Alkaline earth metals Questions in English

(A) The density of alkaline earth metals ($IIA$ group) generally increases down the group as the atomic mass increases more significantly than the atomic volume. However,there is an anomaly for $Ca$ and $Mg$. The correct order of density for the first three elements is $Ca < Mg < Be$. The complete order for the group is $Ca < Mg < Be < Sr < Ba$.

(D) When $CaCO_3$ is treated with $CO_2$ and excess $H_2O$, it forms calcium bicarbonate, which is soluble in water:
$CaCO_3(s) + CO_2(g) + H_2O(l) \rightarrow Ca(HCO_3)_2(aq)$.
Thus, $A = Ca(HCO_3)_2$.
When $Ca(HCO_3)_2$ is heated, it decomposes back into calcium carbonate, water, and carbon dioxide:
$Ca(HCO_3)_2(aq) \xrightarrow{\Delta} CaCO_3(s) + H_2O(l) + CO_2(g)$.
Thus, $B = CaCO_3$.
Therefore, the correct option is $D$.

(A) Anhydrous magnesium chloride $(MgCl_2)$ cannot be prepared by simple heating of its hydrated form $(MgCl_2 \cdot 6H_2O)$ because it undergoes hydrolysis to form magnesium oxychloride $(Mg(OH)Cl)$ and $HCl$.
To prevent this hydrolysis,$MgCl_2 \cdot 6H_2O$ is heated in a current of dry $HCl$ gas.
The reaction is: $MgCl_2 \cdot 6H_2O \xrightarrow{\text{dry } HCl \text{ gas, heat}} MgCl_2 + 6H_2O$.
Therefore,option $A$ is correct.

(C) The reactivity of alkaline earth metals with water increases down the group.
$Be$ is the least reactive among alkaline earth metals and does not react with water or steam even at high temperatures because of its high ionization enthalpy and small size.
$Mg$ reacts with hot water.
$Na$ and $Li$ are alkali metals and react with water.
Therefore,the reaction $Be + H_2O \xrightarrow{\text{Cold}}$ does not take place.

(A) Dolomite is a double carbonate mineral with the chemical formula $MgCO_3 \cdot CaCO_3$.
Therefore,the metals present in dolomite are magnesium $(Mg)$ and calcium $(Ca)$.

(A) The density of alkaline earth metals ($IIA$ group) generally increases down the group as atomic mass increases faster than the atomic volume.
However,there is an anomaly in the case of $Ca$ and $Mg$.
The densities of the elements are: $Be$ $(1.85 \ g/cm^3)$,$Mg$ $(1.74 \ g/cm^3)$,and $Ca$ $(1.55 \ g/cm^3)$.
Wait,checking standard values: $Be$ $(1.85)$,$Mg$ $(1.74)$,$Ca$ $(1.55)$,$Sr$ $(2.64)$,$Ba$ $(3.51)$.
Actually,the density order for the first three elements is $Ca < Mg < Be$.
Therefore,the correct order is $Ca < Mg < Be$.

(C) When alkaline earth metals or their salts are heated,electrons are excited to higher energy levels due to the absorption of energy.
When these electrons return to their ground states,they emit energy in the form of visible light,imparting a characteristic colour to the flame.
$Be$ and $Mg$ atoms have very small sizes and high ionization enthalpies,meaning their electrons are too strongly bound to be excited by the energy of a Bunsen flame.
Therefore,$Be$ and $Mg$ do not impart any colour to the flame.
$Ca$,$Sr$,and $Ba$ impart brick red,crimson (blood red),and apple green colours,respectively.
Thus,the pair that does not impart colour is $BeCl_2$ and $MgCl_2$.

(A) The basicity of alkaline earth metal hydroxides increases down the group as the metallic character increases and the $M-OH$ bond becomes more ionic.
As we move from $Mg$ to $Ba$ in Group $2$,the ionization enthalpy decreases,making it easier to release $OH^-$ ions.
Therefore,the order of basicity is $Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$.
Thus,$Mg(OH)_2$ is the least basic.

(C) Plaster of Paris is $CaSO_4 \cdot \frac{1}{2}H_2O$.
When it is mixed with water,it undergoes hydration to form a hard mass of gypsum $(CaSO_4 \cdot 2H_2O)$.
The reaction is: $CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$.
Thus,the hardening process involves combining with water.

