Acids & Bases

1. Acids & Bases

2. Weaver ant’s belly contains formic acid Coca Cola contains phosphoric acid Wasp stings are alkaline Antacids contain magnesium hydroxide

3. ACIDS Scarred by acid in BangladeshSaturday, 11 July, 2009 Source: Al Jazeera

4. Definition of acid (Arrhenius) • An acid is a substance which produces hydrogen ions as the only positive ions when it is dissolved in water HClmolecules dissolved in water to give hydrogen ion and chloride ion

5. ACIDS – Strong Acids

6. ACIDS – Weak Acids

7. Strength of an acid • The strength of an acid depends on its degree of dissociation / ionisationin water to form hydrogen ions. HClmolecules dissolved in water to give hydrogen ion and chloride ion

8. ACIDS – Strong Acids • Strong acids are acids that ionise / dissociate completely in water to produce hydrogen ions (H+). • There are no molecules left. • The properties and reactions of acids are due to these hydrogen ions. Pure HCl (g)  hydrogen chloride (covalent) HCl (aq)  H+ (aq) + Cl- (aq) (ionic, therefore acidic)

9. ACIDS – Strong Acids • Pure HNO3 (l) : covalent HNO3 (aq) → H+ (aq) + NO3- (aq) • Pure H2SO4 (l) : covalent H2SO4 (aq) → 2H+ (aq) + SO42- (aq)

10. ACIDS – Weak Acids • A weak acid is one that ionises / dissociatesincompletely / partiallyin water to produce few hydrogen ions (H+). • Most of the acid molecules remain as molecules. CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq)

11. ACIDS – Dissociation of H+ • The strength of an acid depends on its degree of ionisation / dissociation in water to form hydrogen ions. Higher degree of ionization / dissociation more H+ ions stronger acids Lower degree of ionization / dissociation weaker acids few H+ ions

12. ACIDS – Strength & Concentration • Strength - depends on its degree of ionisation / dissociation in water to form hydrogen ions. • Concentration - amount of a solute dissolved in a unit volume of the solution • For same concentration, 1 mol/dm3 of HCl is stronger than 1 mol/dm3 of ethanoic acid

13. Concentration • A concentrated acid is one in which the proportion of acid to water is very high. • A diluted acid is one in which the proportion of acid to water is low.

14. Importance of water for acidity • Pure acids exist as molecules instead of ions. • Pure acids do not behave as acids as the properties of acids are due to the presence of hydrogen ions. • When acids are mixed with water, ionisation of acids occurs, and hydrogen ions are produced. Therefore, acids can only behave as acids when they are dissolved in water.

15. ACIDS - Properties of Acids • Acids have a sour taste. • Acids dissolve in water to form colourless solutions which conduct electricity. • Acids turn blue litmus red. • Acids react with metals, carbonates & bases

16. The hydrogen gas causes bubbling during the reaction HCl Zinc ACIDS – Reaction with metal Acid + metal  salt + hydrogen 2 HCl (aq) + Zn (s)  ZnCl2 (aq) + H2 (g) Hydrogen gas burns with a ‘pop’ sound

17. ACIDS – Reaction with metal Acid + metal  salt + hydrogen 2 HCl (aq) + Zn (s)  ZnCl2 (aq) + H2 (g) Observations: • Effervescence observed. Colourless, odourless gas evolved which extinguished a lighted wooden splint with a ‘pop’ sound. • Grey solid reduced in size (when in excess). Grey solid dissolves to form a colourless solution • Heat liberated.

18. ACIDS – Exceptions • Metals less reactive than hydrogen is not reactive enough to react with acids.

19. Reactivity Series

20. Solubility of salts EXCEPT EXCEPT EXCEPT

21. ACIDS – Reaction with carbonate • Acids react with carbonates to produce a salt, carbon dioxideand water. Eg: H2SO4 (aq) + Na2CO3 (s)  Na2SO4 (aq) + CO2 (g) + H2O (l) Observations: • Effervescence occurs. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate. • white solid dissolves to form colourless solution Acid + carbonate  salt + carbon dioxide + water

22. ACIDS – Reaction with base • Acids react with bases (or alkalis) to form salt and water only. Eg: H2SO4 (aq) + MgO (s)  MgSO4 (aq) + H2O(l) Observations: • White solid dissolves to form colourless solution • NOTE: no effervescence Acid + base  salt + water

23. ACIDS – Reaction with base Eg: KOH (aq) + HCl (aq)  KCl (aq) + H2O (l) Observations: • Heat is given out • No visible change • NOTE: no effervescence, no colour change • NOTE: Since acids react with bases to form water and salt only, we can say that acids neutralized by bases and vice versa.

