Salts and Electrolysis

Salts and Electrolysis

1. Acids & Alkalis

   • The pH scale is a measure of how acidic or alkaline a solution is. It ranges from 0 - 14. With 0 being the strongest acid, 14 the strongest alkali and 7 being neutral.
   • A Universal Indicator is a combination of dyes which give colours to show the pH of a solution
   • An ACID is a corrosive substance with a pH less than 7, that forms H+ ions in water.
   • An ALKALI is a soluble base with a pH greater than 7, that forms OH- ions in water.
   • A BASE is any substance with a pH greater than 7, that will therefore neutralise an acid.
   • Neutralisation: Acid + Base -> Salt + Water
   • Can also be written: H+ + OH- -> H20

2. Acids reacting with Metals

   • Acid + Metal -> Salt + Hydrogen
   • The more reactive the metal, the faster the reaction will go, shown by the rate at which the bubbles of hydrogen are given off. Can be tested using a burning splint test. Metals below hydrogen in the reactivity series won't react like copper.
   • The name of the salt produced depends on the combination of metal and acid used. Hydrochloric acid -> Chloride Salts, Sulfuric acid -> Sulfate Salts, Nitric acid -> Nitrate Salts. (when Neutralised.)

3. Bases

   • Metal Oxides and Metal Hydroxides are examples of BASES - will neutralise an acid.
   • Ammonia dissolves in water to give a slightly alkaline solution, when reacted with nitric acid, it forms ammonium nitrate salt.
   • Ammonia (aq) + Nitric Acid (aq) -> Ammonium Nitrate (aq)
   • Ammonium nitrate is a good fertiliser because it has nitrogen from 2 sources (the ammonia & nitric acid) Plants need nitrogen to form proteins.

4. Making Salts

   • Soluble
     • Most Chlorides, Sulfates & Nitrates are soluble (exceptions are: Lead sulfate & chloride, and Silver chloride.)
     • Using a METAL or INSOLUBLE BASE
       • 1) Pick the appropriate acid and metal or base
       • 2) Add them together, the solid will dissolve in acid as it reacts. When the acid is neutralised, excess solid will sink to the bottom.
       • 3) Filter out and crystallise salt solution (evaporate slowly) to get salt crystals.
     • Using an ALKALI
       • 1) Have to add exactly the right amount of alkali to neutralise the acid using an indicator to show when it's finished (green.)
       • 2) Then evaporate water and crystallise salt as normal.
   • Insoluble
     • Most Oxides and Hydroxides are insoluble.
     • Using a PRECIPITATION reaction
       • 1) Pick 2 salt solutions containing the ions you need.
       • 2) Mix them together and once the insoluble salt has precipitated out, you just filter, wash and dry.
       • Precipitation reactions can be used to remove poisonous ions from drinking water and in treating effluent.

5. Electrolysis & Half Equations

   • Electrolysis is the splitting up of an ionic substance in to the elements it's made up of using electricity.
   • It requires an electrolyte (a liquid to conduct the electricity) that contains free ions, which allow the whole thing to work.
   • During electrolysis, positively charged electrons are attracted to the cathode, where they GAIN electrons (Reduction) and negatively charged ions move to the anode where they LOSE electrons (Oxidation) The losing and gaining of electrons at the electrodes is what allows current to flow.
   • The ionic substance must be MOLTEN or in SOLUTION.
   • Sometimes there are more than 2 types of free ions in an electrolyte. If a salt is dissolved in water, they'll be H+ and OH- ions as well.
   • If the metal ion is more reactive than hydrogen, hydrogen is formed at the cathode. If it's less reactive, then the metal ion is formed.
   • At the positive electrode, if OH- and halide ions (Cl-, Br -, I-) are present, then the halidade ion will be formed. If no halide present, then oxygen is formed.
   • Uses of the products formed from electrolysis of sodium chloride, are Hydrogen -> margarine, Chlorine -> Bleach, Plastics, Sodium Hydroxide -> strong alkali used in industry (makes soap)
   • HALF-EQUATIONS show the reactions at the electrodes. They make sure the number of electrons is the same for both half-equations.
   • Half-equations for sodium chloride are: NE = 2H+ + 2e- -> H2 PE = 2Cl- -> 2e- + Cl2

6. Extraction of Aluminium

   • Main ore is Bauxite and after mining and purifying, a white powder is left -> pure aluminium oxide. Aluminium has to then be extracted using electrolysis.
   • Aluminium oxide is dissolved in molten cryolite to lower the melting point from around 2000 degrees to 900. This makes it cheaper and easier.
   • Aluminium Oxide -> Aluminium (-ve electrode) + Oxygen (+ve electrode)
   • Half-equation: NE = Al3+ + 3e- -> Al, PE = 2O2- -> O2 + 4e-

7. Electoplating

   • ELECTROPLATING is the coating of one metal with another metal using electrolysis.
   • Negative electrode is the metal you want to plate, the Positive electrode is the metal you want to plate it with. You also need an electrolyte containing ions of the plating metal.
   • Used for DECORATION - cheaper but attractive and for CONDUCTION - metals like copper plate other metals for electrical circuits and computers.

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