6b. Making a soluble salt from an acid and insoluble base or metal

Doc Brown's Chemistry GCSE/IGCSE/O level Science-Chemistry Revision Notes

The pH scale of acidity and alkalinity, acids, alkalis, salts and neutralisation

6. Methods of making salts

6b. Making a soluble salt from an acid and an insoluble base (oxide, hydroxide, carbonate) or metal

 

 

Method (a) Making a salt by neutralising a soluble acid with a soluble base (alkali) - neutralisation reaction

Method (b) preparing a salt by reacting an acid with a metal or with an insoluble base - oxide, hydroxide or carbonate (this page)

Method (c) Preparing an insoluble salt by mixing solutions of two soluble compounds

Method (d) Making a salt by directly combining its constituent elements

GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids

See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index

6. METHODS of MAKING SALTS - salt preparation procedures

Salt solubility affects the method you choose to make a salt, the table below will help you decide on the method

A solubility guide for salts and other compounds

Information required to decide on the method used to prepare a salt

salts and other compounds

solubility?

common salts of sodium, potassium and ammonium ions usually soluble in water
common sulfates (sulphates) usually quite soluble except for calcium sulfate (slightly soluble), lead sulfate and barium sulfate are both insoluble
common chlorides (similar rule for bromides and iodides) usually soluble except for insoluble lead(II) chloride and silver chloride
common nitrates all soluble
common carbonates most metal carbonates are insoluble apart from sodium & potassium carbonate.  Ammonium carbonate is also soluble
common hydroxides most metal hydroxides are insoluble apart from sodium, potassium and ammonium hydroxide

One important point is to recognise that one of the reactants is insoluble here, which is why you can't use a titration procedure to work out how much of the acid is to be added to a given amount of the solid. However, you can add the solid reactant to the acid until no more reacts and dissolves and then filter off the excess solid leaving a solution of the desired salt.

6b. A 2nd Method of Making a Water Soluble Salt

6b. METHOD (b) Reacting an acid with a metal or with an insoluble base to give a soluble salt

e.g. an insoluble base such as a metal oxide, metal hydroxide or a metal carbonate, often of a Group 2 metal like calcium, magnesium or a Transition Metal like nickel, copper or zinc. Copper metal won't dissolve in acids, but its oxide and carbonate will. Using the same procedure you can also start with a metal that has a low reactivity towards water e.g. magnesium, zinc or iron.

Typical common insoluble bases used for preparing soluble salts:

MgO magnesium oxide, MgCO3 magnesium carbonate, CaO Calcium oxide, CaCO3 calcium carbonate, Ca(OH)2 calcium hydroxide, NiO nickel(II) oxide, ZnO zinc oxide, Zn(OH)2, zinc hydroxide, ZnCO3 zinc carbonate, CuO copper(II) oxide, CuCO3 copper(II) carbonate, PbCO3 lead(II) carbonate (with nitric acid to make lead(II) nitrate), FeCO3 iron(II) carbonate (to make iron(II) salts), MnCO3 manganese(II) carbonate

Typical examples shown by the word and symbol equations below include ...

copper(II) oxide + sulphuric acid ==> copper(II) sulphate + water

CuO + H2SO4 ==> CuSO+ H2O

CuO(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l)

magnesium hydroxide + sulfuric acid ==> magnesium sulfate + water

Mg(OH)2 + H2SO4 ==> MgSO+ 2H2O

Mg(OH)2(s) + H2SO4(aq) ==> MgSO4(aq) + 2H2O(l)

 

magnesium hydroxide + hydrochloric acid ==> magnesium chloride + water

Mg(OH)2(s) + 2HCl(aq) ==> MgCl2(aq) + 2H2O(l)

 

Zinc carbonate + nitric acid ==> zinc nitrate + water + carbon dioxide

ZnCO3 + 2HNO3 ==> Zn(NO3)2 + H2O + CO2

ZnCO3(s) + 2HNO3(aq) ==> Zn(NO3)2(aq) + H2O(l) + CO2 (g)

 

zinc oxide + hydrochloric acid ==> zinc chloride + water

ZnO(s) + 2HCl(aq) ==> ZnCl2(aq) + H2O(l)

Similar for many other Group 2 and Transition metal oxides  e.g. Mg, Ca, Ba and Co, Ni, Cu instead of Zn

 

zinc + sulfuric acid ==> zinc sulfate + hydrogen

Zn + H2SO4 ==> ZnSO4 + H2

Zn(s) + H2SO4(aq) ==> ZnSO4(aq) + H2(g)

 

calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

CaCO3(s) + 2HCl(aq) ==> CaCl2(aq)+ H2O(l) + CO2(g)

Similar for many other Group 2 and Transition metal carbonates e.g. Mg, Sr, Ba and Ni, Co, Zn instead of Ca

 

  • Carbonates are frequently used in this method of salt making, e.g. using copper carbonate to make copper salts

    • The equations are give with, and without sate symbols.