(D) As we move down the group,the size of the metal cation increases,which leads to a decrease in the polarising power of the cation.
According to Fajan's rule,a decrease in polarisation leads to an increase in the thermal stability of the carbonate.
Among the given options,$Ba^{2+}$ is the largest cation,making $BaCO_3$ the most thermally stable carbonate.
Hence,the correct option is $D$.

(A) The melting point of ionic compounds depends on the lattice energy,which is inversely proportional to the interionic distance.
For alkaline earth metal oxides $(BeO, MgO, CaO, SrO)$,as the size of the cation increases down the group,the lattice energy decreases,leading to a decrease in the melting point.
Thus,the order $BeO > MgO > CaO > SrO$ is correct.
In option $B$,the order is incorrect because $CaCl_2$ has a higher melting point than $SrCl_2$.
In option $C$,the order is incorrect as $Al_2O_3$ has the highest melting point due to high lattice energy.

(B) Milk of lime is a suspension of calcium hydroxide,$Ca(OH)_2$,in water.
When calcium oxide $(CaO)$ reacts with water,it forms calcium hydroxide $(Ca(OH)_2)$.
$A$ saturated solution of calcium hydroxide in water is known as lime water,while a suspension of calcium hydroxide in water is known as milk of lime.

(A) The solubility of alkaline earth metal sulphates decreases down the group from $Be$ to $Ba$.
This is because the hydration energy decreases more rapidly than the lattice energy as the size of the cation increases down the group.
Since the sulphate ion is very large,the lattice energy remains relatively constant,while the hydration energy decreases significantly.
Therefore,the correct order of solubility is $MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$.

(A) The chemical formula for calcium oxide is $CaO$.
It is commercially known as Quick lime.

(C) The thermal stability of alkaline earth metal hydroxides increases down the group as the size of the metal cation increases.
As the size of the cation increases,the lattice energy changes,and the polarizing power of the cation decreases.
Therefore,the thermal stability increases in the order: $Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$.

(D) According to Fajan's rule,covalent character increases as the size of the cation decreases and its charge density increases.
Among the alkaline earth metal ions ($Mg^{2+}$,$Ca^{2+}$,$Sr^{2+}$,$Ba^{2+}$),the $Mg^{2+}$ ion has the smallest size.
Therefore,$MgCl_2$ exhibits the most significant covalent character among the given halides.

(D) The reaction of alkaline earth metal oxides with water is exothermic. The enthalpy of hydration (or heat of reaction) depends on the lattice energy of the oxide and the hydration energy of the resulting hydroxide.
As we move down the group from $Mg$ to $Ba$,the lattice energy of the oxides decreases significantly due to the increase in the size of the metal cation.
Since the hydration energy of the metal ions also decreases down the group but at a slower rate than the lattice energy,the net enthalpy change (heat liberated) is highest for the oxide with the highest lattice energy.
$MgO$ has the smallest cation size among the given options,resulting in the highest lattice energy. Therefore,the reaction $MgO + H_2O \to Mg(OH)_2$ releases the most heat per mole.

(D) The hydration energy decreases as the size of the alkaline earth metal cation increases down the group.
$Mg^{2+}$,$Ca^{2+}$,and $Sr^{2+}$ ions have smaller sizes and higher charge density,allowing them to form hydrated salts like $Mg(NO_3)_2 \cdot 6H_2O$,$Ca(NO_3)_2 \cdot 4H_2O$,and $Sr(NO_3)_2 \cdot 4H_2O$.
Due to the relatively large size of the $Ba^{2+}$ ion,its hydration energy is insufficient to hold water molecules in the crystal lattice,so $Ba(NO_3)_2$ crystallizes as an anhydrous salt.

(C) $Be$ (Beryllium) is used to make $X$-ray tube windows.
This is because $Be$ has a very low atomic number ($Z = 4$), which allows $X$-rays to pass through it with minimal absorption.
Therefore, it is highly transparent to $X$-rays.

(A) An amphoteric substance is one that can react with both acids and bases.
$Be(OH)_2$ is amphoteric in nature because it reacts with both acids and bases to form salts and water.
For example:
$Be(OH)_2 + 2HCl \rightarrow BeCl_2 + 2H_2O$ (acts as a base)
$Be(OH)_2 + 2NaOH \rightarrow Na_2[Be(OH)_4]$ (acts as an acid)
Other hydroxides like $Mg(OH)_2$,$Ca(OH)_2$,and $Sr(OH)_2$ are basic in nature.

