Inorganic chemistry: Group 2 (II)

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5.3 & 5.4 Group 2
What is the outcome from syllabus?
Candidates should be able to:
(a) describe the reactions of the elements with oxygen and water
(b) describe the behaviour of the oxides with water
(c) describe the thermal decomposition of the nitrates and carbonates
(d) interpret, and make predictions from, the trends in physical and
chemical properties of the elements and their compounds
(e) explain the use of magnesium oxide as a refractory lining material
and calcium carbonate as a building material
(f) describe the use of lime in agriculture
5.3 Group 2
Alkaline metal:
ns2
Be
Mg
Ca
Sr
Ba
Ra
12+
Mg
5.3 Group 2
BERYLLIUM
STRONTIUM
MAGNESIUM
BARIUM
CALCIUM
RADIUM
5.3 Group 2
The Flame Color:
Element
Color
Element
Color
Li
Scarlet
Be
-
Na
Yellow
Mg
-
K
Lilac
Ca
Brick-red
Rb
Red
Sr
Crimson
Cs
Blue
Ba
Apple-green
5.3 Group 2
Increasing electronegativity
Increasing
electronegativity
I
II
III
IV
Li
Be
B
C
Period 2
Na
Mg
Al
Si
Period 3
Diagonal relationships (对角线规则)
The untypically large electronegativities of Period 2 elements
(caused by their small size) mean that they are in some ways
more typical of elements one group to the right than to elements
of their own group.
For example: Be(OH)2 + 2H+ → Be2+ + 2H2O
Be(OH)2 + 2OHˉ→ [Be(OH)4]2ˉ
5.3 Group 2
The physical properties of Group 2:
Electron
Metallic
First +
Tm/K
arrangement radius/nm Second IE/
kJ mol-1
Tb/K
Density
/g cm-3
Mg
Magnisum
[Ne]3s2
0.160
2189
922
1380
1.74
Ca
Calcium
[Ar]4s2
0.197
1735
1112
1757
1.54
Sr
Strontium
[Kr]5s2
0.215
1614
1042
1657
2.60
Ba
Barium
[Xe]6s2
0.224
1468
998
1913
3.51
5.3 Group 2
Magnesium oxide
2Mg (s) + O2 (g)
2MgO (s)
① burns very vigorously
② bright white flame
③ white solid produced
a. in the air
b. in oxygen
5.3 Group 2
.
..¨
O.
¨
.
..¨
¨
O—O .
¨¨
. ..
..O—O
¨
¨¨
oxygen ion
peroxide ion
superoxide ion
2
2
The reason that there are different types of oxides is related to the
sizes of the ions:
O2ˉ > O22ˉ > O2ˉ
If the cation is too small, it is not easy for enough peroxide or
superoxide ions to cluster round it to form a stable crystal
lattice. For example, Lithium can only forms the ‘normal’ oxide.
5.3 Group 2
The closer the anions with cations, the
more stable the ionic compounds crystal.
The ‘normal’ oxide, MO(M2+ + O2ˉ), is formed when the metals
are heated in oxygen. Strontium and Barium also form
peroxides. As the M2+ ions are smaller than the M+ ions in
Group I, peroxides do not form until lower down the group
II than in Group I.
5.3 Group 2
♦ Reaction with water
Mg (s) + 2H2O (l)
Mg(OH)2 (aq) + H2 (g)
slowly
Mg (s) + H2O (g)
MgO (s) + H2 (g)
steam
rapidly
Beryllium does not react directly with water all. The rest of the Group
II metals react with increasing rapidity on descending the group.
5.3 Group 2
♦ Oxide reaction with water
MgO (s) + H2O (l)
Mg(OH)2 (aq)
Partially soluble
In the saturated solution, pH(Mg(OH)2) = 10
The rest of the Group II oxides react with increasing rapidity on
descending the group.
5.3 Group 2
♦ Reaction with acids
Mg (s) + 2HCl (aq)
MgCl2 (aq) + H2 (g)
Mg (s) + H2SO4(aq)
MgSO4 (aq) + H2 (g)
The reaction is more vigorous as we go down the group.
5.3 Group 2
Thermal stability describes how easily or otherwise a compound will
decompose on heating. Increased thermal stability means a higher
temperature is needed to decompose the compound.
