Topic 4 - Inorganic Chemistry and the Periodic Table

• Group 2 elements become more reactive moving down the group
• When magnesium is combined with steam, it rapidly reacts to form magnesium oxide and hydrogen
• Group 2 oxides increase is in solubility going down the group
• To test for sulfate ions, you can add a solution containing barium ions . Any sulfate ions will react with barium to form a white precipitate
• Group 2 nitrates and carbonates decompose under heat while group 1 chlorides do not because:

  • The charge on a group 2 cation is 2+ while group 1 is only 1+
  • The ionic radius of a group 2 cation is smaller that a group 1 cation
  • Nitrate and carbonate anions are more complex than chlorides
• Magnesium slowly reacts with oxygen, forming a thin layer of magnesium oxide
• A reason for rate of reaction not being what expected is that magnesium may have a thin layer of magnesium oxide
• Magnesium reacts with cold water to form magnesium hydroxide
• Group 2 metals with react with cold water, increasing vigorous going down the group, to form metal hydroxides
• Group 2 hydroxides make the water alkaline
• Observations of group 2 metals in cold water:

  • Fizzing
  • The metal dissolving
  • The solution heating up
  • A white precipitate for calcium only
• Group 2 metal hydroxides act as a Bronsted-Lowry base
• Mg(OH)2 is only slightly soluble so less OH- ions will be disassociated, making it a weaker base as opposed to Ca(OH)2
• Group 2 hydroxides become more soluble moving down the group
• When not soluble, all group 2 hydroxides appear as white precipitates
• Magnesium hydroxide is used in medicine to neutralise stomach acid (milk of magnesia)
• Calcium hydroxide is used in agriculture to neutralise acidic soils
• Aqueous calcium hydroxide is limewater
• To test for carbon dioxide, bubble through limewater. If it turns cloudy, carbon dioxide is present
• Barium hydroxide is very soluble and is a very strong base
• Group 2 sulfates become less soluble moving down the group
• Barium reacts slowly with sulfuric acid because the insoluble barium sulfate will cover the surface of the metal
• Thermal decomposition is the use of heat to break up a compound into more than one product
• Group 2 carbonate become more thermally stable moving down the group
• Smaller group 2 cations have a higher charge density, they have a polarising effect and distort the carbonate ion which weakens the C-O bond
• Group 1 carbonates other than lithium do not decompose as the charge density is not high enough
• Group 2 carbonate thermal decomposition produces carbon dioxide and group 2 oxides
• Group 2 nitrates thermally decompose to produce group 2 oxides, oxygen, and nitrogen dioxide
• Nitrogen dioxide is a brown gas that's toxic, so thermal decomposition of group 2 nitrates must be done in a fume cupboard
• Thermal stability of group 2 nitrates increases going down the group, smaller group 2 nitrates have a higher charge density so they have a polarising effect and distort the nitrate ion, weakening the N-O bond
• Group 1 nitrates (except for lithium) decompose to give nitrate(III) salt and oxygen
• Flame test method:

  1. Dip the nichrome wire in HCl and pass through a roaring flame to clean
  2. Dip the nichrome wire in the metal and place in the flame
  3. Observe the colour
• Lithium gives a scarlett red flame
• Sodium gives a yellow flame
• Potassium gives a lilac flame
• Rubidium gives a red flame]
• Magnesium gives no flame colour
• Calcium gives a brick red flame
• Strontium gives a red flame
• Barium gives an apple green flame