Preparation of Salts.
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PREPARATION OF SALTS 1. INSOLUBLE SALTS Soluble salt 1(aq) + soluble salt 2(aq) insoluble salt + soluble salt (aq) acid 1(aq) acid 2(aq) Pb(NO3)2(aq) + Na2SO4(g) PbSO4(s) + 2NaNO3(aq) Pb2+(aq) + SO2-4(aq) PbSO4 To prepare a salt, you should firstly thoroughly mix the solution, then you should filter it to remove any precipitate. Next, you should, wash with distilled water and finally leave to dry in the oven. 2. SOLUBLE SALTS a. Filtration i. acid + base salt + water H2SO4(aq) + CuO(s) CuSO4(aq) + H2O(l) H2SO4(aq) + Mg(OH)2(s) MgSO4(aq) + H2O(l) ii. acid + insoluble metal carbonate salt + water + carbon dioxide H2SO4(aq) + CuCO3(s) CuSO4(aq) + H2O(l) + CO2(g) iii. acid + reaction metal salt + hydrogen H2SO4(aq) + Mg(s) MgSO4(aq) + H2(g) The base or the metal carbonate must be insoluble so that we can remove the excess through the method of filtration. The metals that we can use must react with acids but must not react with water as this would contaminate the salt with metal hydroxide.
However, we want large crystals. To do this we allow the solution to evaporate at room temperature for about a week, until there are only a few cubic centimetres remaining. These are filtered off because they contain soluble impurities. The crystals are than washed with a little distilled water and dried using filter paper. b. Titration If the metal hydroxide or the metal carbonate are soluble, (i.e. alkalis), we cannot remove the excess via the process of filtration. Therefore, we have to react these substances in their correct stoichiometric amounts (i.e. there is no excess of either of them). This is done by a process known as titration. All sodium (Na) and potassium (K) salts have to be prepared by titration as their hydroxides and carbonates are soluble. i. base of soluble metal hydroxide + acid salt + water NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ii. metal carbonate + acid salt + water + carbon dioxide Na2CO3(aq) + H2SO4(aq) Na2SO4(aq) + H2O(l) + CO2(g) Here are some pictures of the equipment that we need for this experiment: graduation point The pipette is used for measuring out with a fixed volume of solution.
We repeat the whole thing using the same volumes but no indicators so the final colour of the solution is not changed. We then crystallise the solution as normal. Here are some results from this experiment overleaf: ATTEMPT ROUGH ONE TWO FINAL 22.30 21.00 42.10 INITIAL -1.20 0.00 21.00 TITRE 21.10 21.00 21.10 c. Evaporation Another method of making ammonium nitrate is done when either the acid or the alkali is volatile (evaporates easily). An example of this is for an acid -hydrochloric acid or for an alkali - ammonia. In these cases we do not need to use the process of titration to prepare the salt. We can simply add the excess of the one that is volatile and the excess will evaporate when we come to evaporate the solution. NH3(aq) + HNO3(aq) NH4NO3(aq) We measure out approximately 20cm3 of nitric acid into an evaporating basin. We then add ammonia solution while stirring until in excess. We can check for this by its smell and confirm it is in excess by using red litmus paper which should turn blue. We then crystallise the solution as usual where upon the excess ammonia will evaporate. 1 BY ANTON NYMAN 4GO
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