Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
4. INORGANIC CHEMISTRY AND THE PERIODIC TABLE
4A: THE ELEMENTS OF GROUPS 1 AND 2
1. understand reasons for the trend in ionisation energy down Group 2
Ionisation energy decreases down the group for Group 2 metals.
- Despite the increase in number of protons, as the group is descended the
atomic radius increases and the outermost electron experiences a greater
shielding effect.
- The electrostatic force of attraction between the outermost electron and the
nucleus decreases.
- Therefore less energy is needed to remove one electron from each atom in a
mole of gaseous atoms = ionisation energy decreases down the group.
2. understand reasons for the trend in reactivity of the Group 2
elements down the group
Reactivity of the Group 2 elements increases down the group.
- Ionisation energy decreases as you descend the group (see 1).
- The sum of the first ionisation energy and the second ionisation energy [IE 1 +
IE2] is greater for atoms further down the group.
- Less energy is needed to remove the electrons as you descend the group.
- Therefore reactivity increases down the group.
3. know the reactions of the elements Mg to Ba in Group 2 with oxygen,
chlorine and water
Reactions of Group 2 elements with oxygen:
- Group 2 metals react with oxygen to produce metal oxides.
- Magnesium burns with an intense white flame.
o 2 Mg (s) + O2 (g) 2 MgO (s)
- The others react very similarly.
o 2 Ca (s) + O2 (g) 2 CaO (s)
o 2 Sr (s) + O2 (g) 2 SrO (s)
o 2 Ba (s) + O2 (g) 2 BaO (s)
- Strontium and barium can also react with oxygen to form peroxides.
o Sr (s) + O2 (g) SrO2 (s)
o Ba (s) + O2 (g) BaO2 (s)
- Strontium forms strontium peroxide if it is heated in oxygen under high
pressure.
,- Barium forms barium peroxide just on normal heating in oxygen. Mixtures of
barium oxide and barium peroxide will be produced.
- The peroxide ion is O22-.
Reactions of Group 2 elements with chlorine:
- All Group 2 metals react with chlorine to form metal chlorides, MCl 2, which are
white in colour.
o Mg (s) + Cl2 (g) MgCl2 (s)
o Ca (s) + Cl2 (g) CaCl2 (s)
o Sr (s) + Cl2 (g) SrCl2 (s)
o Ba (s) + Cl2 (g) BaCl2 (s)
Reactions of Group 2 elements with water and steam:
- Beryllium reacts with steam at high temperatures to give white beryllium
oxide and hydrogen.
o Be (s) + H2O (g) H2 (g) + BeO (s)
o Beryllium is very unreactive (see 2).
o Furthermore, beryllium has an insoluble, resistant layer of oxide on its
surface which lowers its reactivity at ordinary temperatures. The oxide
layer breaks up above 750oC and exposes the beryllium metal surface
below it, and so the protection then fails.
- Magnesium burns in steam to produce white magnesium oxide and hydrogen
gas.
o Mg (s) + H2O (g) H2 (g) + MgO (s)
- Very clean magnesium ribbon has a very slight reaction with cold water.
o Mg (s) + 2 H2O (l) H2 (g) + Mg(OH)2 (s)
o However, the reaction quickly stops because the magnesium hydroxide
formed is almost insoluble in water and forms a barrier on the
magnesium preventing further reaction.
- Calcium, strontium and barium react with cold water with increasing vigour
(see 2) to give the metal hydroxide and hydrogen.
o Ca (s) + 2 H2O (l) H2 (g) + Ca(OH)2 (aq/s)
o Sr (s) + 2 H2O (l) H2 (g) + Sr(OH)2 (aq)
o Ba (s) + 2 H2O (l) H2 (g) + Ba(OH)2 (aq)
- Group 2 metal hydroxides become more soluble in water as the group is
descended. Calcium hydroxide is slightly soluble in water.
- Test for H2: a lit splint placed into a jar or the gas should go out with a
squeaky pop.
- The solution formed (for Ca, Sr and Ba) should be alkaline due to the OH - ions
produced.
- A white precipitate, Ca(OH)2, should form for the reaction of Ca with water.
- This reaction can be classified as a redox reaction.
o The oxidation number of hydrogen decreases from +1 to 0 and the
oxidation number of the metal increases from 0 to +2.
o The hydrogen is reduced and the metal is oxidised simultaneously.
4. know the reactions of the oxides of Group 2 elements with water and
dilute acid, and their hydroxides with dilute acid
, Reactions of Group 2 metals with a base:
- Acid + base a salt + water
o E.g. Mg (s) + 2 NaOH (aq) Mg(OH)2 (s) + H2O (l)
Reactions of Group 2 metal oxides with water:
- The Group 2 metal oxides react with water to form the metal hydroxide.
o E.g. MgO (s) + H2O (l) Mg(OH)2 (s)
o E.g. CaO (s) + H2O (l) Ca(OH)2 (aq)
o E.g. SrO (s) + H2O (l) Sr(OH)2 (aq)
o E.g. BaO (s) + H2O (l) Ba(OH)2 (aq)
Reactions of Group 2 metal oxides with dilute acid:
- The Group 2 metal oxides react with dilute acid to form a salt and hydrogen.
- All Group 2 metals react with dilute hydrochloric acid to form bubbles of
hydrogen and a colourless solution of the metal chloride.
o X (s) + 2 HCl (aq) XCl2 (aq) + H2 (g)
- The Group 2 metals react with dilute sulphuric acid to form hydrogen and
either a colourless solution (for Be and Mg) or a white precipitate (Ca, Sr, Ba).
o Mg (s) + H2SO4 (aq) MgSO4 (aq) + H2 (g)
o Ca (s) + H2SO4 (aq) CaSO4 (s) + H2 (g)
- The Group 2 metals react with dilute nitric acid to form hydrogen and a
colourless solution.
o Mg (s) + 2 HNO3 (aq) Mg(NO3)2 (aq) + H2 (g)
Reactions of Group 2 metal hydroxides with dilute acid:
- Group 2 metal hydroxides react with dilute acids to form water and a salt.
o Mg(OH)2 (s) + 2 HNO3 (aq) Mg(NO3)2 (aq) + H2O (l)
o Sr(OH)2 (s) + 2 H2SO4 (aq) SrSO4 (aq) + 2 H2O (l)
o Ba(OH)2 (s) + 2 HCl (aq) BaCl2 (aq) + H2O (l)
5. know the trends in solubility of the hydroxides and sulfates of Group
2 elements
Group 2 hydroxides become more soluble as the group is descended and Group
2 sulfates get less soluble as the group is descended.
- As a general rule, the greater the difference in size between the cation and
the anion, the more soluble the compound is in water.
o Compounds tend to be insoluble if both of its ions are small or if both of
its ions are big.
- OH- is a relatively small anion and SO42- is a relatively large anion.
o As the group is descended, the size of the metal cation increases.
o Therefore the difference in size between the metal cation and OH -
increases down the group, and the difference between the metal cation
and SO42- decreases down the group.
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