Group II
Carbonates,
CO32ˉ
Nitrates,
NO3ˉ
Mg
Ca
Sr
Ba
Mg
Ca
Sr
Ba
MgCO3 → MgO + CO2
Same pattern but higher
temperatures needed for
decomposition
The charge density (Z/r) of
the cations (polarization)
will affect the decomposition
temperature.
M(NO3)2 → MO +
2NO2 + 1/2O2
The larger value of Z/r,
The easier breaking up of
distorted anions:
CO32- → CO2 + O2NO32- → NO2 + O2-
Same pattern but higher
temperatures needed for
decomposition
5.3 Group 2
Which one of the following equations represents the
reaction that occurs when calcium nitrate is heated
strongly?
A. Ca(NO3)2 → Ca(NO2)2 + O2
B. 2Ca(NO3)2 → 2CaO + 4NO2 + O2
√
C. Ca(NO3)2 → CaO + N2O + 2O2
D. 3Ca(NO3)2 → Ca3N2 + 4NO2 + 5O2
E. Ca(NO3)2 → CaO2 + 2NO2
5.3 Group 2
Which one of the following elements is likely to have
an electronegativity similar to that of aluminium?
A. Barium
B. Beryllium diagonal relationship
√
C. Calcium
D. Magnesium
E. Strontium
5.3 Group 2
Which one of the following statements is true?
A. All nitrates of Group II metals are decomposed by heat
to give the oxide NO2
√
B. Aqueous sodium nitrate in acidic to litmus.
C. Aqueous ammonium nitrate is alkaline to litmus.
D. The alkali metal nitrites are insoluble in water.
E.
Metals dissolve in concentrated nitric acid to give
hydrogen.
5.4 Compounds of Group II Elements
coins
Ship
cosmetics
pipe
5.4 Compounds of Group II Elements
Table 1: the melting points of the oxides of the Group II elements.
Oxide
Melting point/℃
MgO
2852
CaO
2614
SrO
2430
BaO
1918
refractory
material
As M2+ cationic size increases down the Group, the ionic bonds
become weaker, hence, less energy is needed to break the bonds
and a low melting point is expected.
5.4 Compounds of Group II Elements
Magnesium oxide is used to line industrial furnaces
because it has a very high melting point. Which type of
bond needs to be broken for magnesium oxide to melt?
A. co-ordinate
B. covalent
√
C. ionic
D. metallic
5.4 Compounds of Group II Elements
CaCO3(limestone)
Δ
+ CO2
Ca(OH)2(slaked lime)
+ H2 O
CaO(lime)
5.4 Compounds of Group II Elements
Ca(OH)2 (s) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)
Acid + Base → Salt + Water
This is a base and is used in agriculture to treat acidic soil.
5.4 Compounds of Group II Elements
A farmer spreads lime on land which has already been
treated with a nitrogenous fertilizer. Which reactions will
occur over a period of time?
1. Ca(OH)2 + CO2 → CaCO3 + H2O
√
2. Ca(OH)2 + 2H+ (aq) → Ca2+(aq) + 2H2O
√
+(aq) → Ca2+(aq) + 2NH + 2H O
3.
Ca(OH)
+
2NH
2
4
3
2
√
5.4 Compounds of Group II Elements
When decomposing in water, organic refuse is oxidised to
form carboxylic acids. The water becomes acidic and
aquatic life is destroyed.
Which additives are suitable to remove this acid pollution?
√
1. calcium carbonate
√2. calcium hydroxide
3. potassium nitrate
5.4 Compounds of Group II Elements
Soft water: Ca2+, Mg2+, HCO32-
“temporary hardness”
Δ
Ca2+ (aq) + 2HCO3ˉ(aq) → CaCO3(s) + CO2(g) + H2O(l)
Δ
Mg2+ (aq) + 2HCO3ˉ(aq) → MgCO3(s) + CO2(g) + H2O(l)
Hard water: Ca2+, Mg2+, SO42-, ClCa2+(aq) + SO42-(aq) → CaSO4(s)
“permanent hardness”
5.4 Compounds of Group II Elements
Ca2+ (aq) + 2C17H35COOˉ(aq) → Ca(C17H35COO)2 (s)
stearate
calcium stearate
Mg2+ (aq) + 2C17H35COOˉ(aq) → Mg(C17H35COO)2 (s)
stearate
magnesium stearate
scum
5.4 Compounds of Group II Elements
A number of methods can be used for softening water:
♦ Boiling removes temporary hardness, but is expensive.
