Transcript Inorganic chemistry: Group 2 (II)
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
Be Mg Ca Sr Ba Ra
Alkaline metal: ns 2
12+ Mg
5.3 Group 2
BERYLLIUM MAGNESIUM CALCIUM STRONTIUM BARIUM RADIUM
5.3 Group 2
The Flame Color: Element
Li Na K Rb Cs
Color
Scarlet Yellow Lilac Red Blue
Element
Be Mg Ca Sr Ba
Color
－ － Brick-red Crimson Apple-green
5.3 Group 2
Increasing electronegativity I II III IV Increasing electronegativity 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 + → Be 2+ + 2H 2 O Be(OH) 2 + 2OHˉ→ [Be(OH) 4 ] 2 ˉ
5.3 Group 2
The physical properties of Group 2: Mg Magnisum Electron arrangement [Ne]3
s
2 Metallic radius/nm 0.160
First + Second IE/ kJ mol -1 T m /K 2189 922 T b /K 1380 Ca Calcium [Ar]4
s
2 0.197
1735 1112 1757 [Kr]5
s
2 0.215
1614 1042 1657 Sr Strontium Ba Barium [Xe]6
s
2 0.224
1468 998 1913 Density /g cm -3 1.74
1.54
2.60
3.51
5.3 Group 2
Magnesium oxide 2Mg (s) + O 2 (g) 2MgO (s)
a. in the air b. in oxygen ① ② ③ burns very vigorously bright white flame white solid produced
5.3 Group 2
.
O
¨ .
2
oxygen ion
¨ ¨ ¨ ¨ .
2
peroxide ion
¨ ¨ ¨
superoxide ion
The reason that there are different types of oxides is related to the sizes of the ions:
O 2 ˉ > O 2 2ˉ > O 2ˉ 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(M 2+ + O 2ˉ ), is formed when the metals are heated in oxygen. Strontium and Barium also form peroxides. As the M 2+ 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) + 2H 2 O (l) Mg(OH) 2 (aq) + H 2 (g)
slowly
Mg (s) + H 2 O (g) MgO (s) + H 2 (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) + H 2 O (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) MgCl 2 (aq) + H 2 (g) Mg (s) + H 2 SO 4 (aq) MgSO 4 (aq) + H 2 (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, CO 3 2ˉ Nitrates, NO 3 ˉ Mg Ca Sr Ba
MgCO 3 → MgO + CO 2
Same pattern but
higher temperatures
needed for decomposition The
charge density (Z/r) of the cations (polarization)
will affect the decomposition temperature.
Mg Ca Sr Ba
M(NO 3 ) 2 → MO + 2NO 2 + 1/2O 2
Same pattern but
higher temperatures
needed for decomposition The larger value of Z/r, The easier breaking up of distorted anions: CO 3 2 → CO 2 NO 3 2 → NO 2 + O 2 + O 2-
5.3 Group 2
Which one of the following equations represents the reaction that occurs when calcium nitrate is heated strongly?
A. Ca(NO 3 ) 2 → Ca(NO 2 ) 2 + O 2
√
3 ) 2 C. Ca(NO 3 ) 2 → 2CaO + 4NO 2 + O 2 → CaO + N 2 O + 2O 2 D. 3Ca(NO 3 ) 2 → Ca 3 N 2 + 4NO 2 + 5O 2 E. Ca(NO 3 ) 2 → CaO 2 + 2NO 2
5.3 Group 2
Which one of the following elements is likely to have an electronegativity similar to that of aluminium?
A. Barium diagonal relationship C. Calcium D. Magnesium E. Strontium
5.3 Group 2
Which one of the following statements is true?
B.
to give the oxide NO 2 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 MgO CaO SrO BaO Melting point/℃ 2852 2614 2430 1918
refractory material
As M 2+ 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 D. metallic
5.4 Compounds of Group II Elements
CaCO 3 (limestone) + CO 2 Ca(OH) 2 (slaked lime) + H 2 O Δ CaO(lime)
5.4 Compounds of Group II Elements
Ca(OH) 2 (s) + 2HNO 3 (aq) → Ca(NO 3 ) 2 (aq) + 2H 2 O(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?
