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

pH scale of acidity and alkalinity, acids, bases-alkalis, salts and neutralisation

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

Part 5 Covers 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, metals salts to give hydroxide precipitates.

GCSE/IGCSE Sub-index: (c) doc b Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of acid-alkali chemistry : 2. pH scale, indicators, ionic theory of acids-alkali neutralisation : 3. pH examples of acid, neutral or alkaline solutions : 4. Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions : 5. Reactions of bases-alkalis like sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation : 8. Important formulae, salt solubility and water of crystallisation : 9. Further examples of word/symbol equations for salt preparations : 10. More on Acid-Base Theory and Weak and Strong Acids

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


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

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

  • Reaction of sodium hydroxide with ammonium salts

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

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

      • NaOH(aq) + NH4Cl(aq) ==> NaCl(aq) + H2O(l) + NH3(g)

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

      • NaOH(aq) + NH4NO3(aq) ==> NaNO3(aq) + H2O(l) + NH3(g)

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

      • 2NaOH(aq) + (NH4)2SO4(aq) ==> Na2SO4(aq) + 2H2O(l) + 2NH3(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) + NH3(g)

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

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

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

    • e.g. sodium hydroxide + copper(II) sulphate ==> sodium sulphate + copper(II) hydroxide

    • 2NaOH(aq) + CuSO4(aq) ==> Na2SO4(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.

  • 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) + CO2(g)  ==> NaHCO3(aq)

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

      • 2NaOH(aq) + CO2(g)  ==> Na2CO3(aq) + H2O(l)

      • 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) + CO2(g) ==> CaCO3(s) + H2O(l)

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

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keywords formula equations: NaOH(aq) + NH4Cl(aq) ==> NaCl(aq) + H2O(l) + NH3(g) * NaOH(aq) + NH4NO3(aq) ==> NaNO3(aq) + H2O(l) + NH3(g) * 2NaOH(aq) + (NH4)2SO4(aq) ==> Na2SO4(aq) + 2H2O(l) + 2NH3(g) * NH4+(aq) + OH-(aq) ==> H2O(l) + NH3(g) * 2NaOH(aq) + CuSO4(aq) ==> Na2SO4(aq) + Cu(OH)2(s) * NaOH(aq) + CO2(g) ==> NaHCO3(aq) * 2NaOH(aq) + CO2 (g) ==> Na2CO3(aq) + H2O(l) * Ca(OH)2(aq) + CO2(g) ==> CaCO3(s) + H2O(l) * NaOH + NH4Cl ==> NaCl + H2O + NH3 * NaOH + NH4NO3 ==> NaNO3 + H2O + NH3 * 2NaOH + (NH4)2SO4 ==> Na2SO4 + 2H2O + 2NH3 * NH4+ + OH- ==> H2O + NH3 * 2NaOH + CuSO4 ==> Na2SO4 + Cu(OH)2 * NaOH + CO2 ==> NaHCO3 * 2NaOH + CO2 ==> Na2CO3 + H2O * Ca(OH)2 + CO2 ==> CaCO3 + H2O *

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