Alkaline earth metals Questions in English

(D) The solubility of ionic compounds in water is determined by the balance between lattice enthalpy and hydration enthalpy.
For a compound to be soluble,the hydration enthalpy must be greater than the lattice enthalpy.
Alkaline earth metals have smaller ionic radii and higher charges compared to alkali metals,which results in significantly higher lattice enthalpies for their compounds.
Although their hydration enthalpies are also higher,the increase in lattice enthalpy is more dominant,making the overall dissolution process less favorable compared to alkali metal compounds.

(B) The reaction between $CaCO_3$ and $HCl$ is:
$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$
Among the products,$CO_2$ gas is passed into slaked lime $(Ca(OH)_2)$.
The reaction is:
$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$
Thus,the product $X$ formed is $CaCO_3$ (calcium carbonate).

(A) $1$. Thermal decomposition of calcium carbonate: $CaCO_3 \stackrel{\Delta}{\rightleftharpoons} CO_2 + CaO (X)$.
$2$. Slaking of lime: $CaO (X) + H_2O \longrightarrow Ca(OH)_2 (Y)$.
$3$. Reaction with chlorine to form bleaching powder: $Ca(OH)_2 (Y) + Cl_2 \longrightarrow CaOCl_2 (Z) + H_2O$.
Thus,$X = CaO$,$Y = Ca(OH)_2$,and $Z = CaOCl_2$.

(B) Oxoacid salts are salts derived from oxoacids (acids containing oxygen).
From the given list:
$(i)$ $CaCO_3$ is a salt of carbonic acid $(H_2CO_3)$.
$(ii)$ $MgSO_4$ is a salt of sulfuric acid $(H_2SO_4)$.
$(iv)$ $Sr(NO_3)_2$ is a salt of nitric acid $(HNO_3)$.
Compounds $(iii)$ $BaCl_2$,$(v)$ $MgBr_2$,and $(vi)$ $MgCl_2$ are halides,which are not oxoacid salts.
Therefore,the oxoacid salts are $(i)$,$(ii)$,and $(iv)$.

(A) During the manufacture of cement,$2-3\%$ of gypsum $(CaSO_4 \cdot 2H_2O)$ is added to the clinker.
This addition is done to slow down the process of hydration of the tricalcium aluminate $(C_3A)$,which is responsible for the initial setting of cement.
By slowing down this reaction,the setting time of the cement is increased,allowing sufficient time for mixing and placing the concrete.

(A) The chemical formula of Gypsum is $CaSO_4 \cdot 2H_2O$ and Plaster of Paris $(POP)$ is $CaSO_4 \cdot \frac{1}{2}H_2O$.
$POP$ is prepared by heating gypsum at $393 \ K$: $CaSO_4 \cdot 2H_2O(s) \xrightarrow{393 \ K} CaSO_4 \cdot \frac{1}{2}H_2O(s) + \frac{3}{2}H_2O(g)$.
Option $A$ is correct because the molar mass of $CaSO_4 \cdot 2H_2O$ is $172 \ g/mol$ and $CaSO_4 \cdot \frac{1}{2}H_2O$ is $145 \ g/mol$. The percentage of calcium in gypsum is $(40/172) \times 100 \approx 23.25\%$,while in $POP$ it is $(40/145) \times 100 \approx 27.58\%$. Thus,gypsum has a lower percentage of calcium.

(C) Statement $(i)$ is correct: Beryllium is not readily attacked by acids due to the formation of a protective oxide layer on its surface.
Statement $(ii)$ is correct: $BeO$ is amphoteric in nature,meaning it reacts with both acids and bases.
Statement $(iii)$ is correct: Due to its small size and availability of vacant $2p$ orbitals,Beryllium can exhibit a coordination number greater than $4$ (e.g.,in $[Be(H_2O)_4]^{2+}$ or complex fluorides).
Statement $(iv)$ is incorrect: $BeO$ is amphoteric,not purely acidic.
Therefore,statements $(i)$,$(ii)$,and $(iii)$ are correct.

(B) The $Q$ shell corresponds to the principal quantum number $n=7$,which indicates the element is in the $7^{th}$ period of the periodic table.
$Ra$ (Radium) has the electronic configuration $[Rn] 7s^2$,meaning it has two electrons in the $7^{th}$ $(Q)$ shell.