24. Uses of Acids Sulphuric acid: • in making detergents • in making fertilizers • in car batteries Ethanoic acid: • (in vinegar) to prepare food • in making adhesives such as glues Hydrochloric acid: • in leather processing • for cleaning metals

25. Indicators Each indicator has one colour in an acidic solution and anothercolour in an alkalinesolution.

26. Litmus • An acid turns blue litmus redin the presence of H+.

27. Methyl Orange • Strong acid turns methyl orange red. • Alkali turn methyl orange yellow • Colour change at pH 4 pH 2 pH3 pH4 pH5

28. Phenolphthalein • Strong alkali turns phenolphthalein pink • Remains colourless in acid • Colour change at pH 9

29. Universal Indicator • Strong acid – red • Weak acid – yellow • Neutral – green • Weak alkali – turquoise • Strong alkali – violet

30. Chemistry 5072

31. LITMUS PAPER METHYL ORANGE PHENOLPHTHALEIN Acids turn litmus red Alkalis turn litmus blue Methyl Orange is RED in strongly acidic solution Phenolphthalein is COLOURLESS in strongly acidic solution ... Indicators Each indicator has one colour in an acidic solution and another colour in an alkaline solution. Chemistry 5072

32. pH (Advanced) • The pH of a solution is defined as the negative logarithm to the base 10 of the hydrogen ion (H+) concentration in mol dm-3. pH = - log10[H+]

33. pH • pH values depend on the concentration of acid/base and degree of dissociation. The use of pH in measuring the strength of an acid is limited since its value changes with concentration.

34. BASES AND ALKALIS Soaps and detergents are mild BASES. Toothpaste are basic. The BASE neutralises acids on our teeth produced by bacteria. Floor cleaners often contain a BASE. Your stomach is lined by cells that produce a BASE.

35. BASES and ALKALIS • A base is a substance that reacts with an acid to form a salt and wateronly. • An alkali is a soluble base that produces hydroxide ions (OH-) when dissolved in water. Arrhenius proposed that substances that react with an acid to give a saltand water only be called BASES.

36. BASES and ALKALIS - Examples • Bases are hydroxides and oxides of metals. • Insoluble base Egs: CuO, Al2O3, ZnO, FeO, Cu(OH)2, Zn(OH)2

37. BASES alkalis BASES and ALKALIS - Examples • Soluble base / alkali Egs: Na2O, K2O, NaOH, KOH, Ca(OH)2 (slightly soluble), NH4OH ALL alkalis are BASES BUT NOT all bases are ALKALIS. An ALKALI is a BASE that is SOLUBLE in WATER.

38. ALKALIS – Strong Alkalis • Strong alkalis are alkalis that ionise / dissociates completely in water to produce hydroxide ions (OH-). The properties and reactions are due to these hydroxide ions. Egs: NaOH (aq)  Na+ (aq) + OH- (aq) KOH (aq)  K+ (aq) + OH- (aq)

39. ALKALIS – Weak Alkalis • Weak alkalis are alkalis that ionise / dissociates incompletely / partially in water to produce few hydroxide ions. The reactions are reversible. • The strength of an alkali depends on its degree of ionization / dissociation.

40. BASES and ALKALIS - Properties • Alkalis have a bitter taste and soapy feel. • Bases react with acids to form salt and water only. Eg: H2SO4 (aq) + ZnO (s)  ZnSO4 (aq) + H2O (l) NOTE: Since acids react with bases to form water and salt only, we can say that acids neutralised by bases and vice versa. Base + acid  salt + water

41. Neutralization Reaction between an Acid and an Alkali • In neutralization reaction between an aqueous acid solution and an aqueous alkali solution, the hydrogen ions from the acid combine with the hydroxide ions from the alkali to form water. • Heat is given during the process. H+ (aq) + OH- (aq)  H2O (l)

42. sodium + hydrochloric  sodium + water hydroxideacid chloride ALKALI + ACID  Salt + WATER Sodium chloride is the salt produced from the above reaction. Example

43. Ammonia gas turns moist red litmus paper blue. Ammonium sulphate Copper hydroxide BASES and ALKALIS - Properties • Bases react with ammonium salts to form salt, ammonia and water only. Eg: Cu(OH)2 (s) + (NH4)2SO4 (aq)  CuSO4 (aq) + 2 NH3 (g) + 2 H2O (l) • Alkalis turn red litmus paper blue. • Alkalis change the colours of indicators. Base + ammonium salt  salt + ammonia + water

44. Cu(OH)2 (s) + (NH4)2SO4 (aq)  CuSO4 (aq) + 2 NH3 (g) + 2 H2O (l)

45. BASES and ALKALIS - Properties • Alkalis can react with a solution of one metal salt to give metal hydroxide and another metal salt. Eg: CuSO4 (aq) + 2NaOH (aq) → Cu(OH)2 (s) + Na2SO4 (aq)

46. BASES and ALKALIS - Uses • Ammonia solution: • in window cleaning solutions • in fertilizers • Calcium oxide: • in neutralising acidic solution • to make iron, concrete and cement • Magnesium hydroxide: • in toothpaste to neutralise acid on teeth • in antacids, to relieve digestion