Group 2 and Titrations

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Oxidation and Reduction

  • Oxidation is the addition of oxygen, or loss of hydrogen or electrons.
  • Reduction is the loss of oxygen, or addition of hydrogen or electrons.
  • An oxidation number tells us how oxidised an element is in a compound. 
  • When an element's oxidation number increases, it is oxidation.  When an element's oxidation number has decreased, it is reduction.
  • An oxidising agent has a high oxidation number and can oxidise other atoms, while itself being reduced. 
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Oxidation and Reduction (cont)

  • Reduction and oxidation always take place together in a reaction.
  • These reactions are called redox reactions, where at least one element changes its oxidation number).
  • We break redox reactions into 2 half equations to represent oxidation and reduction. 
  • Remember, group 1 and group 2 elements always have +1 and +2 oxidation numbers respectively. 
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More Redox Reactions

  • A reaction where one of the reactants is both oxidised and reduced is called disproportionation.
  • The oxidation number of the reactant will have increased in one product and decreased in another. 

                                  (http://library.thinkquest.org/C006669/media/Chem/img/disprop.gif)

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More Redox Reactions (cont)

  • You need to be able to balance redox equations using oxidation numbers. 
  • The only way to show this is through example, go to page 79 is AS revision guide. 
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Properties and Reactions of Group 2 Elements and C

  • First ionization energy decreases down group 2.
  • Atomic radius increases down the group as additional electrons fill higher energy levels. So, the distance between the +ve nucleus and the -ve electrons increases and the force of attraction is less.
  • Shielding of the nucleus increases. There are more inner shell electrons which shield the outer electrons from the nucleus, further reducing force of attraction.
  • These two factors outweigh the increase in nuclear charge down a group. 
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Properties and Reactions of Group 2 Elements and C

  • Reactivity increases down group 2, because ionisation energy decreases. 
  • All group 2 solids burn in oxygen to form solid metal oxides. This is the same for burning in chlorine, to form solid metal chlorides.
  • Group 2 solids from calcium and below react with water to form hydrogen and a metal hydroxide (aq).
  • Magnesium reacts very slowly with liquid water, but reacts vigorously with steam to produce insoluble magnesium oxide. 
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Properties and Reactions of Group 2 Elements and C

  • Solid group 2 oxides from calcium and below react with water to form a metal hydroxide  (no hydrogen).
  • All solid group 2 metal oxides neutralise hydrochloric or nitric acid to form a salt (aq) and water.
  • This is the same for group 2 metal hydroxides. 
  • Solubility of group 2 hydroxides increases down the group. 
  • Solubility of grouo 2 sulphates decreases down the group. 
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Properties and Reactions of Group 2 Elements and C

  • Thermal stability means the stability of a compound when it is heated. 
  • All group 2 carbonates undergo thermal decomposition to give the metal oxide (solid) and carbon dioxide (gas).
  • Down the group, more heat is required to make the group 2 carbonates decompose. 
  • All group 2 nitrates will undergo thermal decomposition to give solid metal oxide, brown NO2 gas and oxygen gas. Down the group, more heat is required to get group 2 nitrates decompose.
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Properties and Reactions of Group 2 Elements and C

  • So, the thermal stability of group 2 carbonates and nitrates increases down the group.
  • This is because down the group the +ve cation gets larger (ionic radius), and so its polarizing power decreases. 
  • Less polarizing power means the anion is more stable, as its electron cloud is less distorted.
  • So a more stable anion is more difficult to decompose, requiring more heat energy. 
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Properties and Reactions of Group 2 Elements and C

                (http://www.chemguide.co.uk/inorganic/group2/carbonate3.gif)

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Acid-Base Titrations

  • An acid-base titration uses an accurate volume of substance to find the concentration of an acid or alkali solution.
  • The substance of known concentration goes in the burette. The substance of unknown concentration goes in a conical flask- use a pipette to take a volume
  • For strong acids, use methyl orange indicator. It is red in acid and yellow in alkali, but its end point is orange.
  • For weak acids, use phenylphelein. It is colourless in acid and pink in alkali, but its end point is pale pink. 
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Acid-Base Titrations (cont)

  • You record readings to the nearest 0.05cm^3. This is to the limit of precision of the burette.
  • To improve the reliability, the experiment should be repeated until you have 2 concordant titres, within 0.20.
  • Most systematic errors are from poor technique- always read the burette at eye-level. 
  • Measurement uncertanties are caused by random errors, caused by the limits of accuracy of your apparatus. Repeating the experiment improves reliability, but accuracy won't be improved.
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