(A) Beryllium $(Be)$ has a very small atomic size and high ionization enthalpy compared to other alkaline earth metals.
According to Fajans' rules,the small size and high charge density of the $Be^{2+}$ ion result in high polarizing power,which gives its halides $(BeX_2)$ a predominantly covalent character.
Other alkaline earth metal halides are primarily ionic.

(B) When magnesium powder is burnt in air,it reacts with both oxygen and nitrogen present in the atmosphere.
$2Mg(s) + O_2(g) \to 2MgO(s)$
$3Mg(s) + N_2(g) \to Mg_3N_2(s)$
Therefore,the products formed are $MgO$ and $Mg_3N_2$.

(A) The thermal stability of alkaline earth metal carbonates increases as the size of the cation increases down the group.
As the size of the cation increases,the polarizing power of the cation decreases,which makes the $M-O$ bond stronger and the carbonate ion more stable.
Therefore,the order of thermal stability is $MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3$.

(D) The reaction of calcium carbonate with carbon dioxide in the presence of excess water leads to the formation of calcium bicarbonate:
$CaCO_3(s) + CO_2(g) + H_2O(l) \rightarrow Ca(HCO_3)_2(aq)$.
Thus, $A = Ca(HCO_3)_2$.
When calcium bicarbonate is heated, it decomposes back into calcium carbonate, water, and carbon dioxide:
$Ca(HCO_3)_2(aq) \xrightarrow{\Delta} CaCO_3(s) + H_2O(l) + CO_2(g)$.
Thus, $B = CaCO_3$.
Therefore, the correct option is $D$.

(C) The element is $Ca$ (Calcium).
$Ca$ reacts with $H_2$ to form calcium hydride,$CaH_2$ (compound $A$).
$CaH_2 + 2H_2O \rightarrow Ca(OH)_2 + 2H_2 \uparrow$.
Calcium hydride reacts with water to liberate hydrogen gas.

(D) Amphoteric compounds are those that can react with both acids and bases.
$Zn(OH)_2$,$BeO$,and $Al_2O_3$ are all well-known amphoteric substances.
Therefore,none of the given options represent a compound that lacks amphoteric behaviour.

(A) The hydration energy of an ion is directly proportional to its charge density,which is defined as $\frac{\text{charge}}{\text{size}}$.
For $Mg^{2+}$ and $Al^{3+}$,both have small ionic radii,but $Al^{3+}$ has a higher charge $(+3)$ compared to $Mg^{2+}$ $(+2)$.
Therefore,the charge density of $Al^{3+}$ is greater than that of $Mg^{2+}$,resulting in higher hydration energy for $Al^{3+}$.
Thus,the hydration energy of $Mg^{2+}$ is lesser than that of $Al^{3+}$.

(B) Moving down the group from $Be$ to $Ba$ in alkaline earth metals:
$(i)$ Atomic radius increases due to the addition of new electron shells.
$(ii)$ Ionisation energy decreases because the valence electrons are further from the nucleus and more shielded,making them easier to remove.
$(iii)$ Nuclear charge increases because the number of protons in the nucleus increases as the atomic number increases.
Therefore,both atomic radius and nuclear charge increase down the group.

(B) Dolomite is $CaCO_3 \cdot MgCO_3$.
It is a double carbonate of calcium and magnesium.
Therefore,it contains both $Ca$ and $Mg$.

(B) The metal $X$ is $Ca$.
$1$. Calcium is obtained by the electrolysis of fused $CaCl_2$.
$2$. $Ca$ reacts with hydrogen to form calcium hydride $(CaH_2)$,which is a colourless solid.
$3$. Calcium hydride reacts with water to release dihydrogen gas: $CaH_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + 2H_2(g)$.

(A) The melting point of $MgO$ is very high $(2852 \ ^{\circ}C)$.
Refractory materials are substances that can withstand very high temperatures without melting or decomposing.
Due to its exceptionally high melting point and chemical stability,$MgO$ is widely used as a refractory material.

