Alkali metals Questions in English

(A) The alkali metals $(Group \ IA)$ exhibit a characteristic flame color because they possess low ionization potential.
When heated in a Bunsen burner,the valence electrons absorb energy and are excited from a lower energy level to a higher energy level.
As these excited electrons return to their ground state,they emit the absorbed energy in the form of visible light,which corresponds to a specific color.

(A) The correct option is $(A)$.
Alkali metals belong to Group $1$ of the periodic table and have a general valence shell electronic configuration of $ns^1$.
Since their valence electron is present in the $s-$orbital,they lose this electron to form $M^+$ ions.

(A) Among the alkali metals,only $Li$ reacts directly with atmospheric nitrogen to form lithium nitride $(Li_3N)$ due to its small size and high charge density.
The reaction is: $6Li + N_2 \to 2Li_3N$.

(D) The densities of alkali metals increase down the group from $Li$ to $Cs$.
$Li$,$Na$,and $K$ have densities less than water $(1.00 \ g/cm^3)$.
Specifically,the densities are: $Li$ $(0.53 \ g/cm^3)$,$Na$ $(0.97 \ g/cm^3)$,$K$ $(0.86 \ g/cm^3)$,and $Rb$ $(1.53 \ g/cm^3)$.
Since $1.53 \ g/cm^3 > 1.00 \ g/cm^3$,$Rb$ is heavier than water.
Therefore,the correct option is $D$.

(B) Sodium $(Na)$ is a highly reactive alkali metal that reacts vigorously with water to form sodium hydroxide $(NaOH)$ and hydrogen gas $(H_2)$.
Because of this property,$Na$ metal is used to remove trace amounts of water from organic solvents like benzene $(C_6H_6)$ or ether,which do not react with sodium.
Alcohols react with sodium to form alkoxides,so $Na$ cannot be used to dry alcohols.
Ammonia solution contains water,and $Na$ would react with both,making it unsuitable.
Therefore,it is specifically used for drying inert organic solvents like benzene.

(B) The metallic character of alkali metals increases down the group as the ionization enthalpy decreases and the atomic size increases.
The order of metallic character is: $Cs > Rb > K > Na > Li$.
Therefore,$Cs$ (Cesium) is the most metallic alkali metal.

(D) The reactivity of alkali metals with water increases down the group due to the decrease in ionization enthalpy and increase in electropositive character.
The order of reactivity is: $Li < Na < K < Rb < Cs$.
Among the given options,$Rb$ (Rubidium) is the lowest in the group and thus reacts with water at the highest rate.
The reaction is: $2Rb + 2H_2O \to 2RbOH + H_2$.

(B) Alkali metals belong to Group $1$ of the periodic table.
They have one electron in their outermost $s$-orbital.
Therefore,the general valence shell electronic configuration of alkali metals is $ns^1$.

(B) The alkali metals are the elements belonging to Group $1$ of the periodic table.
These elements are Lithium $(Li)$,Sodium $(Na)$,Potassium $(K)$,Rubidium $(Rb)$,Cesium $(Cs)$,and Francium $(Fr)$.
Therefore,the set $Li, Na, K, Rb, Cs$ correctly represents the alkali metals.

(B) The element with atomic number $11$ is Sodium $(Na)$.
Sodium is an alkali metal belonging to group $1$ of the periodic table.
Alkali metals form oxides with the general formula $M_2O$,which react with water to form strong bases.
The oxide of sodium is $Na_2O$.
The reaction with water is: $Na_2O + H_2O \to 2NaOH$.
Since $NaOH$ is a strong base,the oxide $Na_2O$ is basic in nature.

(C) Sodium carbonate $(Na_2CO_3)$ is produced commercially by the Solvay process.
In this process,ammonia $(NH_3)$ and carbon dioxide $(CO_2)$ are passed through a concentrated solution of sodium chloride $(NaCl)$ to form sodium bicarbonate $(NaHCO_3)$,which is then heated to obtain sodium carbonate.