♦ Calcium hydroxide is cheap and can be added to precipitate out
temporary hardness as calcium carbonate.
Ca(HCO3)2(aq) + Ca(OH)2(s) → 2CaCO3(s) + 2H2O(l)
♦ Sodium carbonate may be added to precipitate out calcium or
magnesium ions.
Mg2+ (aq) + Na2CO3(aq) → MgCO3(s) + 2Na+(aq)
♦ Use ion exchange resins: plastic beads which contain sodium ions.
5.4 Compounds of Group II Elements
River water in a chalky agricultural area may contain Ca2+, Mg2+,
CO32-, HCO3-, Cl- and NO3- ions. In a waterworks, such water is
treated by adding a calculated quantity of calcium hydroxide.
Which will be precipitated following the addition of calcium
hydroxide?
A. CaCl2
B. CaCO3
√
C. MgCO3
D. Mg(NO3)2
5.4 Compounds of Group II Elements
Table 2: Active Ingredients in Commercial Antacid Tablets
Chemical Name
Magnesium
Hydroxide
Calcium Carbonate
Sodium Bicarbonate
Aluminum
Hydroxide
Dihydroxyaluminum
Sodium Carbonate
Chemical
Formula
Chemical Reaction
Mg(OH)2
Mg(OH)2 + 2H+ →
Mg2+ + 2H2O
CaCO3
CaCO3 + 2H+ →
Ca2+ + H2O + CO2 (g)
NaHCO3
NaHCO3 + H+ →
Na+ + H2O + CO2 (g)
Al(OH)3
Al(OH)3 + 3H+ →
Al3+ + 3H2O
NaAl(OH)2CO3
NaAl(OH)2CO3 + 4H+
→ Na++ Al3++ 3H2O +
CO2(g)
5.4 Compounds of Group II Elements
The metals of Group II react readily with oxygen to from
compounds of general formula MO. When each of these
oxides is added to water, which forms the most alkaline
solution?
A. MgO
B. CaO
C. SrO
D. BaO
√
5.4 Compounds of Group II Elements
The solubility of some Group II metal
compounds in mmol·dm-3
CO32-
SO42-
CrO42-
C2O42-
Mg2+
1.5
1830
8500
5.7
Ca2+
0.13
47
870
0.05
Sr2+
0.07
0.71
5.9
0.29
Ba2+
0.09
0.009
0.01
0.52
decreases
down the
group
5.4 Compounds of Group II Elements
定义: 由无限远离的气态正负离子, 在标准状态下形成1mol
离子晶体时的焓变, 叫该种晶体 的晶格能DHlatt 。
The enthalpy change when 1 mole of an ionic compound is
formed from its gaseous ions under standard conditions
(298K,100 kPa)
Na+ (g) + Clˉ (g)  NaCl(s)
D H latt = -781 kJ•mol-1
5.4 Compounds of Group II Elements
Hydration Enthalpy(DHhyd ,水合热): The amount of
energy relaeased when one mole of aqueous ions is
formed from its gaseous ions.
Na+ (g) + aq  Na+ (aq)
DHhyd = -406 kJ·mol-1
5.4 Compounds of Group II Elements
When an ionic solid is dissolved in water, two processes
are taking place. They are the breakdown of the ionic
solid, and subsequent stabilization of the ions by water
molecules (hydration).
NaCl (s)
DHsolu
Na+(aq) + Clˉ(aq)
DHlatt =
-776 kJ·mol-1
DHhyd =
-772 kJ·mol-1
Na+(g) + Clˉ(g)
DHsolu = DHhyd -DHlatt
5.4 Compounds of Group II Elements
For MSO4, SO42- is quite large compared with M2+.Going
down the group II, the increase in size of the cations DHlatt does
not cause a significant change in the but the DHhyd become less
and less negative down the group. As a result, the dissolution
process becomes less and less exothermic and the solubility of
the sulphates(VI) of Group II metals decreases down the group.
For M(OH)2, OH- and M2+ are of the same order of magnitude,
Going down the group II, the increase in size of the cations
DHhyd does not cause a significant change in the but the DHlatt
become less and less negative down the group. As a result, the
dissolution process becomes less and less exothermic and the
solubility of the hydroxides of Group II metals increases down
the group.

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