Acids, bases and salts

• Indicators


  Substances that change colour when they are added to acids and alkalis
• Litmus


  • The most well-known indicator
  • Turns red in acid and blue in alkalis
• Universal indicator


  • Most commonly used in the laboratory
  • Changes colour to show the pH of the solution
• pH scale


  • Acidic = pH < 7
  • Neutral = pH 7
  • Alkaline = pH > 7
• Acids and alkalis are commonly used both in industry and at home
• Acids


  1. Produce hydrogen ions, H+, when they dissolve in water

  2. e.g. for hydrochloric acid: HCl(aq) → H+(aq) + Cl–(aq)
• Bases


  • Chemically opposite to an acid
  • A base that dissolves in water is called an alkali
• Alkalis


  1. Produce hydroxide ions, OH–, when they dissolve in water

  2. e.g. for sodium hydroxide: NaOH(aq) → Na+(aq) + OH–
• Some acids and alkalis are dangerous, others, such as those in vinegar or lemon juice, can be sprinkled on our food!
• Dangerous acids and alkalis have the corrosive hazard warning symbol on their containers
• Neutralisation


  1. Happens when an acid and an alkali 'cancel each other out'

  2. Reaction always produces a salt and water

  3. H+ ions from the acid react with the OH– ions from the alkali to form water (HT only)
• Salt


  The name depends upon the acid and alkali that react
• Strong acids


  • Fully dissociate (ionise) in water
  • Have high numbers of H+ ions in their solutions, so low pH values
  • React more quickly than weak acids
• Weak acids


  • Only partially dissociate
• Temperature rise during the reaction between magnesium and a strong acid

  Higher than that with a weak acid
• Acid strength and concentration are not the same thing
• Acid strength


  Measure of the degree of its dissociation
• Acid concentration


  Measure of the number of moles of acid in 1 dm3 of solution
• Acids react with


  1. Metals

  2. Bases

  3. Carbonates
• Salt


  Name has two parts: first part from the metal, base or carbonate, second part from the acid
• Acids and metals


  1. Acids will react with metals to make a salt and hydrogen gas

  2. e.g. hydrochloric acid + zinc → zinc chloride + hydrogen
• The hydrogen causes bubbling during the reaction, and the reaction is exothermic
• The more reactive the metal, the faster the reaction is, resulting in more bubbling and a bigger temperature rise
• Acids and alkalis/bases


  1. Acids react with alkalis and bases to make a salt and water

  2. Alkalis are commonly metal hydroxides

  3. Bases are commonly metal oxides
• The reactions of acids with alkalis and bases are exothermic
• Acids and carbonates


  1. Acids will react with carbonates to make a salt, water and carbon dioxide gas

  2. e.g. nitric acid + sodium carbonate → sodium nitrate + water + carbon dioxide
• The carbon dioxide causes bubbling during the reaction, and the reaction is exothermic
• Acids


  React with metals, bases and carbonates to form salts
• Preparation of salt crystals from acids


  Depends on whether the substances they are reacting with are soluble or insoluble
• Preparation of a salt from a metal or insoluble base/carbonate

  1. Add excess metal/base/carbonate to the acid

  2. Heat and stir

  3. Filter the mixture

  4. Evaporate the salt solution to collect the crystals
• Insoluble base/carbonate


  Metals, bases and certain metal carbonates are insoluble in water
• Preparation of a salt from an alkali or soluble carbonate


  Use a titration to produce the salt from an acid and alkali
• Titration method


  1. Measure 25 cm3 of alkali into a conical flask

  2. Add indicator

  3. Fill burette with acid

  4. Slowly add acid until indicator changes colour

  5. Record volume of acid added

  6. Repeat without indicator
• Insoluble salt


  Made when solutions of two soluble salts react in a precipitation reaction
• Preparing insoluble salts


  1. Mix the two soluble salt solutions

  2. Filter the insoluble precipitate

  3. Wash and dry the precipitate
• Concentration


  The number of moles per dm3
• Titration calculation (1:1 mole ratio)


  1. Convert volumes to dm3

  2. Calculate moles of known substance

  3. Use mole ratio to calculate unknown concentration
• Titration calculation (2:1 mole ratio)


  1. Convert volumes to dm3

  2. Calculate moles of known substance

  3. Use mole ratio to calculate unknown concentration
• Key terms


  • excess
  • solid
  • acid
  • salt
  • solution
  • unreacted
  • crystals
  • burette
  • conical
  • flask
  • moles
  • concentration (mol/dm3)
  • volume (dm3)