Chemical Ideas 11'2

1
Chemical Ideas 11.2

• The s block Groups 1 2

2
The s block Groups 1 2

• The s block contains two groups of reactive
  metals.
• Group 1 metals (alkali metals)
• Group 2 metals (alkali earth metals)
• Elements become more metallic as you go down a
  group for this reason most reactive metals are
  found at the bottom of the groups
• Elements become less metallic as you go across a
  period from left to right for this reason Gp 1
  metals are more reactive than Gp 2 metals in the
  same period

3
Physical properties

• S block are only really typical metals in there
  chemical properties
• Metals are not widely used in element form as the
  more familiar d block metals
• Tend to be
• Soft
• Weak
• Low melting points
• Too reactive with water and oxygen to have many
  uses
• Compounds of s-block elements are very important

4
Chemical reactions

• Group 1 2 metals are all v. reactive
• Never found in nature in there native, uncombined
  state
• Compounds of s block metals are very common
  throughout nature
• A lot of the ground beneath our feet in made from
  compounds of Ca and Mg!

5
Chemical reactions

• Group 1 2 show patterns of reactivity as you go
  down the group
• There are similarities between the reaction of
  the elements in the group, also differences these
  difference show up in patterns or trends
• The similarities are because elements in the same
  group have the same number of es in the outer
  shell
• Differences are because as you go down a group
  the size of the atom increases.

6
Some chemical properties of Group 2 elements and
their compounds

• Magnesium, calcium, Strontium and Barium are most
  well known
• Elements are all reactive
• Form compounds containing ions with a 2 charge
  such as Mg2 and Ca2

7
Reactions of group 2 elements with water

• All react with water to form hydroxides and
  hydrogen
• Steady increase in reactivity as you go down
  group
• Mg reacts slowly even when water is heated
• Ba reacts rapidly
• None react as vigorously as Gp 1 metals
• General equation
• M(s) 2H20 (l) ? M(OH)2 (aq) H2 (g)

8
Oxides and hydroxides

• General formula of the oxides is MO
• General formula of the hydroxides is M(OH)2
• In water, oxides and hydroxides form alkaline
  solutions (although not very soluble)
• Typical of metals, unlike non-metals whose oxides
  form acidic solutions
• Strongest alkaline solutions are formed by
  elements at bottom of group
• Oxides and hydroxides react with acids to form
  salts
• MO (s) 2HCl (aq) ? MCl2 (aq) H20 (l)
• M(OH)2 (s) H2SO4(aq) ? MSO 4(aq) 2H20
  (l)
• This neutralising effect is used by farmers when
  they put lime (calcium hydroxide) on their fields
  to neutralise soil acidity

9
Effect of heating carbonates

• The general formula of Gp 2 carbonates is MCO3
• Heating carbonates makes them decompose, forming
  the oxide and releasing carbon dioxide MCO3
  (s) ? MO (s) CO 2(g)
• Carbonates more difficult to decompose the
  further you go down the group
• We say the thermal stability of calcium carbonate
  is greater than that of magnesium carbonate
• Decomposition of calcium carbonate (limestone) is
  an important process used to manufacture calcium
  oxide (quicklime)

10
Solubility of compounds

• Solubility of hydroxides
• More soluble as you go down the group
• Pattern is repeated for most Gp 2 compounds where
  the negative ion has a single charge (1-)
• Solubility of carbonates
• Less soluble as you go down the group
• Pattern is repeated for most Gp 2 compounds where
  the negative ion has a double charge (2-)

11
Solubility of compounds

• Numbers in table dont show a perfect trend
• This rule is generally correct but and is useful
  in making predictions
• As in all science the predictions cant be
  confirmed until you or someone else confirms it
  through experiments