METAL SALTS：金属盐.doc

METAL SALTSpH SCALEWhen a substance dissolves in water, it forms an aqueous solution, which may be acidic, alkaline or neutral. Indicators, for example litmus, can be used to show whether a solution is acidic, alkaline or neutral by the way their colours change. Universal indicator can show, in addition, how acidic or alkaline a solution is.The acidity or alkalinity of a solution is expressed as its pH number:pH0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 increasing acidity neutral increasing alkalinityAcids have a pH less than 7 whereas alkalis/bases have a pH greater than 7.Acids dissolve in water to produce H+ ions. This was what makes the solution acidic. Alkalis are soluble bases which dissolve in water to produce OH- ions. Acids and alkalis can be used to neutralise each other as they react to produce water (pH7).H+ + OH- H20METHODS FOR MAKING SALTSThere are several methods for making salts. The method chosen depends on: whether the salt reacts with water whether the salt is soluble or insoluble in water whether the metal contained in the salt is a reactive or unreactive metal1. PRECIPITATIONWhen a salt is insoluble in water, it is prepared by precipitation. A solution of a soluble salt containing the desired metal ion (usually the metal nitrate) is added to a solution containing the desired negative ion (usually as its sodium salt). The two ions combine immediately to give a precipitate (a solid). The precipitate is filtered off, washed with water and dried.e.g. to make lead(II) iodide, a solution of lead(II) nitrate is added to a solution of sodium iodidePb(NO3)2(aq) + 2NaI(aq) PbI2(s) + 2NaNO3(aq)Pb2+(aq) + 2I-(aq) PbI2(s)e.g. to make barium sulphate, a solution of barium nitrate is added to a solution of sodium sulphateBa(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2NaNO3(aq)Ba2+(aq) + SO42-(aq) BaSO4(s)2. SOLUBLE SALTSThere are three methods for making soluble salts: the reaction of an acid with a reactive metal the reaction of an acid and an insoluble metal oxide (or hydroxide) the reaction of an acid and an insoluble metal carbonateThe method followed in each case is the same. a) The reaction of an acid with a reactive metalThe reaction of an acid with a metal can only be used for metals which are neither too reactive nor too unreactive, i.e. those between Ca and Sn inclusive in the reactivity series.For example, magnesium sulphate, MgSO4, can be made by the reaction of magnesium with sulphuric acid:magnesium + sulphuric acid magnesium sulphate + hydrogenMg(s) + H2SO4(aq) MgSO4(aq) + H2(g)The method is: warm the dilute sulphuric acid add magnesium ribbon a little at a time until no more will react, and a small amount of the metal remains undissolved filter off the excess magnesium evaporate off most of the water from the filtrate and leave the solution to cool and crystallise collect magnesium sulphate crystals by filtration wash the crystals with a little cold water and leave them to drydilute sulphuric acidmagnesiumexcess magnesiumb) The reaction of an acid with a baseThe reaction of an acid and an insoluble metal oxide (or hydroxide) can be used to prepare almost all soluble salts. For example, copper(II) sulphate, CuSO4, can be made by the reaction of copper(II) oxide with sulphuric acid:copper oxide + sulphuric acid copper sulphate + waterCuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)The method is: heat dilute sulphuric acid almost to boiling add solid copper oxide a little at a time until no more will react, and a small amount of the solid remains undissolved filter off the excess copper oxide evaporate off most of the water from the filtrate and leave the solution to cool and crystallise collect magnesium sulphate crystals by filtration wash the crystals with a little cold water and leave them to drydilute sulphuric acidcopper oxideexcess copper oxide3. TITRATIONWhen the metal oxide or metal carbonate is soluble in water, it is not possible to use the method described in section 3, because it is not possible to see when an excess has been added, nor is it possible to filter off the excess.When the metal is very reactive, it is not possible to use the acid + metal method described in section 3.Therefore, none of the three usual methods for making soluble salts can be used for making sodium and potassium salts. Instead titration is used.For example, sodium chloride can be made by titrating hydrochloric acid with sodium hydroxide solution.The method is: measure out a known volume of hydrochloric acid using a pipette add one drop of phenolphthalein indicator add sodium hydroxide solution a little at a time from a burette, until the indicator just turns a pale pink colour, showing that neutralisation is complete and the end-point has been reached read from the burette the amount of sodium hydroxide solution needed repeat the titration using identical volumes of hydrochloric acid and sodium hydroxide solution, but leaving out the indicator evaporate