SALTS A salt is a compound formed when thehydrogen ions in an acid are replaced by metal ions or by ammonium ions. Compounds in which the H+ ions in an acidhave been replaced by ammonium ions (NH4+) are called ammonium salts.
NOTE:•An anion is a negatively charged ion. Hence:-•Hydrochloric acid gives chlorides. E.g. sodium chloride,ammonium chloride.•Nitric acid gives nitrates. E.g. barium nitrate, copper nitrate.•Sulphuric acid gives sulphates. E.g. silver sulphate, iron (ii)sulphate.•Phosphoric acid gives phosphates. E.g. sodium phosphate,ammonium phosphate.•Each acid gives rise to a series of salts named by the ANIONwhich they contain.
•Some acids can donate more than one replaceable hydrogen ion(proton)•E.g. sulphuric acid (H2SO4) and phosphoric acid (H3PO4).•Sulphuric acid can give rise to two series of salts and phosphoric acid tothree series of salts depending on how many protons are replaced.•i.e. (a) H2SO4 → NaHSO4 → Na2SO4 (b) H3PO4 → NaH2PO4 → Na2HPO4 → Na3PO4
•NORMAL SALTS- These are salts which do notcontain replaceable hydrogen. E.g. sodiumchloride (NaCl).•ACIDIC SALTS-These are salts which containreplaceable hydrogen. This is because when thehydrogen is replaced they are acting like an acid.E.g. sodium hydrogen sulphate (NaHSO4).•The method chosen to prepare a salt depends onits solubility.•Solubility depends on the combination of positiveand negative ions.
SOLUBILITY RULES SOLUBLE INSOLUBLE1 All nitrates All carbonates except for sodium, potassium and ammonium carbonates2 All chlorides except for silver chloride and All sulphides except lead (ii) chloride for sodium, potassium and ammonium sulphides3 All sulphates except for calcium, barium All oxides except for and lead (ii) sulphates sodium potassium and ammonium oxides4 All sodium, potassium (even other group I All hydroxides except elements), ammonium and nitrate for sodium, compounds are soluble potassium(group I), ammonium and calcium hydroxides
NOTE:•Insoluble salts are prepared by PRECIPITATION.•Soluble salts are prepared by FILTRATION andCRYSTALLISATION method or by TITRATTION.PREPARATION OF INSOLUBLE SALTS•Insoluble salts are prepared by mixing solutionscontaining their positive and negative ions usingthe method of precipitation (Doubledecomposition).•The reactants are chosen so that on exchangingions the unwanted product is still soluble but thegiven insoluble salt will form as a precipitate.•I.e. soluble salt + soluble salt → soluble salt +insoluble salt (precipitate).
NOTE:•A precipitate is an insoluble solid formed when achemical reaction occurs between two dissolved ionicsubstances.CHOOSING REACTANT SOLUTIONS•The other one must contain the negative ion of theinsoluble salt required.PROCEDURE OF PRECIPITATION•Dissolve each reactant separately in water.•Mix chemically equivalent quantities of the reactantsolutions.•Filter the solution and wash the precipitate in warmdistilled water.•One must contain the positive ion of the insoluble saltrequired.
•Dry the solid salt that was produced in an oven(105oC).EXAMPLES•PREPARATION OF BARIUM SULPHATE (BaSO4)•We can use barium nitrate and sodium sulphate.REACTANT IONSBa2+ NO3− → NaNO3 (soluble)Na+ SO42− → BaSO4 (Insoluble)
•PREPARATION OF SILVER CHLORIDE(AgCl)•We can use silver nitrate and sodium chloride.REACTANT IONSAg+ NO3− → NaNO3 (soluble)Na+ Cl− →AgCl (insoluble)NOTE:•The easiest salt solution containing the positive ionsis the metal nitrate since all nitrates are soluble.•For a solution containing the negative ions, we canuse the sodium salt as all sodium salts are soluble.