(B) Flash bulbs are filled with oxygen gas and a thin wire of $Mg$ (magnesium). When the circuit is closed,the $Mg$ wire burns in the presence of oxygen to produce a brilliant white light. The chemical reaction is: $2Mg(s) + O_2(g) \rightarrow 2MgO(s) + \text{light energy}$.

(D) The $Be^{2+}$ cation has a very small size,which results in a very high hydration energy that significantly exceeds its lattice energy.
Consequently,$BeF_2$ is highly soluble in water.
In contrast,the fluorides of other alkaline earth metals have high lattice energies,making them relatively insoluble in water.
The decreasing order of solubility for alkaline earth metal fluorides is $BeF_2 > MgF_2 > CaF_2 > SrF_2 > BaF_2$.

(C) The element with the highest occupied energy level $7s^2$ is Radium $(Ra)$.
Group $2$ elements are alkaline earth metals.
$A$: Group $2$ elements exhibit a $+II$ oxidation state.
$B$: Being highly reactive,they react with water to liberate $H_2$ gas.
$C$: Hydroxides of group $2$ elements are basic in nature,not amphoteric. As we move down the group,basicity increases.
$D$: Thermal stability of carbonates increases down the group,so $RaCO_3$ is more stable than $CaCO_3$.

(D) The solubility of alkaline earth metal hydroxides increases down the group from $Mg(OH)_2$ to $Ba(OH)_2$.
This is because the lattice energy decreases more rapidly than the hydration energy as the size of the metal cation increases.
Since $K_{sp}$ is directly related to the solubility of the compound,$Ba(OH)_2$ has the highest solubility and consequently the highest $K_{sp}$ value among the given hydroxides.

(A) The solubility of alkaline earth metal hydroxides increases down the group from $Be(OH)_2$ to $Ba(OH)_2$.
This is because the lattice energy decreases more rapidly than the hydration energy as the size of the metal cation increases.
Therefore,$Be(OH)_2$ has the lowest solubility in water.

(D) The thermal stability of alkaline earth metal carbonates increases as we move down the group due to the decrease in the polarizing power of the metal cation.
As the size of the cation increases $(Mg^{2+} < Ca^{2+} < Sr^{2+} < Ba^{2+})$,its ability to polarize the large $CO_3^{2-}$ ion decreases.
Consequently,the covalent character decreases and the ionic character increases,leading to higher thermal stability.
Therefore,the order of decreasing thermal stability is $BaCO_3 > SrCO_3 > CaCO_3 > MgCO_3$.

(A) Lime mortar is a mixture of one part of slaked lime $Ca(OH)_2$,three parts of sand $(SiO_2)$ and water.
It is converted into calcium silicate with time and becomes hardened.
Hence,it is used as a building material.
$Ca(OH)_2 + SiO_2 \rightarrow CaSiO_3 + H_2O$
Therefore,the correct mixture is $Ca(OH)_2$,silica and water.

(A) When $MgCl_2 \cdot 6H_2O$ is strongly heated,it undergoes hydrolysis to form $MgO$.
On heating,$MgCl_2 \cdot 6H_2O$ first loses water of crystallization. However,due to the high charge density of the $Mg^{2+}$ ion,the remaining water molecules undergo hydrolysis with $MgCl_2$ instead of simple evaporation.
The reaction is as follows:
$MgCl_2 \cdot 6H_2O \xrightarrow{\Delta} MgO + 2HCl + 5H_2O$
Thus,the final product obtained upon strong heating is magnesium oxide $(MgO)$.

(C) Plaster of Paris,which is $CaSO_4 \cdot \frac{1}{2}H_2O$,reacts with water to form gypsum.
The chemical reaction is: $CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$.
This product,$CaSO_4 \cdot 2H_2O$ (calcium sulphate dihydrate),forms a hard,solid mass upon setting.

(C) Plaster of Paris is a hemihydrate of calcium sulfate.
Its chemical formula is $CaSO_4 \cdot \frac{1}{2}H_2O$ or $2CaSO_4 \cdot H_2O$.
Option $C$ represents this composition correctly.

(A) $MgO$ (magnesium oxide) is used as a refractory material for lining furnaces because it possesses a very high melting point $(2852 \ ^{\circ}C)$.
This property allows it to withstand the extreme temperatures generated inside industrial furnaces without melting or decomposing.

