metals and Non-metals Comparison of physical properties of metals and non metals PHYSICAL PROPERTY METALS NON-METALS Physical State Metallic lustre (having a shining surface) Mostly solids (Liquid -mercury) Lustrous Mostly solids or gases (Liquid -bromine) Dull (Iodine -lustrous) Density High Low Melting Point &boiling point Hardness Malleability (ability to be beaten into thin sheets) Ductility (ability to be drawn into thin wires) Sonority (ability to produce a ringing sound on striking a hard surface) Heat Conductivity Electrical Conductivity High (Gallium, Caesium, Li, Na, K low) Hard (Alkali metals- Li, Na, K- soft) Malleable Best- gold & silver Ductile Best- Gold Sonorous Very good conductors of heat Best- silver & copper Very good conductors of electricity Low (Carbon: diamond high) Soft (Diamond- hardest) Non-malleable Non-ductile Non-sonorous Poor conductors of heat Poor conductors of electricity (Graphite- good conductor) Sheeba Musthafa Page 1

2 COMPARISON OF CHEMICAL PROPERTIES OF METALS AND NON METALS PROPERTY METALS NON-METALS Chemical nature Electropositive (tendency to lose electrons) Electronegative (tendency to gain electrons) Action with Oxygen Forms basic metallic oxides Forms acidic non-metallic oxides Action with acids Reactive metals liberate H 2 gas Does not liberate Hydrogen gas Chemical properties of Metals 1. Reactive Metal + Oxygen Metal Oxides (Basic) Bases / Alkalis (Turns Red litmus Blue) 2. Reaction of metals with Water Metal + Water Metal Oxides + Hydrogen (Bases) 3. Reactive metals + dilute acids salt of the metal + Hydrogen 4. Reactive metal A + Salt solution of B Salt solution of A + Metal B (Metal A is more reactive than metal B) Important!!! The thin protective oxide coating on the surface of Mg, Al, Zn, Pb prevent the metal from further oxidation. Anodising is a process of forming a thick oxide layer of Aluminium. During anodizing, a clean Al article is made the anode and is electrolyzed using dilute H 2 SO 4. Oxygen gas produced at the anode reacts with Al and forms protective thick oxide layer. It helps to resist corrosion and gives the Al articles an attractive finish. Amphoteric oxides are metal oxides that react with both acids and bases to produce salt and water. Eg: Aluminium oxide, Zinc oxide Sheeba Musthafa Page 2

3 Eg: Al 2 O HCl 2 AlCl H 2 O Al 2 O NaOH 2 NaAlO 2 + H 2 O Iron does not burn on heating, but iron filings burn vigorously when sprinkled in the flame of the burner. Copper does not burn on heating, but gets covered with a black coloured layer of CuO. Reaction of K & Na with water is exothermic that the evolved H 2 immediately catches fire. Ca & Mg floats on the surface of water due to the bubbles of Hydrogen sticking to its surface. Nitric acid being a strong oxidizing agent, does not give hydrogen gas with metals. It oxidises H 2 to H 2 O and gets reduced to any of the nitrogen oxides (N 2 O, NO, NO 2 ). Reactivity / activity series The series showing the arrangement of metal in the decreasing order of their activity. Any metal in the series is less reactive than its preceding metals. Sheeba Musthafa Page 3

4 Stability of elements Noble gases have a completely filled valence shell and are hence stable. Octet electronic configuration similar to noble gases is found to be stable. The elements differ in their reactivity due to their tendency to attain a completely filled valence shell. How do metals and non-metals react? Metals Metals have 1, 2 or 3 valence electrons Metals show a tendency to lose electrons from their valence shell. Metals are electropositive Non-metals Non-metals have 5, 6 or 7 valence electrons Non-metals show a tendency to gain electrons to their valence shell. Non-metals are electronegative Eg: Formation of Sodium chloride Ionic or electrovalent bond. The chemical bond formed by the transfer of electrons from the valence shell of a metal to the valence shell of a non-metal, thereby both elements acquiring nearest noble gas configuration is called ionic or electrovalent bond. An ionic bond is formed between metals and non-metals. The compounds formed through ionic bonding are called ionic compounds or electrovalent compounds. eg NaCl, CaO, MgO, MgCl 2, ZnCl 2 etc Sheeba Musthafa Page 4