(D) The correct answer is $D$.
$A$: $LiOH$ is a weaker base than $NaOH$ because the $Li-O$ bond is stronger than the $Na-O$ bond due to the smaller size of $Li^+$,making it harder to release $OH^-$ ions. This is correct.
$B$: Salts of $Be$ undergo hydrolysis because $Be^{2+}$ has a high charge density and small size,which polarizes water molecules. This is correct.
$C$: Calcium bicarbonate,$Ca(HCO_3)_2$,exists only in aqueous solution and is soluble in water. This is correct.
$D$: Hydrolysis of beryllium carbide $(Be_2C)$ produces methane $(CH_4)$,not acetylene $(C_2H_2)$. The reaction is: $Be_2C + 4H_2O \rightarrow CH_4 + 2Be(OH)_2$. Thus,this statement is incorrect.

(D) The fluorides of alkaline earth metals,except for $BeF_2$,are generally insoluble in water due to their high lattice energy compared to their hydration energy.
Among the given options,$NaF$ and $KF$ are alkali metal fluorides and are soluble in water.
$BeF_2$ is covalent and soluble in water.
$MgF_2$ is an alkaline earth metal fluoride that is insoluble in water.

(B) The hydration energy of alkaline earth metal ions decreases as the size of the ion increases $(Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+})$.
$MgCl_2$ forms a hexahydrate $(MgCl_2 \cdot 6H_2O)$,whereas $CaCl_2$ forms a hexahydrate,$SrCl_2$ forms a hexahydrate,and $BaCl_2$ forms a dihydrate $(BaCl_2 \cdot 2H_2O)$.
Among the given options,$MgCl_2$ has the highest tendency to coordinate water molecules due to its small ionic radius and high charge density,allowing it to form stable hexahydrates.

(C) Beryllium forms an alloy with copper known as beryllium copper or $Cu-Be$ alloy.
This alloy is widely used in the manufacturing of high-strength springs,electrical connectors,and non-sparking tools due to its excellent mechanical properties and conductivity.

(B) Portland cement is primarily composed of calcium silicates and aluminates.
Specifically,the composition of Portland cement is approximately:
$1.$ Dicalcium silicate $(Ca_2SiO_4)$: $26 \%$
$2.$ Tricalcium silicate $(Ca_3SiO_5)$: $51 \%$
$3.$ Tricalcium aluminate $(Ca_3Al_2O_6)$: $11 \%$
Therefore,the major ingredient $(51 \%)$ is tricalcium silicate $(Ca_3Al_2O_5)$.

(B) The hydrides of alkaline earth metals,specifically $BeH_2$ and $MgH_2$,exhibit a polymeric structure due to the presence of electron-deficient three-center two-electron $(3c-2e)$ bonds.
These structures allow the metal atoms to achieve a stable coordination environment through bridging hydrogen atoms.

(B) Amphoteric oxides are those that react with both acids and bases to form salt and water.
$BeO$ (Beryllium oxide),$ZnO$ (Zinc oxide),and $Sb_2O_3$ (Antimony$(III)$ oxide) are amphoteric.
$CO$ is neutral.
$CaO$ is basic.
$SO_2$ and $SO_3$ are acidic.
Therefore,there are $3$ amphoteric oxides.

(D) Beryllium $(Be)$ has the smallest atomic size in group $2$. Due to its high ionization enthalpy,its outermost electrons are tightly held and do not get excited to higher energy levels by the energy provided in a flame test. Therefore,$Be$ does not impart any characteristic color to the flame.

(C) In the same period,alkaline earth metals $(Group \ 2)$ have a higher nuclear charge than alkali metals $(Group \ 1)$.
Due to the higher nuclear charge,the electrons in alkaline earth metals are pulled more strongly towards the nucleus,resulting in smaller ionic radii compared to alkali metals.
Therefore,the Assertion is true.
However,the Reason states that alkali metals have a higher nuclear charge than alkaline earth metals,which is incorrect; alkali metals have a lower nuclear charge.
Thus,$(A)$ is true but $(R)$ is false.

(A) Beryllium is a member of Group $2$ (alkaline earth metals). When powdered Beryllium is heated in air,it reacts with both oxygen $(O_2)$ and nitrogen $(N_2)$ present in the air.
The reactions are as follows:
$2Be(s) + O_2(g) \rightarrow 2BeO(s)$
$3Be(s) + N_2(g) \rightarrow Be_3N_2(s)$
Thus,the products formed are Beryllium oxide $(BeO)$ and Beryllium nitride $(Be_3N_2)$.