(B) Gypsum $(CaSO_4 \cdot 2H_2O)$ is added to cement clinker during the grinding process. It acts as a retarder,which slows down the hydration process of tricalcium aluminate $(C_3A)$,thereby increasing the setting time of cement and preventing flash set.

(C) In the Solvay process,$Ca(OH)_2$ is used to recover $NH_3$ from $NH_4Cl$ as follows: $2NH_4Cl + Ca(OH)_2 \rightarrow 2NH_3 + CaCl_2 + 2H_2O$.
When $CO_2$ is passed through an aqueous solution of $Ca(OH)_2$ (lime water),it turns milky due to the formation of insoluble $CaCO_3$: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O$.
$Ca(OH)_2$ is also used in whitewashing because of its disinfectant properties.
Therefore,the compound $(A)$ is calcium hydroxide,$Ca(OH)_2$.

(B) The electronic configurations of the given elements are:
$Be (Z=4): 1s^2 2s^2$
$Mg (Z=12): 1s^2 2s^2 2p^6 3s^2$
$Ca (Z=20): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$
All these elements belong to Group $2$ (alkaline earth metals) and have a valence shell configuration of $ns^2$.
They do not contain $p$-electrons in their outermost energy level (valence shell). Therefore,option $B$ is not a similar characteristic.

(D) $1$. For $Ca(OH)_2, Sr(OH)_2, Ba(OH)_2$: Solubility increases down the group due to the decrease in lattice energy being more significant than the decrease in hydration energy. Thus,$Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$ is correct.
$2$. For $Li_2CO_3, Na_2CO_3, K_2CO_3$: Solubility of alkali metal carbonates increases down the group. Thus,$Li_2CO_3 < Na_2CO_3 < K_2CO_3$ is correct.
$3$. For $CsNO_3, RbNO_3, KNO_3$: Solubility of alkali metal nitrates decreases down the group as the hydration energy decreases more rapidly than the lattice energy. Therefore,the correct order is $CsNO_3 < RbNO_3 < KNO_3$. This is correct.
$4$. For $BaS_2O_3, MgS_2O_3, CaS_2O_3$: The solubility of alkaline earth metal thiosulfates generally decreases down the group as the size of the cation increases. The correct order is $CaS_2O_3 > MgS_2O_3 > BaS_2O_3$. Thus,the given order $BaS_2O_3 < MgS_2O_3 < CaS_2O_3$ is incorrect.

(C) The atomic radius of $Na$ is greater than that of $Mg$ because $Na$ has a lower nuclear charge than $Mg$ across the period. This statement is correct.
Due to the higher nuclear charge and two valence electrons in $Mg$ compared to one in $Na$,the metallic bond in $Mg$ is stronger than in $Na$. This statement is correct.
Magnesium has lower melting and boiling points than $Ca$. This is because $Ca$ has a larger atomic size and different crystal structure,but generally,the metallic bonding trends in alkaline earth metals are influenced by their packing and electronic configuration. Specifically,$Mg$ has a hexagonal close-packed structure,while $Ca$ has a face-centered cubic structure. The melting point of $Mg$ $(923 \ K)$ is lower than that of $Ca$ $(1124 \ K)$. Thus,the statement that $Mg$ has higher melting and boiling points than $Ca$ is incorrect.
$Mg$ and $Ca$ are indeed the most abundant alkaline earth metals in the Earth's crust. This statement is correct.

(A) The thermal stability of alkaline earth metal carbonates $(MCO_3)$ increases as we move down the group from $Be$ to $Ba$.
This is because the electropositive character of the metal increases down the group,which increases the ionic character of the $M-O$ bond.
As the size of the cation increases,the polarizing power of the cation decreases,leading to greater stability of the carbonate ion $(CO_3^{2-})$.
Therefore,the order of thermal stability is $BeCO_3 < MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3$.

(B) The solubility of alkaline earth metal hydroxides increases down the group because the lattice energy decreases more rapidly than the hydration energy as the size of the cation increases.
Therefore,the statement that the solubility of hydroxides decreases down the group is incorrect.
- Solubility of sulphates decreases down the group due to the decrease in hydration energy.
- Thermal stability of carbonates increases down the group due to the increase in the size of the cation.
- Basic nature increases down the group as the electropositive character increases.