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Acid and bases

This is a summary of the topic "Acids and bases" in the GCE O levels subject: Chemistry. Students taking either the combined science (chemistry/physics) or pure chemistry will find this useful. These slides are prepared according to the learning outcomes required by the examinations board.

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Acid and bases

  2. 2. WHAT IS AN ACID? Definition: An acid is a substance that produces hydrogen ions, H+, in aqueous solution.
  3. 3. ACIDS Acid Formula Ions produced in aqueous solution hydrochloric acid HCl H+(aq) Cl-(aq) nitric acid HNO3 H+(aq) NO3 -(aq) sulfuric acid H2SO4 H+(aq) SO4 2-(aq)
  4. 4. ACIDS hydrochloric acid  hydrogen ion + chloride ion HCl(aq)  H+(aq) + Cl-(aq) sulfuric acid  hydrogen ion + sulfate ion H2SO4(aq)  2H+(aq) + SO4 2-(aq) The hydrogen ions produced gives the properties of acids.
  5. 5. PROPERTIES OF ACIDS 1. Acids have a sour taste. 2. Acids dissolved in water can conduct electricity. 3. Acids turns damp blue litmus red.
  6. 6. CHEMICAL REACTIONS OF ACIDS General equations: 1. metal + acid  salt + hydrogen 2. metal carbonate + acid  salt + water + carbon dioxide 3. metal oxide/metal hydroxide + acid  salt + water
  7. 7. CHEMICAL REACTIONS OF ACIDS metal + acid  salt + hydrogen magnesium + dilute sulfuric acid  magnesium sulfate + hydrogen Mg(s) + H2SO4(aq)  MgSO4(aq) + H2(g)
  8. 8. CHEMICAL REACTIONS OF ACIDS metal carbonate + acid  salt + water + carbon dioxide calcium carbonate + dilute hydrochloric acid  calcium chloride + water + carbon dioxide CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2)(l) + CO2(g)
  9. 9. CHEMICAL REACTIONS OF ACIDS metal oxide + acid  salt + water zinc oxide + dilute sulfuric acid  zinc sulfate + water ZnO(s) + H2SO4(aq) ZnSO4(aq) + H2O(l)
  10. 10. CHEMICAL REACTIONS OF ACIDS metal hydroxide + acid  salt + water zinc hydroxide + dilute nitric acid  zinc nitrate + water Zn(OH)2(s) + 2HNO3(aq)  Zn(NO3)2(aq) + 2H2O(l)
  11. 11. USES OF ACIDS Sulfuric acid is one of the most widely used acids. It is used to: 1. Manufacture fertilizers. 2. Manufacture detergents. 3. Produce car batteries.
  12. 12. WHAT IS A BASE? Definition: A base is any metal oxide or hydroxide that reacts with an acid to produce a salt and water only. Most bases are insoluble. A base is a combination of a metal with either oxide ions, O2- or hydroxide ions, OH-.
  13. 13. BASES Base Formula sodium oxide Na2O zinc oxide ZnO copper (II) oxide CuO magnesium hydroxide Mg(OH)2 aluminum hydroxide Al(OH)3
  14. 14. CHEMICAL REACTIONS OF BASES base + acid  salt + water copper(II) oxide + dilute sulfuric acid  copper(II) sulfate + water CuO(s) + H2SO4(aq)  CuSO4(aq) + H2O(l)
  15. 15. ALKALIS: A SPECIAL CLASS OF BASE Definition: An alkali is a substance that produces hydroxide ions, OH-, in aqueous solution. It is a base that is soluble in water.
  16. 16. ALKALIS: A SPECIAL CLASS OF BASE Alkali Formula Ions produced in aqueous solution sodium hydroxide NaOH Na+(aq) OH-(aq) potassium hydroxide KOH K+(aq) OH-(aq) calcium hydroxide Ca(OH)2 Ca2+(aq) OH-(aq) barium hydroxide Ba(OH)2 Ba2+(aq) OH-(aq)
  17. 17. ALKALIS sodium hydroxide  sodium ion + hydroxide ion NaOH(aq)  Na+(aq) + OH-(aq) calcium hydroxide  calcium ion + hydroxide ion Ca(OH)2(aq)  Ca2+(aq) + 2OH-(aq)
  18. 18. PROPERTIES OF ALKALIS 1. Alkalis have a bitter taste and feel soapy. 2. Alkalis turn damp red litmus paper blue.
  19. 19. CHEMICAL REACTIONS OF ALKALIS General equations: 1. Alkalis react with acids to form a salt and water only. 2. Alkalis, when heated with ammonium salts, give off ammonia gas. 3. Alkalis react with a solution of one metal salt to give another metal salt and metal hydroxide.
  20. 20. CHEMICAL REACTIONS OF ALKALIS alkali + acid  salt + water sodium hydroxide + dilute hydrochloric acid  sodium chloride +water NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l) This process is neutralization as only a salt and water is produced. The ionic equation for this: H+(aq) + OH-(aq)  H2O(l)
  21. 21. CHEMICAL REACTIONS OF ALKALIS alkali + ammonium salt  salt + water + ammonia calcium hydroxide + ammonium chloride  calcium chloride + water + ammonia Ca(OH)2(aq) + 2NH4Cl(s)  CaCl2(aq) + 2H2O(l) + 2NH3(g)
  22. 22. CHEMICAL REACTIONS OF ALKALIS alkali (containing metal A) + salt (of metal B)  salt (of metal A) + metal hydroxide (of metal B) sodium hydroxide + iron(II) sulfate  sodium sulfate + iron(II) hydroxide 2NaOH(aq) + FeSO4(aq)  Na2SO4(aq) + Fe(OH)2(s)
