Chemical Changes

• Electrolysis definition


  Using electricity to split up a compound into its elements.
• What is an electrolyte?


  a liquid containing free-moving ions which conducts electricity
• Names of positive and negative electrodes

  Positive-anode

  Negative-cathode
• the process of electrolysis

  When an ionic compound is molten or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes.

  
  

  Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are either reduced or oxidised at the electrodes producing elements. This process is called electrolysis.
• Electrolysis of aluminium oxide

  1. Aluminium is extracted from the ore bauxite by electrolysis. Bauxite contains aluminium oxide

  2. Aluminium oxide has a very high melting point and is therefore mixed with cryolite to lower it.

  3. The molten mixture contains free ions so it will conduct electricity

  4. The positive aluminium ions are attracted to the negative electrode where they each gain three electrons and become neutral aluminium atoms and sink to the bottom of the electrolysis tank.

  5. The negative oxygen ions are attracted to the positive electrode where they lose two electrons and become neutral oxygen atoms which combine forming o2 molecules.
• Electrolysis of Aqueous Solutions


  At the cathode Metal ions more reactive than hydrogen will stay in the aqueous solution as the hydrogen (from the water) is produced.

  If they are less reactive , a solid layer of the pure metal will be formed.

  
  

  At the anode if hydroxide and halide ions are present , molecules or chlorine, bromine or iodine will be formed. If no halide ions are present, then the hydroxide ions are discharged and oxygen will be formed.
• Half equation


  A equation that shows the reactions at the electrodes
• pH scale


  Measure of how acidic or alkaline a solution is
• Acidic pH


  less than 7
• Alkaline pH


  more than 7
• How can we measure ph?

  Using a pH probe or universal indicator
• What do acids form in water?

  H+ ions
• What do alkalis form in water?

  OH- ions
• Neutralisation reaction


  acid + base = salt + water
• strong acid


  Ionise completely in water and dissociates to release h+ ions
• Weak acids


  Do not fully ionise in solution and only a small proportions of acid particles dissociate to release h+ ions
• Strong acids are more or less reactive than weak


  More reactive
• What is Ph


  a measure of the hydrogen ion concentration of a solution
• Rule for ph concentration


  Factor H+ ion concentration changes by =10^-x
• What does acid concentration mean?

  How much acid there is in a certain volume of water
• Reactions between acids and metal oxide


  Salt + water
• Reactions of acid and metal hydroxide

  Salt+water
• Reaction of metal carbonate and acid

  Salt + water + carbon dioxide
• How to make soluble salts using an insoluble base

  Gently warm the dilute acid using a Bunsen burner then turn off the Bunsen burner, then add the insoluble base a bit at a time until no more reacts with the acid. It will be neutralised when excess solid sinks to the bottom of the flask, then filter out excess solid to get salt solution and to get crystals gently heat the solution using a water bath and leave it to cool.
• What is the reactivity series?

  lists metals in order of reactivity towards other substances

  the higher a metal is the more reactive it is (more easily it form positive ions)
• The reactivity series


  Potassium,sodium,lithium,calcium,magnesium,carbon,zinc,iron,hydrogen,copper
• Reaction of meal and acid

  Salt+ hydrogen
• The more reactive a metal is...


  the faster the reaction will go
• reaction of magnesium with acid

  Reacts vigorously and produces lots of bubbles
• Reaction of zinc and acid

  Slow reaction but reacts stronger as you heat them up
• Reaction of iron with acid

  Slow reaction similar to zinc
• Reaction of Metal with Water

  metal + water -> metal hydroxide + hydrogen
• oxidation (reduction reaction)


  Gain of oxygen
• Reduction (reduction reaction)


  Loss of oxygen
• Which metals can be extracted from their ore by reduction with carbon?

  metals that are less reactive than carbon eg zinc, iron, tin, lead
• Which metals can be extracted using electrolysis?

  Metals higher than carbon in the reactivity series as they are more reactive
• oxidation (redox reactions)


  loss of electrons
• Reduction (redox reaction)


  gain of electrons
• Rule for displacement reactions

  A more reactive metal will displace a less reactive metal from its compound
• ionic equation


  Show only the particles that react and the products they form