  • copper(II) carbonate + hydrochloric acid ==> Copper(II) chloride + water + carbon dioxide

    • CuCO3 + 2HCl ==> CuCl2 + H2O + CO2

      • CuCO3(s) + 2HCl(aq) ==> CuCl2(aq) + H2O(l) + CO2(g)

    • and with sulphuric acid a blue solution of copper(II) sulphate is formed.

  • copper(II) carbonate + sulphuric acid ==> Copper(II) sulphate + water + carbon dioxide

    • CuCO3 + H2SO4 ==> CuSO4 + H2O + CO2

      • CuCO3(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l) + CO2(g)

  • copper(II) carbonate + nitric acid ==> Copper(II) nitrate + water + carbon dioxide

    • CuCO3 + 2HNO3 ==> Cu(NO3)2 + H2O + CO2

      • CuCO3(s) + 2HNO3(aq) ==> CuSO4(aq) + H2O(l) + CO2(g)

  • Similar equations for other carbonates to give soluble salts which can be crystallised from solution e.g.

    • calcium carbonate CaCO3, to make two salts - calcium chloride/nitrate (calcium sulfate is not very soluble)

    • iron(II) carbonate FeCO3, to make three salts - iron(II) chloride/sulfate/nitrate

    • magnesium carbonate MgCO3, to make three salts - magnesium chloride/sulfate/nitrate

    • manganese(II) carbonate MnCO3, to make three salts - manganese(II) chloride/sulfate/nitrate

    • zinc carbonate ZnCO3, to make three salts - zinc chloride/sulfate/nitrate

    • lead(II) carbonate PbCO3, only nitric acid to make lead(II) nitrate, lead(II) chloride and lead(II) sulfate are insoluble and must be prepared by method (c)

(c) doc b More examples of neutralization equations are given in section 4.

soluble salt preparation from insoluble base-acid neutralisationMETHOD (b) Procedure for making a soluble salt from an insoluble base, carbonate or metal

(1) The required volume of acid is measured out into the beaker with a measuring cylinder. The excess of insoluble metal, oxide, hydroxide or carbonate is weighed out (*) and the solid added in small portions to the acid in the beaker with stirring. Doing the weighing will minimise trial and error especially if the reaction is slow, as long as you know how to do the theoretical calculation and add on a little excess!

(*) You can avoid doing a calculation and weighing of the insoluble solid reactant by adding small quantities to the hot acid until no more apparently dissolves.

(2) The mixture may be heated to speed up the reaction. When no more of the solid dissolves it means ALL the acid is neutralised and there should be a little excess solid. You should see a residue of the solid (oxide, hydroxide, carbonate) left at the bottom of the beaker.

(3)  The hot solution (with care!) is filtered to remove the excess solid metal/oxide/carbonate, into an evaporating dish.  On filtration, only a solution of the salt is left i.e. the liquid that passes through the filter paper.

(4) You may need to carefully heat the solution to evaporate some of the water. This evaporation can be done safely with an electric heater of a hot water bath. Then hot concentrated solution is left to cool and crystallise. After crystallisation, you collect and dry the crystals with a filter paper.

Note (i) Apparatus used: (1) balance, measuring cylinder, beaker and glass stirring rod. (2) beaker/rod, bunsen burner, tripod and gauze; (3)-(4) filter funnel and filter paper, evaporating (crystallising) dish.

(ii) A measuring cylinder is adequate for measuring the acid volume, you do not need the accuracy of a pipette or burette required in method (a).

(iii) How to calculate amounts required and % yield is dealt with in (c) doc b(c) doc bChemical Calculations Part 14

Salt solubility affects the method you choose to make a salt and so  section 8. contains tables of information-data on salt solubility which will help you decide on the method to prepare a salt.

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