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GCSE Chemistry Notes: Salt formation when elements combine

The apparatus for the preparation of aluminium chlorise (c) doc b

6d. Making a salt by direct combination of elements

e.g. combining a metal with a non-metal element

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

6d. How to make a salt by direct combination of elements?

 

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

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

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

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

6d. Method (d) Making a salt by direct combination of elements

Sometimes it isn't appropriate to prepare a soluble salt by reacting an acid with an insoluble base or alkali, so it may be possible to prepare the salt by directly combining the metal and the non-metal elements.

Two such examples are the preparation of anhydrous aluminium chloride and anhydrous iron(III) chloride (anhydrous here means without any water of crystallisation).

The apparatus for the preparation of aluminium chlorise (c) doc b

Preparation of aluminium chloride AlCl3

 

prep FeCl3

Preparation of iron(III) chloride FeCl3

  • How can we make aluminium chloride? How do we prepare iron(III) chloride?

  • METHOD (d) both preparations illustrated above.

  • Don't forget to wear safety glasses or goggles when doing this preparation - but it is a teacher demonstration.

  • These compounds can be made by direct combination of the elements to form anhydrous salts e.g. if dry chlorine gas Cl2 is passed over heated iron or aluminium, the chloride is produced.

    • These experiment preparations (shown above) should be done very carefully by the teacher in a fume cupboard.

    • The aluminium can burn intensely with a violet flame, white fumes of aluminium chloride sublime from the hot reacted aluminium and the white solid forms on the cold surface of the flask.

      • aluminium  +  chlorine  ==>  aluminium chloride

      • 2Al(s) + 3Cl2(g) ==> 2AlCl3(s)

      • The aluminium chloride is often discoloured yellow from the trace chlorides of copper or iron that may be formed from traces of these metals that might be present in the original aluminium).

    • The iron (e.g. as steel wool) glows red and brown fumes of iron(III) chloride stream off, the brown solid collects on the cold flask surface.

      • iron + chlorine ==> iron(III) chloride

      • 2Fe(s) + 3Cl2(g) ==> 2FeCl3(s)

    • Note (i): Both these chlorides react exothermically and hydrolyse with water to give the metal hydroxide and fumes of hydrogen chloride, and so dry conditions are needed.

    • Note (ii): Both these chlorides cannot be made in an anhydrous form from aqueous solution neutralisation. This is because on evaporation the compounds contain 'water of crystallisation'. On heating the hydrated salt  hydrolyses and decomposes into water, the oxide or hydroxide and fumes of hydrogen chloride, and maybe some impure anhydrous chloride, basically it a mess in terms of trying to make pure AlCl3 and FeCl3 in this way.

  • Theoretically you can make many anhydrous simple salts like chloride by direct combination of elements.

    • e.g. you can burn sodium in chloride gas to make sodium chloride ('salt').

    • sodium  +  chlorine  ===>  sodium chloride

    • 2Na(s)  +  Cl2(g)  ===>  2NaCl(s)

    • BUT, it would be the most ridiculously expensive and impractical way of making it.

    • Its a lot cheaper just to evaporate seawater and recrystallising the salt if you needed a particularly pure sample of sodium chloride.


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