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Acids and Bases

Acids and Bases. You should already know quite a lot about acids and bases: Acids are corrosive chemicals with a characteristic sour taste. They form solutions with a pH < 7 More about pH later!. All acids react in similar ways: Write balanced equations for the following:

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Acids and Bases

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  1. Acids and Bases

  2. You should already know quite a lot about acids and bases: • Acids are corrosive chemicals with a characteristic sour taste. • They form solutions with a pH < 7 • More about pH later!

  3. All acids react in similar ways: • Write balanced equations for the following: • Iron with dilute sulfuric acid • Lead carbonate with nitric acid • Zinc oxide with hydrochloric acid • Calcium hydroxide with nitric acid • Sodium hydrogen carbonate with sulfuric acid • Potassium hydroxide with hydrochloric acid (write an ionic equation!)

  4. Fe(s) + H2SO4(aq)  FeSO4(aq) + H2(g) • PbCO3(s) + 2HNO3(aq)  Pb(NO3)2(aq) + CO2(g) + H2O(l) • ZnO(s) + 2HCl(aq)  ZnCl2(aq) + H2O(l) • Ca(OH)2 + 2HNO3(aq)  Ca(NO3)2(aq) + 2H2O(l) • 2NaHCO3(s) + H2SO4(aq)  Na2SO4(aq) + 2CO2(g) +2H2O(l) • OH-(aq) + H+(aq) H2O(l)

  5. When equal volumes of 2 mol dm-3 sulfuric acid and 2 mol dm-3 aqueous sodium hydroxide are mixed, how can you tell they react? • A gas is evolved • The mixture becomes warm • The solution changes colour • A solid precipitate is formed

  6. When equal volumes of 2 mol dm-3 sulfuric acid and 2 mol dm-3 aqueous sodium hydroxide are mixed, how can you tell they react? • A gas is evolved • The mixture becomes warm • The solution changes colour • A solid precipitate is formed All neutralisation reactions are exothermic!

  7. Definitions of acids and bases • What’s the formula of sulfuric acid • H2SO4 • Hydrochloric acid? • HCl • Nitric acid? • HNO3 • Ethanoic acid? • CH3COOH • Carbonic acid? • H2CO3

  8. What do they all have in common? • The most basic definition of an acid is the Arrhenius definition. • An acid is a substance that splits up in water to give a hydrogen ion and an anion

  9. E.g. • HCl(aq)  H+(aq) + Cl-(aq) • The(aq) bit is important • Acids do not show acidic properties in many non-aqueous solvents. • We can emphasise the importance of water: • HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq) • H3O+ is known as a hydronium ion (or sometimes a hydroxonium ion). • We don’t usually bother showing this!

  10. Because of the importance of the hydrogen ion in acidic reactions, we often omit the spectator ions • (the ions which don’t change during a reaction) • You did lots of examples of this at IGCSE • E.g. • Mg + 2HCl  MgCl2 + H2 • Write it as ions: • Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2

  11. Remove any ions which are the same on both sides: • Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2

  12. Remove any ions which are the same on both sides: • Mg + 2H+ + 2Cl-  Mg2+ +2Cl- + H2 • Leaving the ionic equation: • Mg + 2H+  Mg2+ + H2

  13. How do we define a base? • A base is a substance which reacts with an acid to form water. • i.e. undergoes neutralisation!!! • What’s the difference between an alkali and a base? • An alkali is soluble in water. • This fits in much better with the Arrhenius definition for an acid . . .

  14. An alkali is a substance which gives OH-(aq) ions in water. • E.g. • NaOH(aq)  Na+(aq) + OH-(aq)

  15. A puzzle: • Name a common alkali gas • Ammonia (there is only one) • How does ammonia give hydroxide ions in water? • NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) • Carbonates and hydrogen carbonates act in a similar way.

  16. Bronsted – Lowry Acids • This is just a different way of defining an acid. • An acid is a substance that acts as a proton donor. • Why “proton” ? • A hydrogen atom has 1 proton and 1 electron • To make a H+ ion we take away the electron • So a H+ ion is JUST a single proton

  17. A base is a proton acceptor • [ there is a slight technical difference between the B-L definition and Arrhenius’ definition. In the B-L definition we don’t need to have H+(aq) present at any time – so we can talk about acids and bases without needing an aqueous solvent]

  18. If an acid can donate one proton it is monoprotic • If it can donate 2 protons it is diprotic • 3 is triprotic • Many is polyprotic • A few species can either donate a proton or accept a proton. • These are known as amphiprotic

  19. E.g. • H2SO4 HSO4-SO42-

  20. E.g. • H2SO4 HSO4-SO42- Accept aproton

  21. E.g. • H2SO4 HSO4-  SO42- • Donate a proton

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