(C) reducing agent is a substance that can easily lose electrons to undergo oxidation.
Alkali metals have very low ionisation enthalpy and their standard electrode potential $(E^{\circ})$ values are highly negative,which indicates a strong tendency to lose electrons and get oxidised in an aqueous medium.
Therefore,they act as strong reducing agents.

(C) In the Castner process,molten $NaCl$ is electrolyzed.
The cathode is made of $Fe$ (iron) and the anode is made of $Ni$ (nickel).
Therefore,the anode is made of $Ni$ metal.

(D) Among the $s$-block elements,$Li$ (an alkali metal) and $Be$ (an alkaline earth metal) are known to form nitrides directly by reacting with $N_2$ due to their small size and high charge density. However,$Li$ is the only alkali metal that forms a stable nitride $(Li_3N)$ by direct combination with nitrogen. $Be$ also forms $Be_3N_2$. Given the options,$Li$ is the classic example of an alkali metal showing this property.

(A) The anomalous behavior of $Li$ compared to other alkali metals is primarily due to its exceptionally small atomic and ionic size.
This small size results in a high charge density,which leads to high hydration energy,high polarizing power,and greater covalent character in its compounds.

(B) On heating sodium bicarbonate strongly,it decomposes to give carbon dioxide,water vapour,and sodium carbonate.
Reaction: $2NaHCO_{3(s)} \rightarrow Na_2CO_{3(s)} + CO_{2(g)} + H_2O_{(g)}$

(A) The reducing power of an element depends on its standard oxidation potential. Alkali metals have the highest tendency to lose electrons,making them strong reducing agents. Among the given options,$K$ (potassium) is an alkali metal with the lowest ionization energy,which makes it the strongest reducing agent.

(A) The term 'alkali' is derived from the Arabic word 'al-qaly',which means 'the ashes' or 'plant ashes'. Historically,these metals were obtained from the ashes of plants.

(C) Potassium nitrate $(KNO_3)$ is also known as Indian saltpetre.
This is because it is one of the several nitrogen-containing compounds and is very important as a crude salt in India.
It is a major constituent of gunpowder.

(B) The Solvay process is used for the manufacture of $Na_2CO_3$. The raw materials used are $NaCl$,$CO_2$,$NH_3$,and $H_2O$.
The chemical reactions involved are:
$NH_3 + CO_2 + H_2O \rightarrow NH_4HCO_3$
$NH_4HCO_3 + NaCl \rightarrow NaHCO_3 + NH_4Cl$
$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$
Thus,the correct chemicals are $NaCl, CO_2$ and $NH_3$.

(D) The reaction of alkali metals like $Na$ with ammonia at high temperatures leads to the formation of sodamide.
The balanced chemical equation is:
$2Na + 2NH_3 \xrightarrow{300\ ^oC} 2NaNH_2 + H_2$
Thus,$Na$ is the correct metal.

(D) When sodium is heated in moist air,it first reacts with oxygen to form sodium oxide $(Na_2O)$:
$2Na + \frac{1}{2}O_2 \rightarrow Na_2O$
Then,the sodium oxide reacts with the moisture (water vapor) present in the air to form sodium hydroxide $(NaOH)$:
$Na_2O + H_2O \rightarrow 2NaOH$
Thus,the final mixture contains both $Na_2O$ and $NaOH$.

(D) When sodium metal dissolves in liquid $NH_3$,it undergoes the following reaction: $Na + (x+y)NH_3 \rightarrow [Na(NH_3)_x]^+ + [e(NH_3)_y]^-$.
The ammoniated electrons $[e(NH_3)_y]^-$ are responsible for the blue color,high electrical conductivity,and paramagnetic nature of the solution.
Therefore,the solution becomes paramagnetic,not diamagnetic.
Thus,option $D$ is the correct answer.