•PREPARATION OF LEAD (II) IODIDE(PbI2)I.e. Pb (NO3)2(aq) + 2NaI (aq) → PbI2(s) +2NaNO3 (aq)IONICALLYI.e. Pb2+ (aq) + 2I− (aq) → PbI2(s)
NOTE:•The reactants involved in aprecipitation reaction must be insolution form because the ionsmust be able to move and interactwith one another when thereactants are mixed.•When the ions in the insoluble saltencounter each other, they getattracted and form a solid that willsink to the bottom of the containerand be collected as a PRECIPITATE.
PREPARATION OF SOLUBLE SALTS.•Soluble salts are prepared using twomethods:•Filtration and crystallisation method.i.e neutralising an ACID with EXCESSINSOLUBLE REACTANT.•Titration. i.e. neutralising an ACID withthe EXACT amount of ALKALI.
•FILTRATION AND CRYSTALLISATONMETHOD•This method is used when a suitableinsoluble starting material has been found.•The acid reacts with an excess ofinsoluble reactant that can be:-•Metal•Base (insoluble)•Carbonate•Therefore, to prepare a given salt, weneed to choose the correct acid and asuitable insoluble reactant. (metal, oxide,hydroxide or carbonate).
PROCEDURE•Neutralise an acid with an excess ofthe insoluble reactant.•Filter off any unreacted reagent.•Evaporate the solution to thecrystallisation point.•Cool to produce crystals of the salt.•Filter, wash and dry the crystalsbefore collection.
EXAMPLEPREPARATION OF COPPER (II) SULPHATE•Starting materials include copper (ii)oxide and dilute sulphuric acid.•i.e. CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
STEP 3 AND 4.•Dont evaporate all the water. The filtrate is heated until athin crust of cystals form on the surface of the liquid.
NOTE:•If a metal carbonate is used toprepare a salt using this method,there will be bubbles of carbondioxide gas as the metal carbonate isadded to the acid in step 1.•When there is no more bubbles, itmeans that all the acid has been usedup and we can proceed to the nextstep.
•TITRATION•Soluble salts of ammonium and group Imetals (sodium, potassium and lithium) areprepared using this method.•This is because all their compounds aresoluble (including metals themselves) andvery reactive.•Group I metals are very reactive resultingin too violent reactions that we cannot useexcess reactant.
NOTE:•Titration means using the EXACTquantities of reactants for thereaction.•This method is used when it is notpossible to find a suitable insolublestarting material like a metal, metaloxide or a carbonate that can be easilyfiltered off at the end of the reaction.
INDICATOR•In titration, an indicator is needed toshow the endpoint of one reactantneeded to exactly neutralise a givenvolume of the other reactant.•A common indicator used in thelaboratory is the screened methylorange.
COLOUR CHANGE IN METHYL ORANGE ACIDIC END POINT ALKALINE SOLUTION (NEUTRAL) SOLUTION RED GREY/COLO GREEN URLESS PROCEDURE OF TITRATION •The most common procedure is to react the alkali solution with the dilute acid using a burette. •An indicator is used to determine when the exact amount of reactant has been added.
EXAMPLE: PREPARATION OF SODIUMNITRATE.•Starting materials include sodiumhydroxide and dilute nitric acid.•i.e. NaOH(aq) + HNO3(aq) → NaNO3(aq) +H2O(l)STEP 1.•Place the soluble dilute acid in a conicalflask.•Add a few drops of indicator. E.g. methylorange.
STEP 2.•From a burette, slowly add the alkalisolution. Ensure that the solution is mixedwell.•When the indicator begins to changecolour, the reaction should be slowed to adrip.•At this point, just enough acid is added toneutralise the alkali, all the alkali hasreacted.
STEP 3.•Once the colour change is complete, the reaction iscomplete (END POINT).•The burette should be turned off.•The resulting mixture contains only sodium nitrate andwater.NOTE:•From the titration result, we can know the exactvolume of nitric acid needed to react with 25.0 cm3 ofsodium hydroxide.•From the burette, volume of nitric acid needed will be:Vn=V2 – V1.
STEP 4.•Evaporate the sodium to crystallisation point.•Cool to produce crystals of the salt.•Filter, wash and dry the crystals.NOTE:•In a strict titration, a second titration must be carriedout.•The salt solution obtained in the first titration isthrown away because it is affected by the indicator.•The second titration is done without the indicator. Theexact volume of acid to be added is obtained from thefirst titration.