GCSE C2 Chapter 5

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  • Created by: emma998
  • Created on: 10-03-14 16:49

Acid and alkalis

  • When something is dissolved in water it is in aqueous solution (aq)
  • Pure water has a pH value of 7 meaning it is neutral
  • Acids are substances which produce hydrogen H+ ions when added to water
  • They have a pH value of less than 7
  • Bases react with acids and neutralise them
  • Alkalis are bases that disslove in water to produce alkaline solutions
  • Produce OH- ions
  • Alkalis have a pH value between 7 and 14
  • The pH scale has values between 0 and 14, 0-2 being strong acids and 12-14 being strong alkalis
  • Neutralisation reactions are not visibly detected and therefore indicators are added to show the acidity or alkalinity of substances. They have different colours in acidic conditions to alkaline conditions
  • Universal indicators have different colours at different pH values
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Making salts from metals or bases

  • A salt can be made by reacting a metal or a base with an acid
  • Metal +    acid    =   salt     + hydrogen
  •   Zn   +  H2SO4  =  ZnSO4 +     H2
  • Base +  acid   =    salt    + water
  • MgO +  2HCl  =  MgCl2  +  H2O
  • When a base reacts with an acid a neutralisation reaction occurs
  • A base is a metal compound
  • Making salts from metal or bases that are insoluble in water:
    • A metal or base that is insoluble in water can be added slowly a little bit at a time to the acid until the acid has reacted completley
    • The mixture is then filtered to remove the excess solid reactant leaving a solution of the salt
    • The water is evaporated from the solution
    • Leaving crystals of the salt (crystalisation)
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Making salts from solutions

  • Soluble salts can be made by reacting an acid and an alkali
  • Acid + Alkali = Salt + water
  • H+ + OH- = H2O
  • A solid salt can be obtained from a soluble salt by crystalisation
  • Ammonia solution is an alkali that does not contain a metal
  • It reacts with acids to produce ammonium salts such as ammonium nitrate NH4NO3. 
  • Amonium salts are used as fertilisers
  • Insoluble salts can made by mixing solutions of soluble salts containing ions needed
  • Lead iodide can be made by mixing solutions of lead nitrate and potassium iodide
  • The solutions react and displacement occurs producing lead iodide
  • The lead iodide forms a precipitate 
  • The precipitate is filtered from the solution, washed with distilled water and then dried
  • Pb(NO3)2 (aq)  +  2KI (aq)  =  PbI (s)  + 2KNO3 (aq)
  • Some pollutants in water can be removed in this way, precipitation. 
  • A solution containing specific ions is added to the water
  • A reaction occurs between the ions added and the pollutant forming a precipitate which is filtered from the water.
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Electrolysis

  • Electrolysis is the process of using electricity to break down an ionic compound into elements
  • The substance which is broken down is called the electrolyte
  • The molten electrolyte is placed in a container
  • There is the negative electrode and the positive electrode made of inert substances(don't react) that make contact with the electrolyte
  • When electricity is passed through the electrolyte electrolysis takes place
  • The ions in the electrolyte move to the electrode where they become atoms
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Changes at the elctrodes

  • Positive ions move to the negative electrode (cathode) where they are reduced (gain electrons) to become uncharged metallic atoms
  • Negative ions move to the positive electrode (annode) where they are oxidised (lose electrons) to become uncharged non-metallic atoms - some atoms then form simple molecules such as O2
  • Half equations can represent changes at electrodes
  • Lead bromine - PbBr2
  • At the negative electrode: Pb2+ + 2e- = Pb
  • At the positive elctrode: 2Br- = 2e- + Br2
  • Water contains hydrogen and hydroxide ions
  • When solutions of ions in water are electrolysed:
    • Hydrogen is usually produced at the negative electrode if the solution contains other positive ions of a metal which is more reactive than hydrogen
    • Oxygen is usually produced at the positive electrode unless the solution contains a high concentration of halide ions and then a halogen is produced
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The extraction of aluminium

  • Aluminium is more reactive than carbon so it cannot be extracted from its ore by displacement using carbon
  • It must be extracted from from its ore by electrolysis
  • Aluminium needs to be heated to over 2000C to make it molten
  • Therefore it is mixed with cryolite (an ionic compound) which lowers melting point to 850C
  • Aluminium and oxygen are produced as products of electrolysis, cryolite reamins in the cell as more aluminium oxide is added
  • At the negative electrode aluminium ions are reduced to uncharged atoms by gaining 3 electrons. At the positive electrode oxygen ions are oxidised to uncharged oxygen atoms which join together to form small molecules
  • 2Al2O3 = 4Al + 3O2
  • At the negative elctrode: Al3+(l)  +  3e-  =  Al (l)
  • At the positive electrode: 2O2-  = 4e-  +  O2
  • Electrodes made of carbon which react with the oxygen at high temperatures causing electrodes to gradually burn away meaning they often have to be replaced
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Electrolysis of brine

  • Brine is a solution of sodium chloride which contains hydrogenH+ ions, hydroxideOH- ions, sodium ionsNa+ and chloride ionsCl-
  • Hydrogen is produced at the negative electrode
  • Chlorine is produced at the positive electrode
  • Leaving a solution of sodium hydroxide
  • Sodium hydroxide is a strong alkali and has many uses including: Controlling pH, neutralising acids, making bleach, making paper, making soap
  • Hydrogen has many uses including: Margarine, in hydrochloric acid
  • Chlorine also has many uses including: Treating drinking water and swimming pool water, making bleach and disenfectants, making paper
  • At the negative electrode: 2H+  +  2e-  =  H2
  • At the positive electrode: 2Cl-  =  2e-  +  Cl2
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Electroplating

  • Electroplating uses electrolysis to coat a thin layer of a metal onto an object
  • Can be done to: Make it more attractive, prevent corrosion, make it harder, reduce costs
  • The metal to be plated is used as the negative electrode.
  • The plating metal is the positive electrode.
  • The electrolyte is a solution containing ions of the plating metal
  • At the positive electrode atoms of the plating metal lose electrons to form +ions
  • These ions go into the solution
  • They are attracted to negative elctrode where reduced to form atoms of plating metal
  • These metal atoms are deposited on the negative electrode (object to be plated)
  • Half equations for nickel electroplating:
    • At the positive electrode: Ni(S) = Ni2+(aq) + 2e-
    • At the negative electrode: Ni2+(aq) + 2e- = Ni(S)
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