(C) The solubility of the alkali metal carbonates increases as we go down the group from $Li_2CO_3$ to $Cs_2CO_3$. This is because the lattice energy decreases more rapidly than the hydration energy as the size of the alkali metal cation increases.

(A) Alkali metals have the largest atomic radii in their respective periods due to the presence of only one electron in the outermost shell and the shielding effect of inner shells. Therefore,they possess the highest atomic volume in their respective periods,not low atomic volume. Thus,option $(A)$ is the correct answer.

(A) The tendency for the oxidation half reaction $M \rightarrow M^{+} + e^-$ is determined by the standard oxidation potential,which is the negative of the standard reduction potential.
Lithium has the most negative standard reduction potential $(E^{\circ} = -3.04 \, V)$ among all alkali metals.
Since the oxidation potential is the negative of the reduction potential,lithium has the highest standard oxidation potential $(+3.04 \, V)$.
Therefore,lithium has the greatest tendency to undergo oxidation.

(B) Most metal carbonates decompose on heating to give the corresponding metal oxide and $CO_2$ gas.
$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$
$PbCO_3 \xrightarrow{\Delta} PbO + CO_2$
$Li_2CO_3 \xrightarrow{\Delta} Li_2O + CO_2$
However,alkali metal carbonates (except $Li_2CO_3$) are thermally stable and do not decompose on heating.
$Na_2CO_3$ does not decompose on heating because the lattice energy of $Na_2CO_3$ is very high,making it thermally stable.

(D) $NaOH$ is prepared by the electrolysis of brine in the Castner-Kellner cell.
In this cell,a mercury cathode is used.
The reactions are as follows:
At anode: $2Cl^{-} \to Cl_2 + 2e^{-}$
At cathode: $Na^{+} + e^{-} + Hg \to Na-Hg$ (amalgam)
The sodium amalgam is then treated with water to produce $NaOH$:
$2Na-Hg + 2H_2O \to 2NaOH + 2Hg + H_2 \uparrow$

(C) When sodium metal dissolves in liquid ammonia,it undergoes ionization to form ammoniated cations and ammoniated electrons:
$Na + (x+y)NH_3 \rightarrow [Na(NH_3)_x]^+ + [e(NH_3)_y]^-$.
The deep blue colour of the solution is due to the presence of ammoniated electrons,which absorb energy in the visible region of the spectrum.

(A) The reducing power of an element in an aqueous solution is determined by its standard electrode potential $(E^\circ)$.
$Li$ has the most negative standard electrode potential $(E^\circ = -3.04 \ V)$ among alkali metals,which is due to its very high hydration energy.
Therefore,$Li$ is the strongest reducing agent in an aqueous solution.

(B) The melting points of alkali metals decrease as we move down the group due to the decrease in the strength of metallic bonding as the atomic size increases.
The melting points $(K)$ are as follows:
$Li: 453.5 \ K$
$Na: 370.8 \ K$
$K: 336.2 \ K$
$Rb: 312 \ K$
$Cs: 301.5 \ K$
Thus,with the increase in atomic weight,the melting point decreases.

(A) The reaction of alkali metals like $Na$ and $K$ with water is highly exothermic in nature.
$2Na(s) + 2H_2O(l) \to 2NaOH(aq) + H_2(g) + \text{Heat}$
$2K(s) + 2H_2O(l) \to 2KOH(aq) + H_2(g) + \text{Heat}$
Since heat is released during these reactions,they are classified as exothermic.

(A) The characteristic feature of alkali metals is that they are good conductors of heat and electricity due to the presence of delocalized electrons in their metallic lattice.
Alkali metals have low ionization potentials,not high.
They have low melting points due to weak metallic bonding.
They have high oxidation potentials (they are strong reducing agents),not low.

