Chemical Changes

• Titration Practical


  1. Use the pipette to add 25 cm3 of alkali to a clean conical flask

  2. Add a few drops of indicator and put the conical flask on a white tile

  3. Fill the burette with acid and note the starting volume

  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix

  5. Stop adding the acid when the end-point is reached (this is when the acid has neutralised the alkali and the indicator changes colour)

  6. Note the final volume reading, and calculate how much acid you added in total

  7. Repeat the titration until you get 'concordant results', which means volumes of acid that are within 0.10 cm3 of each other

  8. Use the concordant results to calculate the mean volume of acid required to neutralise the alkali
• Neutralisation reaction

  Part 1 - Obtain a solution of the soluble salt that we want

  1. Place dilute acid in a beaker and heat gently.
  2. Add the solid base bit by bit until it stops reacting, which means it's in excess.
  3. Isolate the salt solution by filtering out the excess solid base using filter paper and a funnel.

  Part 2 - Isolate the soluble salt crystals from the solution 

  1. Heat the salt solution gently in a water bath until crystals start to form.
  2. Let the solution cool further, which will cause more crystals to precipitate.
  3. Filter out the soluble salt crystals using filter paper and funnel
• Oxidation


  When a substance gains oxygen
• Reduction


  When a substance loses oxygen
• Reactivity series of metals

  • Shows the metals in order of their reactivity
  • Metals above H2 in reactivity series react with acid to produce H2. The more reactive the metal is, the quicker and more violent reaction with acid occurs.
  • Metals below H2 don't react with acids.
  • Not all metals above H2 react with water - mostly Group I and II metals. Aluminium is the borderline case.
• Displacement reaction


  A reaction where a more reactive metal displaces a less reactive metal from a compound
• Unreactive metals are found in their natural state in Earth
• Extracting metals less reactive than carbon

  Reduction with carbon. Carbon displaces the metal in a metal oxide - gets oxidised to carbon oxides. Metal from the metal oxide gets reduced to the pure metal.
• Extracting metals more reactive than carbon


  By electrolysis
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Reaction between metals and acids

  Metal + acid → salt + hydrogen
• Type of reaction between metals and acids

  Redox reaction, also a displacement reaction
• Metals above hydrogen in the reactivity series will react with acid
• Neutralisation reaction


  Base + acid → salt + water
• Reaction between metal carbonate and acid

  Metal carbonate + acid → salt + water + carbon dioxide
• Reaction between metal oxides and acids


  Metal oxide + acid → a salt + water
• Redox reaction


  A reaction where both oxidation and reduction occurs
• When magnesium reacts with hydrochloric acid, magnesium has been oxidised (Mg to Mg2+) and the hydrogen in HCl has been reduced (H+ to H2)
• Forming a soluble salt


  React the excess acid with some insoluble chemical (e.g. metal oxide)

  1. Filter off the leftovers
  2. Crystallise the product
• Acids


  Produce hydrogen ions in aqueous solutions
• Alkalis


  Produce hydroxide ions in aqueous solutions
• Bases


  Compounds that neutralise acids
• Alkalis


  Soluble bases - produce hydroxide ions in aqueous solutions
• pH scale


  Measure of acidity/alkalinity of a solution; neutral solution has pH 7
• Neutralisation reaction (ionic form)

  H+ + OH− → H2O
• Strong acid


  Completely ionised in aqueous solution
• Weak acid


  Only partially ionised in aqueous solution
• As concentration of H+ increases


  pH decreases
• pH of 7


  Measure of acidity/alkalinity of a solution; neutral solution
• Neutralisation reaction


  H+ + OH− → H2O
• Concentrated acid


  Has more moles of acid per unit volume than dilute
• Dilute acid


  Solution of low concentration
• Concentration is not the same as strength of an acid
• As pH is decreased by one unit


  Hydrogen ion concentration increases by a factor of 10
• Electrolysis


  Passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged (they lose/gain electrons) at electrodes to produce these
• Electrolyte


  The liquid/solution which conducts electricity
• Cathode


  Negative electrode
• Anode


  Positive electrode
• Electrolysis


  1. Reduction occurs at the cathode

  2. Oxidation occurs at the anode