MOD 6: ACID-BASE REACTIONS

• Properties of acids


  • Sour tasting
  • Sting/burn
  • Solutions are conductive
  • Turn blue litmus to red
• Properties of bases


  • Soapy/slippery feel
  • Bitter taste
  • Solutions are conductive
  • Turn red litmus blue
• Alkalis


  Soluble bases
• Acid


  A proton (H+) donor. An acid will dissociate and give up its H+
• Base


  A proton (H+) acceptor
• Low pH


  Lots of H+
• Lots of H+


  High pH
• Inorganic acid


  An acid that does not contain carbon
• Bases containing O or OH


  Named using the rules for naming ionic compounds
• Red cabbage solution


  Acts as an indicator which contains a chemical called anthocyanin
• Anthocyanin


  A natural acid-base indicator
• Properties of acid-base indicators

  • Change colour depending on pH
  • Display a constant colour in acids or bases
  • Colour change occurs over a small pH range
• Colours of common indicators

  • Methyl orange (Acid: Red, Neutral: Yellow, Base: Yellow)
  • Methyl red (Acid: Red, Neutral: Yellow, Base: Yellow)
  • Bromothymol blue (Acid: Yellow, Neutral: Blue, Base: Blue)
  • Litmus (Acid: Red, Neutral: Blue, Base: Blue)
  • Phenolphthalein (Acid: Clear, Neutral: Clear, Base: Pink)
  • Indigo Carmine (Acid: Blue, Neutral: Blue, Base: Yellow)
  • Universal indicator (Acid: Red, Neutral: Green, Base: Purple)
  • Thymol blue (Acid: Pink, Neutral: Yellow, Base: Blue)
  • Phenol red (Acid: Yellow, Neutral: Pink, Base: Pink)
• Universal indicator


  A mixture of several indicators (phenolphthalein, bromothymol blue, methyl red and thymol blue)
• Uses of indicators


  • At home (Aquarium water testing, Pool water testing, Soil testing)
  • In industry (To determine when a reaction is complete, Monitoring waste solutions for disposal)
• Reversibility of pH indicators

  • Colour change caused by the dissociation of the H+ ion
  • Indicators are weak acids
  • Dissociation of the indicator causes the solution to change colour
• Indicators in acidic solutions

  • Surplus of H+ ions, meaning there will be a lot of HIn molecules (red colour)
• Indicators in basic solutions

  • Small amount of H+ ions, meaning there will be a lot of In- molecules (yellow colour)
• Acid-base reactions (neutralisation)


  Acid + base → salt + water
• Acid-base reactions


  • HCL(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  • H2SO4(aq) + MgO(s) → MgSO4(aq) + H2O
• Acid-carbonate reactions


  Acid + carbonate → salt + carbon dioxide + water
• Acid-carbonate reactions


  • 2HCL(aq) + CaCO3(s) → CaCl2(s) + CO2(g) + H2O(l)
  • H2SO4(aq) + CaCO3(aq) → CaSO4(aq) + CO2(g) + H2O(l)
  • HCL(aq) + NaHCO3(aq) → NaCl(aq) + CO2(g) + H2O
  • H2SO4(aq) + 2NaHCO3(s) → Na2SO4(aq) + 2CO2(g) + 2H2O(l)
• Acid-metal reactions


  Acid + metal → salt + hydrogen
• Acid-metal reactions


  • 2HCL(aq) + Mg(s) → MgCl2(aq) + H2(g)
  • H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g)
  • Net ionic: 2H+(aq) + Mg(s) → Mg2+(aq) + H2(g)
• Neutralising excess stomach acidity e.g., acid reflux treated with antacid tablets
• Stomach


  Produces HCL to aid with the digestion of food and to kill harmful micro-organsims
• Excess stomach acidity


  Results in either heartburn or indigestion
• Antacid


  Contains either magnesium hydroxide or aluminium hydroxide which neutralises stomach acid
• Adjusting and managing the pH of wastewater before it goes in to lakes
• Wastewater processes


  Either acidic or alkaline, causes signficant environmental damage if water enters creeks and lakes untreated
• Methods of neutralising acidity in wastewater

  • Supplementing wastewater with either sodium hydroxide or sodium carbonate
  • Passing acidic water through a calcium carbonate (limestone) bed
• Adjusting soil acidity for optimal plant growth → hydrangeas
• Hydrangea flowers


  Colour changes depending on whether the soil is either acidic or alkaline (basic)
• Hydrangea flowers in acidic soil

  Produce blue flowers
• Hydrangea flowers in alkaline (basic) or neutral soil

  Produce pink flowers
• Enthalpy of neutralisation


  Any neutralisation reaction will always form the net ionic equation H+(aq) + OH-(aq) → H2O
• Enthalpy


  The total energy possessed by a chemical substance
• Enthalpy of neutralisation


  The enthalpy change when solutions of an acid or alkali react together under standard conditions
• Arrhenius' theory


  An acid was a subsance which produced hydrogen ions in an aqueous solution
• Limitations of Arrhenius' theory

  • Can only be applied to aqueous solutions
  • His definiton could not explain why ammonia is a base
  • Describes the behaviour of a base as they produce hydroxide ions as the only negative ion in an aqueous solution