(A) The melting points of alkaline earth metals $(Be, Mg, Ca, Sr, Ba)$ do not show a regular trend due to differences in their crystal structures. However,$Be$ has a significantly higher melting point $(1560 \ K)$ compared to the other alkaline earth metals $(Ca = 1124 \ K, Sr = 1045 \ K, Ba = 1002 \ K)$ due to its small size and strong metallic bonding. Therefore,$Be$ has the highest melting point among the given options.

(D) Assertion $(A)$: $MgO$,$CaO$,$SrO$,and $BaO$ are oxides of alkaline earth metals. While $MgO$ is sparingly soluble,$CaO$,$SrO$,and $BaO$ react with water to form hydroxides $(M(OH)_2)$,which are soluble to varying degrees. Therefore,the statement that they are all insoluble is incorrect.
Reason $(R)$: The basic strength of alkaline earth metal oxides increases down the group as the electropositive character of the metal increases with the increase in atomic number. Thus,$BaO > SrO > CaO > MgO$ in terms of basicity. This statement is correct.
Conclusion: Since $(A)$ is incorrect and $(R)$ is correct,the correct option is $(D)$.

(C) Amphoteric hydroxides are those that react with both acids and bases.
Among the given options,$Be(OH)_2$ is amphoteric in nature.
It reacts with acids to form salts and with alkalis to form beryllates.
For example:
$Be(OH)_2 + 2HCl \rightarrow BeCl_2 + 2H_2O$
$Be(OH)_2 + 2NaOH \rightarrow Na_2BeO_2 + 2H_2O$

(A) Assertion $(A)$: $Be$ and $Mg$ have very high ionization enthalpies due to their small atomic size.
Reason $(R)$: Because of their high ionization enthalpies,the electrons in $Be$ and $Mg$ are too strongly bound to be excited to higher energy levels by the energy available in the Bunsen flame.
Consequently,they do not impart any characteristic colour to the flame.
Therefore,both $(A)$ and $(R)$ are true,and $(R)$ is the correct explanation of $(A)$.

(A) The properties described are characteristic of Beryllium $(Be)$.
$1$. $BeSO_4$ is soluble in water.
$2$. $BeO$ is amphoteric in nature (reacts with both acids and bases).
$3$. $Be(OH)_2$ is insoluble in water but dissolves in $NaOH$ solution to form beryllate,$Na_2[Be(OH)_4]$,due to its amphoteric nature.
Therefore,the metal $M$ is $Be$.

(A) The solubility of alkaline earth metal sulphates decreases down the group from $Be$ to $Ba$.
This is because the hydration enthalpy decreases more rapidly than the lattice energy as the size of the cation increases.
For $BeSO_4$,the hydration enthalpy is significantly higher than its lattice energy,making it highly soluble in water.
Therefore,the correct option is $BeSO_4$.

(C) $Be(OH)_2$ is amphoteric in nature.
It reacts with bases to form beryllate ions: $Be(OH)_2 + 2 OH^{-} \longrightarrow [Be(OH)_4]^{2-}$. This reaction is feasible.
It reacts with acids to form beryllium ions: $Be(OH)_2 + 2 H^{+} \longrightarrow Be^{2+} + 2 H_2O$. This reaction is feasible.
Therefore,both reactions $(i)$ and $(ii)$ are feasible.

(D) The covalent character of alkaline earth metal halides decreases as the size of the cation increases down the group.
According to Fajan's rule,smaller cations have higher polarizing power,leading to greater covalent character.
Among the given options,$Mg^{2+}$ has the smallest ionic radius compared to $Ca^{2+}$,$Sr^{2+}$,and $Ba^{2+}$.
Therefore,$MgCl_2$ exhibits the highest covalent character among the choices provided.
Due to its covalent nature,$MgCl_2$ is soluble in organic solvents such as ethanol.

(B) The general thermal decomposition reaction for alkaline earth metal carbonates $(MCO_3)$ is:
$MCO_3(s) \xrightarrow{\Delta} MO(s) + CO_2(g)$
Where $M$ represents an alkaline earth metal $(Be, Mg, Ca, Sr, Ba)$.
From the reaction,it is clear that the products are a metal oxide $(II)$ and carbon dioxide $(I)$.
Therefore,the correct option is $B$.

(C) The process of "slaking of lime" involves the addition of water to quicklime $(CaO)$ to produce slaked lime $(Ca(OH)_2)$.
The chemical equation for this reaction is: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$.
Therefore,option $C$ represents the slaking of lime.

