acids  bases by mr victor lee

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1. Acids & BasesbyMr Victor Lee Version 1 dated 23 Jun 2002

3. Is this an acid? When gaseous hydrogen chloride dissolves in methylbenzene (organic solvent), the resultant solution does not conduct electricity. This indicates that hydrogen chloride does not break up into ions in methylbenzene; rather it continues to exist as discrete molecules.

4. Role of water in Acidity Acids are covalent compounds. They exist as molecules in organic solvents. Hence they do not behave as acids in the absence of water. Acids show the properties of acids only when they are dissolved in water. This is because water reacts with the acids to produce hydrogen ions which are responsible for the acidic properties.

5. Properties of Acids Generally colourless. Most dilute acids have a sour taste. All dilute acids conduct electricity due to the presence of mobile ions. Acids turn blue litmus red.

6. What tests can show that a given substance is an acid? The properties of acids can be inferred from their reactions with various substances, indicators, metals, metal oxides and hydroxides, and carbonates. In practical, the best test for acids would be the reaction with carbonates to produce carbon dioxide gas.

7. Reactions with Metals Dilute acids react with metals above hydrogen in the electrochemical series to produce hydrogen gas and a salt of the acid. Acid + Metals ? Salt + Hydrogen gas Mg(s) + 2HCl(aq) ? MgCl2(aq) + H2(g)

8. What is a salt? A salt is a compound obtained when the hydrogen of an acid is partly or completely replaced by a metal or an ammonium ion, NH4+.

9. More examples of salts

10. Exception 1 (Oxidising agent) Dilute nitric acid, being an oxidising agent, reacts with metals to give nitrogen dioxide (poisonous brown gas) instead of hydrogen.   4HNO3(aq) + Zn(s) ? Zn(NO3)2(aq) + 2NO2(g) + 2H2O(l)

11. Exception 2 (Reactivity Series) When copper, silver or gold comes into contact with dilute acids, no bubbles of hydrogen gas are given off. This shows that the unreactive metals do not react with dilute acids. Remember : Only metals above hydrogen in reactivity series can react with acid to produce hydrogen gas.

12. Exception 3 (Insoluble salts!!!) Lead does not dissolve in dilute hydrochloric acid and dilute sulphuric acid. A layer of lead(II) chloride or lead(II) sulphate is formed from the initial reaction between lead and the dilute acid. This layer is insoluble in water and quickly forms a coating around the metal, which protects it from further attack by the acid. Calcium does not dissolve in dilute sulphuric acid.

13. Reactions with Carbonates & Hydrogen Carbonates Dilute acids react with carbonates and hydrogen carbonates to produce carbon dioxide, a salt of the acid and water. Acid + Carbonates ? Salt + Water + Carbon dioxide gas Na2CO3(s) + H2SO4(aq) ? Na2SO4(aq) + CO2(g) + H2O(l)

14. Reactions with Bases & Alkali All acids react with bases to form a salt and water only. (Neutralisation process) Acid + Base ? Salt + Water HCl(aq) + NaOH(aq) ? NaCl(aq) + H2O(l) CaO(s) + 2HCl(aq) ? CaCl2(aq) + H2O(I)

15. What is neutralisation?? Neutralisation is a process whereby an acid reacts completely with a specific amount of base to form a salt and water as the only products.

16. Examples of neutralisation Reaction between copper(II) oxide (base) and dilute hydrochloric acid (acid) to form copper(II) chloride (salt) and water 2HCl(aq) + CuO(s) ? CuCl2(aq) + H2O(1)  Ionic : 2H+(aq) + O2–(s) ? H2O(1) Reaction between sodium hydroxide (alkali) and dilute nitric acid (acid) to form sodium nitrate (salt) and water HNO3(aq)+ NaOH(aq) ? NaNO3(aq) + H2O(l) Ionic : H+(aq) + OH–(aq) ? H2O(1)

17. Is this neutralisation? Consider the following reactions which involve reacting lead(II) oxide (PbO) and lead(IV) oxide (PbO2) with acids. PbO(s) + 2HNO3(aq) ? Pb(NO3)2(aq) + H2O(l) (1)   PbO2(s) + 4HC1(aq) ? PbCl2(aq) + 2H2O(l) + C12(g) (2) Which of the above is NOT considered neutralisation?

18. Neutralisation or not? Reaction (2) is not a neutralisation reaction. In this reaction, besides a salt (lead(II) chloride) and water, chlorine is also produced. Reaction (1) is a neutralisation reaction because only a salt (lead(II) nitrate) and water are formed.

19. Strength of an acid The strength of an acid depends only on the concentration of H+ ions in the solution. A strong acid is one that ionizes completely to give hydrogen ions, H+, when dissolved in water. HCl(aq) ? H+(aq) + Cl–(aq) A weak acid is one that ionizes only partially to give hydrogen ions, H+, when dissolved in water. CH3COOH(aq) ? CH3COO–(aq) + H+(aq)

20. Basicity of an acid The basicity of an acid is the number of replaceable hydrogen ions, H+, in one molecule of the acid.

21. Uses of acids Hydrochloric and sulphuric acids are commonly used as a solvent to remove rust from iron and steel objects. Acids are also used in the manufacture of fertilisers, detergents and paints.

22. What is a base/alkali? A base is a substance which reacts with an acid to form a salt and water only. An alkali is a basic hydroxide that is soluble in water.

