Part 5. More on other important chemical reactions of insoluble bases and soluble bases (alkalis) e.g. sodium hydroxide and ammonia

5(a) Summary introduction for Part 5

Apart from the reactions with acids, this section also covers in detail other reactions of bases (acid reactions are covered in Part 4) with other chemicals e.g. the reaction of the alkali sodium hydroxide with ammonium salts, carbon dioxide and metals salts to give hydroxide precipitates.

Word and symbol equations are given for the reaction of the base sodium hydroxide with ammonium salts.

Word and symbol equations are given for the formation of metal hydroxide precipitates when sodium hydroxide is added to solutions of soluble metal salts.

These revision notes on the reactions of soluble bases (alkalis) and insoluble bases should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9, 10 school science courses for ~14-16 year old science students for national examinations in chemistry topics including acids bases alkalis salts preparations reactions

5(b) Some important reactions of Bases (alkali = soluble base)

(a) Reaction reminders from Part 4

Bases, both soluble (alkali) and insoluble, react with acids to form salts.

Such base may be metal oxides, metal hydroxides, metal carbonates and ammonia.

Alkalis (soluble bases) include soluble metal hydroxides, soluble metal carbonates and ammonia.

• Neutralisation of bases with acids is dealt with Part 4, so Part 5 mainly concentrates on other reactions of bases.

  • BUT a few 'reminder' examples wouldn't go amiss here e.g

  • (soluble base) potassium hydroxide + hydrobromic acid ==> potassium bromide + water

  • KOH_(aq) + HBr_(aq) ==> KBr_(aq) + H₂O_(l)

  • (insoluble base) magnesium hydroxide + hydrochloric acid ==> magnesium chloride + water

    • Mg(OH)_2(s) + 2HCl_(aq) ==> MgCl_2(aq) + 2H₂O_(l)

  • (soluble, alkali) ammonia + nitric acid ==> ammonium nitrate

    • NH₃ + HNO₃ ==> NH₄NO₃

    • NH_3(aq) + HNO_3(aq) ==> NH₄NO_3(aq)

    • Similarly ...

    • ammonia + hydrochloric acid ==> ammonium chloride

    • NH₃ + HCl ==> NH₄Cl

    • NH_3(aq) + HCl_(aq) ==> NH₄Cl_(aq)

    • and

    • ammonia + sulfuric acid ==> ammonium sulfate

    • 2NH₃ + H₂SO₄ ==> (NH₄)₂SO₄

    • 2NH_3(aq) + H₂SO_4(aq) ==> (NH₄)₂SO_4(aq)

  • (insoluble base) zinc oxide + hydrochloric acid ==> zinc chloride + water

    • ZnO_(s) + 2HCl_(aq) ==> ZnCl_2(aq) + H₂O_(l)

  • (insoluble) calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

    • CaCO_3(s) + 2HCl_(aq) ==> CaCl_2(aq)+ H₂O_(l) + CO_2(g)

  • (soluble) sodium carbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide

    • Na₂CO_3(s) + 2HCl_(aq) ==> 2NaCl_(aq) + H₂O_(l) + CO_2(g)

  • (slightly soluble) sodium hydrogencarbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide

    • NaHCO_3(s) + HCl_(aq) ==> NaCl_(aq) + H₂O_(l) + CO_2(g)

• Reaction of sodium hydroxide with ammonium salts

  • 

• Ammonium salts are decomposed when mixed with a strong base e.g. the alkali sodium hydroxide produces ammonia gas  ...

  • (i) sodium hydroxide + ammonium chloride ==> sodium chloride + water + ammonia

  • NaOH_(aq) + NH₄Cl_(aq) ==> NaCl_(aq) + H₂O_(l) + NH_3(g)

  • (ii) sodium hydroxide + ammonium nitrate ==> sodium nitrate + water + ammonia

  • NaOH_(aq) + NH₄NO_3(aq) ==> NaNO_3(aq) + H₂O_(l) + NH_3(g)

  • (iii) sodium hydroxide + ammonium sulfate ==> sodium sulfate + water + ammonia

  • 2NaOH_(aq) + (NH₄)₂SO_4(aq) ==> Na₂SO_4(aq) + 2H₂O_(l) + 2NH_3(g)

  • The ammonia is readily detected by its pungent odour (strong smell) and by turning damp red litmus blue.

  • The ionic equation for all three reactions is:

    • NH₄⁺_(aq) + OH^–_(aq) ==> H₂O_(l) + NH_3(g)

    • The chloride, nitrate and sulfate ions are spectator ions, they do not change chemically in these neutralisation reactions.

  • This reaction can be used to prepare ammonia gas and as a simple chemical test for an ammonium salt.

    • The alkaline ammonia gas is tested for by damp red litmus turning blue.

    • See also "Chemical Tests" and "Gas Preparation–Collection" pages

5(d) Use of calcium carbonate and calcium oxides acting as basic metal compounds

• Use of limestone and lime to control soil acidity in neutralisation reactions to decrease acidity, increasing soil fertility, is dealt with in the 1st section on the Extra Industrial Chemistry page.

5(e) The reaction of sodium hydroxide with metal ions

• Alkali's (soluble bases) are used to produce the insoluble hydroxide precipitates of many metal ions from their soluble salt solutions.

  • (aq) indicates soluble, (s) indicates insoluble - precipitate formed e.g.

  • sodium hydroxide + copper(II) sulfate ==> sodium sulfate + copper(II) hydroxide

  • 2NaOH_(aq) + CuSO_4(aq) ==> Na₂SO_4(aq) + Cu(OH)_2(s) a blue precipitate

  • ionically: Cu²⁺_(aq) + 2OH^–_(aq) ==>  Cu(OH)_2(s)

  • This reaction can be used as a simple test to help identify certain metal ions.

    • See "Chemical Tests" page

    • and GCSE/IGCSE page on Transition Metals page for more examples.

• Aqueous solutions of alkalis like sodium hydroxide ('caustic soda') and calcium hydroxide ('limewater') react with the acidic gas carbon dioxide to form carbonate compounds if the gas is bubbled into their solutions.

  • There are two possible reactions with sodium hydroxide and carbon dioxide

  • (i) sodium hydroxide + carbon dioxide ==> sodium hydrogen carbonate + water

    • NaOH_(aq) + CO_2(g)  ==> NaHCO_3(aq)

    • This reaction predominates with excess carbon dioxide.

  • (ii) sodium hydroxide + carbon dioxide ==> sodium carbonate + water

    • 2NaOH_(aq) + CO_2(g)  ==> Na₂CO_3(aq) + H₂O_(l)

    • This reaction predominates with excess sodium hydroxide.

    • This reaction can be used to remove carbon dioxide gas from a mixture of gases.

  • The reaction with 'limewater' is ...

  • calcium hydroxide + carbon dioxide ==> calcium carbonate + water

    • Ca(OH)_2(aq) + CO_2(g) ==> CaCO_3(s) + H₂O_(l)

    • The formation of the white precipitate of calcium carbonate is used as a test for the gas carbon dioxide.