(D) Plaster of Paris is $CaSO_4 \cdot \frac{1}{2}H_2O$.
When mixed with water,it sets into a hard mass of gypsum,which is $CaSO_4 \cdot 2H_2O$.
The crystalline form of gypsum formed during this hardening process is monoclinic.

(C) When an aqueous solution of magnesium bicarbonate is boiled,it undergoes thermal decomposition to form magnesium hydroxide,carbon dioxide,and water.
The balanced chemical equation is:
$Mg(HCO_3)_{2(aq)} \xrightarrow{\Delta} Mg(OH)_{2(s)} + 2CO_{2(g)} + 2H_2O_{(l)}$

(C) Gypsum $(CaSO_4 \cdot 2H_2O)$ reacts with an aqueous solution of ammonium sulphate $((NH_4)_2SO_4)$ to form a double salt known as ammonium calcium sulphate,which is represented by the formula $CaSO_4 \cdot (NH_4)_2SO_4 \cdot 2H_2O$.

(B) $(I)$ Correct: $BeCl_2$ exists as a chlorobridged dimer in the vapour phase.
$(II)$ Correct: $BeSO_4$ is highly soluble in water due to the high hydration enthalpy of $Be^{2+}$ ions.
$(III)$ Incorrect: $BeO$ is amphoteric in nature,not basic.
$(IV)$ Correct: $BeCO_3$ is unstable and decomposes easily,so it is stored in a $CO_2$ atmosphere.
$(V)$ Correct: $BeCO_3$ is the least soluble among group $2$ carbonates due to the high lattice energy of the small $Be^{2+}$ ion.
Therefore,statements $(I)$,$(II)$,$(IV)$,and $(V)$ are correct. However,based on the provided options,the best fit is $(I, IV, V)$ or $(I, II, IV)$. Given the standard chemistry facts,$(I, II, IV)$ is often cited in textbooks.

(B) $I$. Beryllium oxide $(BeO)$ is amphoteric in nature,reacting with both acids and bases. This is a correct statement.
$II$. Beryllium does not react directly with hydrogen to form $BeH_2$. This is an incorrect statement.
$III$. $BeH_2$ is prepared by the reaction of $BeCl_2$ with $LiAlH_4$: $2BeCl_2 + LiAlH_4 \rightarrow 2BeH_2 + LiCl + AlCl_3$. This is a correct statement.
$IV$. Beryllium sulphate $(BeSO_4)$ is highly soluble in water due to the high hydration enthalpy of the small $Be^{2+}$ ion. It is not the least soluble. This is an incorrect statement.
Therefore,statements $I$ and $III$ are correct.

(D) The reaction between dilute $HCl$ and $CaCO_3$ is: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$.
The gas evolved is carbon dioxide $(CO_2)$.
$CO_2$ is a colourless and odourless gas.
$CO_2$ is acidic in nature because it forms carbonic acid $(H_2CO_3)$ when dissolved in water.
$CO_2$ is not considered a poisonous gas (it is a natural component of the atmosphere).
$CO_2$ is moderately soluble in water,not the least soluble gas compared to others like $H_2$ or $N_2$.
Therefore,the statements that it has the least solubility and that it is poisonous are both technically incorrect,but in the context of standard chemistry questions,$CO_2$ is not poisonous.

(D) The thermal stability of alkaline earth metal carbonates increases down the group as the electropositive character of the metal increases.
The order of thermal stability is: $BeCO_3 < MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3$.
$BeCO_3$ is the least thermally stable because the small size of the $Be^{2+}$ ion leads to high polarization of the carbonate ion,making it unstable.
It is so unstable that it must be stored in an atmosphere of $CO_2$ to prevent decomposition: $BeCO_3 \rightarrow BeO + CO_2$.

(B) The solubility of alkaline earth metal sulphates in water decreases down the group.
This is because the hydration energy decreases more rapidly than the lattice energy as the size of the cation increases.
The correct order is: $BeSO_{4} > MgSO_{4} > CaSO_{4} > SrSO_{4} > BaSO_{4}$.

(A) Alkaline earth metals impart characteristic colours to the Bunsen flame due to the excitation of electrons to higher energy levels.
$A$. Calcium $(Ca)$ imparts a brick red colour to the flame $(p)$.
$B$. Strontium $(Sr)$ imparts a crimson red colour to the flame $(r)$.
$C$. Barium $(Ba)$ imparts an apple green colour to the flame $(q)$.
Therefore,the correct matching sequence is $A-p, B-r, C-q$.