√
2 + CO 2 → CaCO 3 + H 2 O
√
2 + 2H + (
aq
) → Ca 2+ (
aq
) + 2H 2 O
√
2 + 2NH 4 + (
aq
) → Ca 2+ (
aq
) + 2NH 3 + 2H 2 O
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?
3. potassium nitrate
5.4 Compounds of Group II Elements
Soft water: Ca 2+ , Mg 2+ , HCO 3 2-
“temporary hardness”
Ca Mg 2+ 2+ ( (
aq aq
) + 2HCO ) + 2HCO 3 ˉ 3 ˉ ( (
aq aq
) → CaCO 3 ) → MgCO ( 3
s
( ) + CO
s
2 ) + CO ( 2
g
( ) + H
g
2 ) + H O( 2
l
O( )
l
) Hard water: Ca 2+ , Mg 2+ , SO 4 2 , Cl Ca 2+ (
aq
) + SO 4 2 (
aq
) → CaSO 4 (
s
)
“permanent hardness”
5.4 Compounds of Group II Elements
Ca 2+ (
aq
) + 2C 17 H 35 COOˉ(
aq
) → Ca(C 17 H 35 COO) 2 (
s
) stearate calcium stearate Mg 2+ (
aq
) + 2C 17 H 35 COOˉ(
aq
) → Mg(C 17 H 35 COO) 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(HCO 3 ) 2 (
aq
) + Ca(OH) 2 (
s
) → 2CaCO 3 (
s
) + 2H 2 O(
l
) ♦ Sodium carbonate may be added to precipitate out calcium or magnesium ions.
Mg 2+ (
aq
) + Na 2 CO 3 (
aq
) → MgCO 3 (
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 Ca 2+ , Mg 2+ , CO 3 2 , HCO 3 , Cl and NO 3 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. CaCl 2
√
3 C. MgCO 3 D. Mg(NO 3 ) 2
5.4 Compounds of Group II Elements
Table 2: Active Ingredients in Commercial Antacid Tablets Chemical Name Chemical Formula Chemical Reaction
Magnesium Hydroxide Calcium Carbonate Sodium Bicarbonate Aluminum Hydroxide Mg(OH) CaCO 3 NaHCO Al(OH) 3 3 2 Dihydroxyaluminum Sodium Carbonate NaAl(OH) 2 CO 3 Mg(OH) 2 Mg 2+ + 2H + 2H 2 O + → CaCO 3 Ca 2+ + 2H + H 2 + → O + CO 2 (g) NaHCO 3 Na + + H 2 + H + → O + CO 2 (g) Al(OH) 3 Al 3+ + 3H + 3H 2 O + → NaAl(OH) 2 CO 3 + 4H + → Na + + Al 3+ + 3H 2 O + CO 2 (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
5.4 Compounds of Group II Elements
Mg 2+ Ca 2+ Sr 2+ Ba 2+ The solubility of some Group II metal compounds in mmol·dm -3 CO 3 2 SO 4 2 CrO 4 2 C 2 O 4 2 1.5 1830 8500 5.7
0.13 47 870 0.05
0.07 0.71 5.9 0.29
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 D
H
latt
。
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) D
H
latt = － 781 kJ•mol -1 NaCl(s)
5.4 Compounds of Group II Elements
Hydration Enthalpy(
D
H
hyd
, 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) D
H
hyd = － 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
) D
H
solu Na + (
aq
) + Clˉ(
aq
) D
H
latt = － 776 kJ·mol -1 D
H
hyd = － 772 kJ·mol -1 Na + (
g
) + Clˉ(
g
) D
H
solu =
D
H
hyd
－ D
H
latt
5.4 Compounds of Group II Elements
For
MSO 4
, SO 4 2 is quite large compared with M 2+ .Going down the group II, the increase in size of the cations D
H
latt does not cause a significant change in the but the D
H
hyd 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 M 2+ are of the same order of magnitude, Going down the group II, the increase in size of the cations D
H
hyd does not cause a significant change in the but the D
H
latt 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.