C7: Acids, bases, salts

  • Acids: proton donors

  • Bases: proton acceptors

  • Indicators: substances that change color if they are put into an acid or alkaline solution.

    • Litmus and methyl orange:

    • Universal indicator: mixture of indication dyes

  • pH scale

    • Acids: pH < 7, more acidic → lower pH

    • Neutral substances: pH 7

    • Bases: pH > 7, more basic → higher pH

  • Bases and neutralization

    • Acids are a source of hydrogen ions, H+

    • Bases (or alkalis) are a sources of  hydroxide ions, OH– 

      • they react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water

    • acid + base → salt + water (neutralization)

      • net ionic equation: H+  (aq) + OH– (aq)⟶ H2O (l)

    • Most bases are insoluble in water

    • Alkalis: bases that are soluble in water

  • Characteristic reactions of acids

    • metal + acid → salt + hydrogen

      • Hydrochloric acid → chloride salt

      • Nitric acid → nitrate salt

      • Sulfuric acid → sulfate salt

    • Acid + base → salt + water

    • Acid + metal carbonate → salt + water + carbon dioxide

  1. Oxides

  • Compounds made from one or more atoms of oxygen combined with another element

  • Acidic oxides:

    • Non-metal + oxygen

    • acid oxide + base → salt + water

    • Produce an acidic solution when dissolved in water

    • Eg. CO2, SO2

  • Basic oxides:

    • Metal + oxygen

    • basic oxide + acid → salt + water

    • Produce a basic solution when dissolved in water

    • Eg. CuO, CaO

  • Amphoteric oxides:

    • Can react with both bases/acids → salt + water

    • Eg. ZnO, Al2O3

  • Neutral oxides:

    • Oxides that do not react with acids/bases

    • Eg. N2O, NO, CO

  1. Preparation of salts

  • Solubility of salts

    • All common sodium, potassium, ammonium salts are soluble

    • all nitrates are soluble

    • most chlorides and sulfates are soluble

    • all carbonates are insoluble

  • Preparing soluble salts:

    • Method A: Acid + solid metal/base/carbonate

      • Warm acid, add excess solid metal/base/carbonate until no more reaction is shown (no longer turns blue litmus paper red for base)

      • Filter mixture to get rid of excess into evaporating dish

      • Gently evaporate to concentrate salt solution (water bath/sand tray)

      • Let crystals from as solution cools → filter, wash, dry

    • Method B: Acid + alkali by titration (neutralization)

      • Acid is poured into a burette. A known volume of alkali is placed in a conical flask using a pipette.

      • A few drops of indicator (methyl orange/thymolphthalein) is added to the flask

      • Acid solution is slowly run into the flask until indicator just changes color

      • Swirl conical flask gently to ensure everything is mixed

      • Activated charcoal can be added to remove indicator and filtered out later

  • Anhydrous: dehydrated salt/powder

  • Preparing insoluble salts: precipitation reaction

    • Dissolve soluble salts in water and mix together using a stirring rod in a beaker

    • Filter to remove precipitate from mixture

    • Wash the residue with distilled water to remove traces of other solutions

    • Leave in an oven to dry