off most of the water from the solution and leave it to cool and crystallise collect sodium chloride crystals by filtration wash the crystals with a little cold water and leave them to dryELECTROLYSISWhen substances which are made of ions are dissolved in water or melted, they can be decomposed (broken down) into simpler substances by passing an electric current through them. This process is called electrolysis. Electrolysis can occur because the ions are free to move about.flow of electronsbattery(+) (-)anode cathodeELECTRODESelectrolyteVOLTAMETERcell(-) ElectrodeDuring electrolysis, positively charged ions, for example metal ions and hydrogen ions, move to the negative electrode. Here, the ions gain electrons (reduction) to form electrically neutral atoms or molecules. If metal atoms are formed, they are deposited on the electrode. If hydrogen molecules are formed, they are given off as a gas.(+) ElectrodeNegatively charged ions move to the positively charged electrode. Here, the ions lose electrons (oxidation) to form electrically neutral atoms or molecules.During the electrolysis of molten lead bromide positive lead ions are attracted to the negative electrode where they are reduced to metallic lead.Pb2+ + 2e- PbAt the positive electrode negative bromide ions are oxidised to bromine.2Br- Br2 + 2e-THE EXTRACTION OF ALUMINIUMThe raw material for the production of aluminium is the ore, bauxite. This is an impure form of aluminium oxide, the main impurity being iron(III) oxide. The ore is purified to give aluminium oxide (sometimes called alumina). For electrolysis to be carried out, the aluminium oxide must be molten, but because its melting point is so high (2000oC), it is dissolved in molten cryolite (Na3AlF6). This lowers the melting point of the melt to 970oC.crust of solidelectrolytemolten electrolytemolten aluminium(-) graphite cathode(+) graphite anodespipe for sucking outmolten aluminium The process is run continuously. As aluminium oxide is used up, more is added at regular intervals from a hopper above the cell, after first breaking the crust of electrolyte. The molten aluminium oxide/cryolite mixture is electrolysed using graphite (carbon) electrodes. The positive aluminium ions (Al3+) are attracted to the cathode, where they gain electrons (reduction) to form aluminium atoms.Al3+ + 3e- AlThe negative oxide ions (O2-) are attracted to the anode, where they lose electrons (oxidation) to form oxygen molecules.2O2- - 4e- O2 Overall 2Al2O3 4Al + 3O2At the high temperatures of the electrolysis, the oxygen produced at the anode reacts with the graphite (carbon) that the electrode is made of to form carbon monoxide and carbon dioxide. This means that the anode burns away and has to be replaced regularly.C + O2 CO2ELECTROLYTES IN AQUEOUS SOLUTIONIn aqueous solutions electrolysis is a little more complex because of the ions from water (H+ and OH-). Where there are two ion with the same charge that are attracted to the same electrode, the LESS REACTIVE ion will be discharged.Pt electrodes(+)(-)hydrogenchlorinebrine containing universal indicatorElectrolysis of Sodium Chloride solution (Brine) The apparatus used for this experiment is called a Hofmann Voltameter.Sodium chloride solution contains four ions. Na+ and Cl- ions are provided by sodium chloride, and H+ and OH- ions are provided by the water.The two positive ions (Na+ and H+) are attracted to the negatively-charged cathode. Since hydrogen is below sodium in the reactivity series, hydrogen ions are easier to reduce than sodium ions. Therefore, they gain electrons (reduction) to form hydrogen atoms. The hydrogen atoms immediately combine to form hydrogen molecules (H2)(-) 2H+ + 2e- H2The two negative ions (OH- and Cl-) are attracted to the positively-charged anode. Since the chloride ions are easier to oxidise than hydroxide ions, they lose electrons (oxidation) to form chlorine atoms. The chlorine atoms immediately combine to form chlorine molecules (Cl2)(+) 2Cl- - 2e- Cl2The initial colour of the universal indicator in the voltameter is green, because sodium chloride solution is neutral.When hydrogen ions are removed at the cathode, this leaves an excess of OH- ions in this area. These make the solution alkaline, so the indicator changes colour to purple.Chlorine is formed at the anode. Chlorine in water is a powerful bleach and bleaches the universal indicator, making it colourless.Overall, when sodium chloride solution is electrolysed, H+ and Cl- ions are discharged at the electrodes, but the Na+ and OH- ions stay in the solution, making sodium hydroxide.The electrolysis of brine is an important industrial process. Solution mining of salt deposits brings brine to the surface. This solution is then taken by pipeline to the factory, where it is electrolysed. The process is carried out on an enormous scale: the electrolysis of brine at the ICI site in Runcorn in Cheshire uses 1% of the total electricity consumed in Great Britain!The three products