GCSE/IGCSE Sub-index: 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

• 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) + 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)

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

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

• 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) + 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

    • See also GCSE/IGCSE page on Transition Metals page

• 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)

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

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

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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