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Presentation On Preparation Of Salts

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Published in: Chemistry | Science
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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The worksheets and topical questions will be discussed during the tutoring sessions

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  1. Preparation of Salts Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • Preparation, separation and purification of soluble salts by reaction of an acid with: (a) an alkali by titration (b) excess metal (c) excess insoluble base (d) excess insoluble carbonate • Preparation of insoluble salts by precipitation
  3. Preparation of Soluble Salts Start Does the metal react with acids? Is the base or carbonate soluble? Yes No Does it react safely ? Yes Method A: can prepare salt by using excess metal and acid. followed by filtration, e.g MgSOtOHP. znC12 Method A can salt by reacting acid with excess, solid. followed by filtration. e.g. cuso,.5H.o Method B: can use titration method e.g. Naa. K,soa, NH.NO, Salt crystals prepared by evaporation and crystall isation Figure 12.7: Flow chart showing which method to use for preparing soluble salts.
  4. Preparation of Soluble Salts (contd.) Method A — acid plus solid metal, base or carbonate: • • Stage 1: An excess of the solid is added to the acid and allowed to react. Using an excess of the solid makes sure that all the acid is used up. If it is not used up at this stage, the acid would become more concentrated when the water is evaporated later (stage 3). Stage 2: The excess solid is filtered out. Stage 3: The filtrate is gently evaporated to concentrate the salt solution. This can be done on a heated water-bath (Figure 12.8) or sand tray (Figure 12.9). Stage 4: When crystals can be seen forming (crystallisation point), heating is stopped and the solution is left to crystallise. Stage 5: The concentrated solution is cooled to let the crystals forrn. The crystals are filtered off and washed with a little distilled water. Then the crystals are dried carefully between filter papers (Figure 12.10),
  5. Preparation of Soluble Salts (contd.) Method A — acid plus solid metal, base or carbonate (Stage 1-4): rod off. Wait no iitmzpaper red. Out on the from bath. and
  6. Preparation of Soluble Salts (contd.) Method A — acid plus solid metal, base or carbonate (Stage 5): Crystals Of zinc nitrate after drying between filter papers.
  7. Preparation of Soluble Salts (contd.) Method B — acid plus alkali by titration: Titration: A method of quantitative analysis using solutions: one solution is slowly added to the known volume of another solution using a burrette until an end point is reached. Method B involves the neutralization of acids using alkali (sodium hydroxide) or a soluble carbonate (e.g., sodium carbonate). Both the reactants and products are colorless so indicator is used to find the neutralization point or end point methyl orange acid yell OW colourless Add acid until the colour just changes. Figure 12.12 a: Cotour changes for Hie indicators methyl orange and thymolphthalein during the titration. b: Actual colours of methyl orange in acid and alkali.
  8. Preparation of Soluble Salts (contd.) Method B — acid plus alkali by titration: • Stage 1: The acid solution is poured into a burette. The burette is used to accurately measure the volume of sOlution added. A known volume of alkali solution is placed in a conical flask using a volumetric pipette. The pipette delivers a fixed volume accurately. A few drops of an indicator (e.g. thymolphthalein or methyl orange) are added to the flask. Stage 2: The acid solution is run into the flask a few drops at a time from the burette until the indicator just changes colour (Figure 12.12). The conical flask must also be swirled after each portion of acid to ensure everything is mixed and the reaction is complete. Having found the end-point for the reaction, the volume of acid (titre volume) added is noted. The experiment is then repeated without using the indicator. The same known volume of alkali is used in the flask. The same volume of acid as noted in the first part is then run into the flask. Alternatively, activated charcoal can be added to remove the coloured indicator. The charcoal can then be filtered off. Stage 3: The salt soMion is evaporated and cooled to form crystals as described in method A.
  9. Preparation of Soluble Salts (contd.) Method B — acid plus alkali by titration: burette end-point has been reached indicator afteradding the indicator of the solution and crystallisation as in method A
  10. Practice Questions Name the salts formed vhen: 4 5 6 a b c a b c d e dilute hydnchloric acid reacts with magnesium calciun•i oxide reacts with dilute nitric acid zinc carbonate reacts •with dilute sulfuric acid. In the methods of preparing a salt using a solid m«ab base or carbonate, Why is the solid used in In such methods, how is the excess solid removed once the reaction has finished? name the two important pieces of graduated glassware used in the titration method of preparing a salt. What colour is the indicator methyl orange ih alkali? Why should the crystals prepared at the end of expedrnents to prepare salts not be heated too strongly when drying them? There are two methods of pr4aring soluble salts depending on the solubility of the reagent reacted with the acid: method A (titration using a burette and an indicator) and method B (addition of an excess of a base ora metal to a dilute acid and removal of the excess sorld). Which method would you use to prQare the soluble salt, zinc sulfate, from the insoluble base, zinc oxide? i Write down the method. il Wife down the reagent to use. iii Write the word equation.
  11. Preparing Insoluble Salts by Precipitation Insoluble salts forms a layer on the surface and the reaction stops. E.g., the reaction between marble chips and sulfuric acid forms a layer of insoluble calcium sulfate on marble chips. layer of insoluble calcium suffate Such insoluble salts cannot be made by crystallization methods. They are generally cant get at made by ionic precipitation. the calcium carbonate Precipitation: The sudden formation of a salt when either two solutions are mixed or gas is bubbled into solution 0 00 0 0 chip 00 0
  12. Preparing Insoluble Salts by Precipitation (contd.) Insoluble white barium sulfate is formed immediately bari•m sodium sulfate BaCb(aq) + NQS04(aq) barium sulfate 4- sodium cmoride BaS04(s) + 2NaC1(aq) The ionic equation will be: Ba2+(aq) + -• Bas04(s) The spectator ions are left out of ionic equation sodium sulfate solution Bal-and SO.* ions combine to form pre#itate of BaSOt; the Nat and Cl- ions in solution. be by
  13. Preparing Insoluble Salts by Precipitation (contd.) Formation of precipitates of silver chloride salt silver nitrate + sodium chloride — silver chloride AgNO 3(aq) -F NaCl(aq) Ag+ (aq) CE (aq) AgCl(s) AgC1(s) sodium nitrate N aN 03 (aq) silver ion silver ions and chloride ions sticking nitrate ion together to make solid silver chloride 0000 000 0 00 00 0 0 00 00 Oqooo 000 sodium ions and nitrate ions are spectator ions 00 chloride ion sodium ion Illustration of which ions take part in the precipitation of silver chloride.
  14. Preparing Insoluble Salts by Precipitation (contd.) Precipitation reactions are often used in analysis to identify salts, such as, chlorides, iodides and sulfates Precipitates of yellow lead iodide are formed when lead nitrate and potassium iodide are mixed Pb2+(aq) + 21-(aq) Pb12(s) preciøitatiø:n Of iodide by solutions.
  15. Practice Questions 7 8 9 What do the following terms mean in connection with chemical reactions to produce salts? a precipration b titration c ionic equation Which of these salts can be prepared by A silver iodide B magnesium C flead(ll) chloride The insoluble salt, bariurn surate, can be prepared from a solution of barium nitrate by precipitation, a b c name a soluble sSit that could be added to the barium nitrate solution to give a precipitate of barium What colour is the precipitate of barium "'fate? Wrtte loric equ*ion (with state "mbols) for the reaction.

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