5 Properties of ionic compounds 1. Ionic compounds are generally hard, brittle crystalline solids (due to strong electrostatic force of attraction). 2. Ionic compounds have high melting and boiling points (due to strong inter-ionic attraction). 3. Ionic compounds are soluble in polar solvents like water and insoluble in nonpolar solvents like kerosene and petrol. 4. Ionic compounds in the solid state do not conduct electricity. They conduct electricity only in the molten state or in aqueous solutions due to the availability of mobile charged ions. Occurrence of metals The metals at the bottom of the reactivity series are least reactive and are found in the elemental form. Example:- Au, Ag, Pt, Cu. Most metals are found in the form of compounds as oxides, sulphides, carbonates etc. 1. Minerals: Elements or compounds found naturally in the earth s crust. 2. Ores: The minerals from which metals can be extracted profitably. 3. Gangue: The earthly impurities present in the ore particles. 4. Flux: The substance used to remove the gangue. 5. Slag: The molten fusible substance formed when flux reacts with gangue. 6. Metallurgy: Several steps involved in the extraction of pure metal from its ores. Steps involved in extraction of metals: 1. Enrichment of ore 2. Extraction of metal from the enriched ore 3. Refining of metals 1. Enrichment / Concentration of the ore o The process of removing gangue from the ores. o Physical or chemical properties of the gangue and the ore decide the method employed for concentration of the ore. Eg: hydraulic washing, froth floatation process, magnetic separation. How are metals classified based on their reactivity? 1. Metals of high reactivity K, Na, Ca, Mg, Al 2. Metals of medium reactivity Zn, Fe, Pb, Cu 3. Metals of low reactivity. Ag, Au Sheeba Musthafa Page 5

6 2. Extraction of metal from the enriched ore Steps involved in extraction of metals (a) Extracting metals low in the reactivity series (Hg, Cu): Ores can be reduced to metal by heating alone. Eg : Mercury from its ore Cinnabar (HgS) a. Conversion of sulphide to oxide 2HgS(s) + 3O 2 (g) heat 2HgO(s) + 2SO 2 (g) b. Decomposition of oxide to metal 2HgO(s) heat 2Hg(l) + O 2 (g) (b) Extracting Metals in the middle of the reactivity series (Fe, Zn, Pb): Step 1. The metal sulphides and carbonates are converted to metal oxides, by roasting or calcination. Step 2.The metal oxides are then reduced to the corresponding metals by using suitable reducing agents like carbon or highly reactive metals like Al. Step 1 Roasting a. Used to convert sulphide ores into oxides. b. Done by heating the sulphide ore strongly in the presence of excess air, so that oxygen gets added to form the corresponding oxides. c. Sulphur impurities escape as SO 2 gas. Calcination a. Used to convert carbonate ores to oxides. b. Done by heating the carbonate ore to a high temperature in the absence of air. Sheeba Musthafa Page 6

7 Step 2. Reduction of metallic oxides to metals The oxide obtained by calcination or roasting is then reduced to the metal using a reducing agent (carbon or highly reactive metals like Al) Metal oxide + reducing agent Metal + oxidized reducing agent 2 ZnO (s) + C (s) Heat 2 Zn (s) + CO 2 (g) Fe 2 O 3 (s) + 2 Al (s) Heat 2Fe (l) + Al 2 O 3 (s) (c) Extraction of metals towards top of the reactivity series Metals high in the reactivity series are very reactive and cannot be obtained from their compounds by heating with carbon. These metals have more affinity for oxygen than carbon. These metals are obtained from their compounds by electrolytic reduction. Sodium, magnesium and calcium are obtained by the electrolysis of their molten chlorides. 3. Refining of metals Corrosion Electrolytic refining - Refining by electrolysis. A thick block of the impure metal is made anode. A thin strip of the pure metal is made cathode. A water soluble salt of the metal to be refined is taken as electrolyte. The process of slow wasting away of metal by the action of atmospheric gases and moisture is called corrosion. Silver objects : Black deposit (Silver sulphide) Copper objects : Green deposit (Basic copper carbonate) Iron objects : Brown flaky deposit (Hydrated ferric oxide) Conditions necessary for corrosion of iron Rusting agents Air (oxygen) Moisture (water). Prevention of corrosion Painting Applying grease or oil Galvanizing Chrome plating Anodising: Alloying Sheeba Musthafa Page 7

8 Alloy Homogeneous mixture of two or more metals or a metal with a non-metal. It is prepared by first melting the primary metal and then dissolving the other elements in it in definite proportions. It is then cooled to room temperature. Why do we make alloys? For altering the properties of metals like: Increasing hardness: Adding carbon to iron. Making corrosion resistant: Adding Ni and Cr to iron (stainless steel) Lowering melting point: Mixing tin + lead (solder ) Reducing electrical conductivity: Mixing Cu and Zn to make Brass Reducing reactivity: Adding mercury to sodium. *************** Sheeba Musthafa Page 8