  23. 23. USES OF BASES AND ALKALIS 1. Magnesium oxide is used as antacid for relieving gastric pain and for making refractory bricks. 2. Sodium hydroxide and potassium hydroxide are used in the preparation of soap. 3. Calcium hydroxide (lime) is used to reduced acidity in soil. 4. Ammonia solution is used to make fertilizers.
  24. 24. STRONG AND WEAK ACIDS The term strength refers to how easily an acid or an alkali ionizes when dissolved in water. We can explain the strength of acids in terms of their extent of ionization.
  25. 25. STRONG AND WEAK ACIDS A strong acid is an acid that is completely ionized in aqueous solution. HCl(aq)  H+(aq) + Cl-(aq) A weak acid is an acid that is only partially ionized in aqueous solution. CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq) The sign “⇌” indicates partial ionization.
  26. 26. THE PH SCALE The strength of an acid or alkali can be shown using the pH scale. The pH of a solution is related to the concentration of hydrogen ions or hydroxide ions present in a solution. An acid with a lower pH value is more acidic as it has a higher concentration of hydrogen ions. A alkaline solution with a higher pH value is more alkaline as it has a higher concentration of hydroxide ions.
  27. 27. THE PH SCALE Universal Indicator The pH value of a solution can be determined by using Universal Indicator.
  28. 28. OTHER INDICATORS Indicator Colour in acidic solution pH range at which indicator changes colour Colour in alkaline solution methyl orange red 3 - 5 yellow screened methyl orange violet 3 - 5 green litmus red 5 - 8 blue bromothymol blue yellow 6 - 8 blue phenolphthalein colourless 8 - 10 pink
  29. 29. IMPORTANCE OF PH Our blood is slightly alkaline and has a pH of around 7.4. Any injection given must have a pH of almost 7.4. If the pH of our blood changes to much, it could cause death. It is important to control the pH of soil because this affects the growth and development of plants. Most plants grow best when the soil is neutral or slightly acidic. We can add chemicals to control the pH of soil. Soil that are too acidic can be treated with bases such as quicklme (calcium oxide) or slaked lime (calcium hydroxide).
  30. 30. TYPES OF OXIDES Many acids and alkalis are formed by dissolving oxides in water. There are four types of oxides: 1. Acidic oxides 2. Basic oxides 3. Amphoteric oxides 4. Neutral oxides
  31. 31. ACIDIC OXIDES Most oxides of non-metals are acidic oxides and they dissolve in water to form acids. sulfur dioxide + water  sulfurous acid SO2(g) + H2O(l)  H2SO3(aq)
  32. 32. ACIDIC OXIDES Acidic oxide Formula Physical states Acid produced in water carbon dioxide CO2 gas carbonic acid H2CO3 sulfur trioxide SO3 gas sulfuric acid H2SO4 phosphorous(V) oxide P4H10 solid phosphoric acid H3PO4
  33. 33. ACIDIC OXIDES Acidic oxides do not react with acids, but they react with alkalis to form a salt and water. sulfur dioxide + sodium hydroxide  sodium sulfite + water SO2(g)n+ 2NaOH(aq)  Na2SO3(aq) + H2O(l)
  34. 34. BASIC OXIDES Most oxides of metals are basic oxides and they are insoluble. Some oxides, such as sodium oxide and potassium oxide dissolve readily in water to form alkalis.
  35. 35. BASIC OXIDES Basic oxides are solids at room temperature and they react with acids to form a salt and water. calcium oxide + nitric acid  calcium nitrate + water CaO(s) + 2HNO3(aq)  Ca(NO3)2(aq) + H2O(l)
  36. 36. AMPHOTERIC OXIDES Amphoteric oxides are metallic oxides that react with both acids and bases to form salt and water. Zinc oxide is an example of an amphoteric oxide. It is able to react with both hydrochloric acid and sodium hydroxide.
  37. 37. AMPHOTERIC OXIDES hydrochloric acid + zinc oxide  zinc chloride + water 2HCl(aq) + ZnO(s)  ZnCl2(aq) + H2O(l) sodium hydroxide + zinc oxide  sodium zincate + water 2NaOH(aq) + ZnO(s)  Na2ZnO2(aq) + H2O(l)
  38. 38. AMPHOTERIC OXIDES Amphoteric oxide Formula aluminium oxide Al2O3 lead(II) oxide PbO
  39. 39. NEUTRAL OXIDES Some non-metals form oxides that are neither basic or acidic in nature. They are called neutral oxides and they are insoluble in water.
  40. 40. NEUTRAL OXIDES Neutral oxides Formula water H2O carbon monoxide CO nitric oxide NO
  41. 41. HOW TO IDENTIFY AN UNKNOWN OXIDE? Unknown oxide Reacts with acid Basic or amphoteric oxide Reacts with alkali Amphoteric oxide Does not react with alkali Basic oxide Does not react with acid Acidic or neutral oxide Reacts with alkali Acidic oxide Does not react with alkali Neutral oxide
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