2.
Group II Elements
2.1: Variations in properties of the elements
metal
atomic number
atomic radius pm
ionic radius 2+ M pm
m.p °C
b.p °C
ionisation energy -1 kJ mol
Density -1 kJ mol
Be Mg Ca Sr Ba Ra
Use your data book to complete the table above Atomic and ionic radii increase down the group because an extra shell of electrons is added. For each 2+ element the M ion is smaller than the atom. This is because of the loss of the outer shell electrons. The tendency to react and form ions increases down the group. The first and second ionisation energies decrease as the group is descended. Atoms change in 2 ways down a group: 1. the charge on the nucleus increases 2. the number of inner full shells increases However, as the group is descended the outer electrons get further and further away from the effective nuclear charge(as atomic size incs.) and the shielding effect increases(inc. in no. of shells), so electrons are less strongly held, and hence, ionisation energy decreases. The atoms in the elements are held together by metallic bonding. A metallic bond is the electrostatic electrostatic force of attraction between metal ions and the delocalised electrons between them. In metallic bonding, the delocalised electrons are relatively non-directional non-directional and the cations tend to be close packed with them. This allows layers of metal ions to shift their position in the metal lattice without the bonds breaking (enables malleability and ductility). Bonding and structure should not be confused; bonding is about the different types of attractive forces between particles and structure refers to the arrangement arrangement of particles.
2.2: Reaction of the elements with oxygen, water and dilute acids
1.
Reactions with oxygen
All of the elements tarnish in air to form a coating of the oxide. The reaction is rapid with elements high atomic number. When heated in air or oxygen they burn vigorously to produce white ionic oxides. 1
In air, Be metal is coated with an inert layer of oxide. This layer of oxide prevents further reaction unless the metal is heated to a high temperature. When heated, Mg burns with a bright white flame, hence the use of Mg powder as an ingredient in flares and fireworks. 2Mg(s) + O 2 (g) → 2MgO (s) Calcium burns brightly in air, with a brilliant red flame: 2Ca(s) + O 2 (g) → 2CaO (s) 2-
Barium burns in excess air or oxygen to form a peroxide, BaO 2 which contains the peroxide ion, O 2 . Equation: _______________________________________
2.
Reactions with water
All of the metals except beryllium react. With Be an oxide layer forms on its surface and this prevents the reaction with water. The reactions are not as vigorous as with group I, but, as in group I the rate of reaction increases down the group. Magnesium reacts very slowly with cold water. With hot water the reaction is slow, but when hot steam is passed over heated Mg an exothermic reaction occurs: Mg(s) + H2O (g) → MgO (s) + H 2 (g) n.b MgO is formed not Mg (OH) 2
The reaction of Mg with hot water gives: _____________________________________ Calcium reacts with cold water: Ca(s) + 2H2O (l) → Ca (OH) 2 (aq) + H2 (g) white ppte. The reaction of the other metals with cold water increases down the group. They all form alkaline solutions. The solubility and pH of the hydroxide solutions formed also increase down the group. e.g. Ca (OH) 2 (aq) with pH ≈ 9-11 will be less soluble and less basic than Ba (OH) 2 (aq).
Why does the reactivity of group II increase down the group? Use ideas about electron loss to explain. 3.
Reactions with dilute acids
Group II elements react with dilute acids to release H 2 (g). As with their reactions with water, their reactivity increases down the group. The reactions are examples of redox reactions. The metal loses + electrons and the H ion gains electrons. In general: +
2+
M(s) + 2H (aq) → M (aq) + H 2 (g) where M= Mg, Ca, Sr, Ba, or Ra. 2
Write the equation for the reaction between Mg and dilute hydrochloric acid: ________________________________________________________________
2.4: Thermal decomposition of nitrates and carbonates
The nitrates and carbonates of group 2 are generally ionic in nature. The difference in properties of these compounds can be explained in terms of two factors: · the charge on the metal ions · the size of the metal ions Carbonates The carbonates decompose as follows: MCO3(s) → MO(s) + CO2 (g)
where M = Be, Mg, Ca, Sr, Ba or Ra. Lattice Enthalpy of MO(s)
Be Mg Ca Sr Ba Ra
decomposes at room temperature (so easily that it cannot be isolated at r.t) decomposes readily on heating (540°C) decomposes on strong heating (900°C –1200°C) decomposes on very strong heating decomposes on very strong heating (1360°C)
1.