23. Can we define base as substance that release OH– ions in water? A base is a substance which releases hydroxide ions in an aqueous solution but many bases are insoluble, we often seldom use the formation of OH– as definition to avoid complication. You must take note that when acid reacts with a base, it is the hydroxide ions from the base that is responsible for it’s basic characteristics.

24. Explanation for the Properties of Alkalis Hydroxide ions (OH–) are found in alkaline solutions. This give the alkaline solutions their distinctive properties. Sodium hydroxide releases hydroxide ions in solution as follows: NaOH(s) ? Na+(aq) + OH– (aq) Note that ammonia and sodium carbonate form hydroxide ions as below: NH3 + H2O ? NH4+ + OH- CO32- + H2O ? HCO3- + OH-

25. Properties Of Bases They are generally colourless. Most alkalis have a bitter taste. Most alkalis have a soapy touch. They change the colour of indicators, e.g. alkalis turn litmus from red to blue and phenolphthalein from colourless to pink. All alkalis conduct electricity due to the presence of mobile ions.

26. Reactions with Acids They neutralise acids to form a salt and water only. Acid + Base ? Salt + Water 2NaOH(aq) + H2SO4(aq) ? Na2SO4(aq) + 2H2O(l) Important : This reaction is also true for insoluble bases. CuO(s) + H2SO4(aq) ? CuSO4(aq) + H2O(l)

27. Reactions with Ammonium Salts When an alkali is warmed with an ammonium salt, ammonia gas is evolved. Base + Ammonium Salts ? Salt + Water + Ammonia gas NaOH(aq) + NH4Cl(aq) ? NaCl(aq) + H2O(l) + NH3(g) Ca(OH)2(s) + 2NH4Cl(s) ? CaCI2(s) + 2H2O(g) + 2NH3(g)

28. Reactions with Metallic Ions When added to metallic ions in aqueous solutions, alkalis precipitate the insoluble hydroxides of these cations. The reaction between hydroxide ions (OH–) and metallic ions is a useful test for the identification of unknown cations. Most cations give a characteristic reaction with alkalis.

29. Precipitation of Insoluble Metal OH– CuSO4(aq) + 2NaOH(aq) ? Cu(OH)2(s) + Na2SO4(aq) blue precipitate FeCl2(aq) + 2KOH(aq) ? Fe(OH)2(s) + 2KCl(aq) dirty green precipitate FeCl3(aq) + 3KOH(aq) ? Fe(OH)3(s) + 3KCl(aq) reddish–brown precipitate

30. Uses Of Alkalis Alkalis are used in the manufacture of most soaps and detergents. The alkali present in the soap or detergent is able to dissolve the dirt and grease and hence helps in the cleaning process. Sodium hydroxide and ammonia are common alkalis used. Alkalis are used to neutralise acids in our daily lives. For example, toothpastes are alkaline and they enable the acid produced by the bacteria present on our teeth to be neutralized. Otherwise, our teeth will be corroded by the acid.

31. Importance of controlling soil pH Agriculturally, alkalis are used to control the acidity of soil. Most plants grow well in neutral soil of pH around 6.5.

32. How to control soil pH? To neutralise soil that is too acidic, calcium hydroxide (slaked lime) is usually added, known as “liming the soil”. To make basic soils more acidic, aluminium sulphate or powdered sulphur is added. The sulphur is converted into sulphurous acid by soil bacteria.

33. Measuring the pH of a solution? An indicator is used to measure the pH of a solution. An indicator is a compound that has one colour in acidic solutions and another colour in alkaline solutions.

35. Colours of indicators

36. Oxides An oxide is formed when an element combines with oxygen. Basically, there are FOUR types of oxides: Basic oxides Amphoteric oxides Acidic oxides Neutral oxides

37. Type 1 : Basic oxides A basic oxide is formed when a metal reacts with oxygen. They react with acids to form a salt and water only. CaO(s) + 2HCl(aq) ? CaCl2(aq) + H2O(l) MgO(s) + H2SO4(aq) ? MgSO4(aq) + H2O(l) Ionic Eqn: O2–(aq) + 2H+(aq) ? H2O(l)

38. Type 2 : Acidic oxides An acidic oxide is formed when a non-metal reacts with oxygen. React with water to form acids. SO2(g) + H2O(l) ? H2SO3(aq) P4O10(s) + 6H2O(l) ? 4H3PO4(aq) React with alkalis to form a salt and water only. CO2(g) + 2NaOH(aq) ? Na2CO3(aq) + H2O(l) SO3(g) + 2KOH(aq) ? K2SO4(aq) + H2O(l)

39. Type 3 : Amphoteric oxides An amphoteric oxide is a metallic oxide that can behave both as a basic oxide and as an acidic oxide. They can react with both acids and alkalis to form a salt and water only. aluminium oxide (Al2O3), zinc oxide (ZnO), lead(II) oxide (PbO) tin(VI) oxide (SnO2).

40. Reactions of amphoteric oxides With acids, it behaves as a basic oxide, forming a salt and water only. ZnO(s) + H2SO4(aq) ? ZnSO2(aq) + H2O(l) With alkalis, it behaves as an acidic oxide, forming complex salts. Al2O3(s) + 2NaOH(aq) + 3H2O(l) ? 2NaAl(OH)4(aq) complex salt (don’t need to memories this equation)

41. Type 4 : Neutral oxides  Most neutral oxides are non-metallic oxides. Neutral oxides do not react with acids or bases. water (H2O), carbon monoxide (CO) nitrogen monoxide (NO) dinitrogen oxide (N2O)

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