3889 3513 3310 3152
Write the equation for the decomposition of CaCO 3
______________________________________________________________________
2. Complete the table. Group II carbonates become more stable down the group and this indicates that the larger the metal ion the more stable the carbonate. Thermal stability (T.S) increases as atomic no. and atomic size increases. The T.S of the carbonate is dependent on the carbonate (MCO 3) lattice compared with the oxide (MO) lattice at the same 2+ temperature. This is due to the fact that as the cation size changes (M ), the lattice enthalpies of the carbonate and those of the oxides change by different factors. The lattice enthalpies of the carbonates change little, because the carbonate ion dominates in size. At the same time, the oxide lattice enthalpy falls faster as the cation size increases and the sum of the radii of the ions increases. This results in T.S increasing in the order: BeCO3<< MgCO3
Nitrates The nitrates of group II (Mg to Ba) are colourless crystalline solids which decompose) on heating to form the metal oxide, brown nitrogen (IV) oxide, and oxygen: E.g. 2M (NO3)2 → 2MO(s) + 4NO2 (g) + O2 (g)
Write the equation for the decomposition of Mg (NO3)2. 2+
The thermal stability of the nitrates increase as ionic radius (M ) increases. They show the same trend in T.S as the carbonates. Thermal decomposition becomes increasingly difficult as the cation size increases. In fact, red heat is required to decompose barium nitrate. The explanations for the trend in thermal stability are the same as for the thermal decomposition of the carbonates.
Show the trend is thermal decomposition of the nitrates. Explain the reasons for the trend in terms of (i) cation size and lattice enthalpy (ii) ionic radius and polarising power
2.3: Variation in the solubility of the Sulphates
The solubility of the sulphates decrease as atomic number increases. This trend can be explained by the energy transfer processes which take place during dissolving.
Solubilities of group II sulphates
Ion
Cation radius pm
Solubility mol per 100g water
ΔHhyd -1 kJ mol
ΔHsoln of MSO4
65
1.83 x 10
1920
-91.2
99
4.66x 10
1650
-17.8
113
7.11 x 10
-5
1480
-8.7
135
9.43 x 10
-7
1360
+19.4
2+
Be
2+
Mg 2+
Ca
2+
Sr
2+
Ba
-1
-3
4
The enthalpy of hydration, ΔH hydration, of the sulphate ion is the same for all of the group II sulphates so it is not responsible for the trend in solubility. However, the lattice enthalpy of the group II sulphates become less exothermic as the atomic number of the metal increases, but the change in ΔH latt from MgSO4 to BaSO4 22+ is quite small because of the large size of the SO 4 ion. Nevertheless, the ΔH hyd of the M ion from Mg to 2+ 2+ Ba is large because of the big difference in ionic radii the very small Mg and the large Ba ion. Hence, it is 2+ the ΔHhyd of the cation (M ) that determines the solubility trend. i.e. the more exothermic ΔH hyd the greater the likelihood of the MSO 4 dissolving. This means that ΔH soln of the sulphate becomes less exothermic down the group and hence solubility decreases down the group. 2.5: Uses of some compounds of magnesium and calcium
Magnesium oxide This is a solid with a high m.p of 2800°C. It is used as a refractory (heat resistant) ceramic to line furnaces. E.g. made into bricks to line kilns. It is also used in blast furnaces since it can withstand high temperatures (≈ 1500 °C). “Milk of magnesia” which is a suspension containing MgO is used to neutralise stomach acid. Calcium compounds Calcium oxide, CaO (quicklime) is used: · in steel making · to refine beet sugar · to improve soil fertility · in the lab to dry ammonia and to remove last traces of water from ethanol Calcium hydroxide Ca (OH) 2(s), as hydrated lime is used: · in agriculture and horticulture to improve soil fertility and to make fungicides · to stabilize soil in the construction industry · to recover ammonia in the Solway process Ca (OH) 2(aq), i.e. Ca (OH) 2 suspended in water is used: · to treat sewage and to soften water · with chlorine to bleach paper and cotton · in the manufacture of leather goods Calcium carbonate as lump limestone used in: · in blast furnaces to extract iron · as an aggregate in concrete as ground limestone: · is heated with sodium carbonate and sand to make glass · used as a filler · used in agriculture to improve soil fertility 5
6