(C) The flame test is a characteristic property of alkali metals.
Potassium $(K)$ compounds impart a violet or lilac colour to the flame.
Since $KCl$ is a compound of potassium (a group $IA$ element),it gives a violet colour in the flame test.
Therefore,the correct substance is $KCl$.

(C) Lithium $(Li)$ and magnesium $(Mg)$ exhibit a diagonal relationship in the periodic table.
This occurs because they have similar ionic sizes and similar charge-to-size ratios,which results in similar polarizing power.
Therefore,they exhibit similar chemical properties.

(B) When sodium bicarbonate $(NaHCO_3)$ is heated,it decomposes to form sodium carbonate $(Na_2CO_3)$,carbon dioxide $(CO_2)$,and water $(H_2O)$.
The balanced chemical equation is:
$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + CO_2 \uparrow + H_2O$

(D) Alkali metals like lithium,sodium,and potassium are highly reactive and react violently with water $(H_2O)$ and carbon dioxide $(CO_2)$.
Specifically,$CO_2$ reacts with these metals to form their respective carbonates and carbon,which can be an exothermic reaction.
Therefore,none of the common fire extinguishers like water or $CO_2$ are suitable.
An asbestos blanket is used to cut off the supply of oxygen,which is the most effective way to extinguish such fires.

(A) The correct answer is $A$.
Alkali metal carbonates,except for $Li_2CO_3$,are highly stable and do not decompose upon heating.
$Na_2CO_3$ and $K_2CO_3$ are thermally stable.
In contrast,alkaline earth metal carbonates (like $MgCO_3$) and other metal carbonates decompose upon heating to form their respective oxides and $CO_2$ gas.
As we move down the group,the thermal stability of carbonates increases.

(A) The basicity of alkali metal hydroxides increases down the group as the electropositive character of the metal increases.
As we move from $Li$ to $Cs$ in the periodic table,the atomic size increases and the ionization energy decreases.
This leads to an increase in the metallic (electropositive) character,making the $M-OH$ bond more ionic and easier to dissociate into $M^+$ and $OH^-$ ions.
Therefore,$CsOH$ is the most basic among the given options.

(A) The photoelectric effect is the emission of electrons when electromagnetic radiation,such as light,hits a material.
Group $I$ elements (alkali metals) are highly electropositive,meaning they have low ionization energy.
As we move down the group from $Li$ to $Cs$,the ionization energy decreases,making it easier for the valence electron to be ejected.
Therefore,$Cs$ (Cesium) has the lowest ionization energy among the given options,making the photoelectric effect maximum in $Cs$.

(B) Lithium $(Li)$ reacts with nitrogen $(N_2)$ to form lithium nitride $(Li_3N)$.
$6Li + N_2 \rightarrow 2Li_3N$ $(A)$
Lithium nitride $(Li_3N)$ on heating at high temperature decomposes back to $Li$ and $N_2$.
$Li_3N$ reacts with water $(H_2O)$ to produce ammonia gas ($NH_3$,which is $B$).
$Li_3N + 3H_2O \rightarrow 3LiOH + NH_3(g)$
Ammonia gas $(NH_3)$ reacts with $CuSO_4$ solution to form a deep blue colored complex,tetraamminecopper$(II)$ sulfate.
$CuSO_4 + 4NH_3 \rightarrow [Cu(NH_3)_4]SO_4$
Thus,$A$ is $Li_3N$ and $B$ is $NH_3$.

(A) Sodium sulphate $(Na_2SO_4)$ is commonly known as salt cake. It is produced as a byproduct in the Mannheim process for the production of hydrochloric acid from sodium chloride and sulphuric acid.

(D) Glauber's salt is the common name for sodium sulphate decahydrate,which has the chemical formula $Na_2SO_4 \cdot 10H_2O$.
It exists as white or colourless monoclinic crystals.
When exposed to dry air,it undergoes efflorescence,losing its water of crystallization to form powdery anhydrous sodium sulphate.

(B) The flame test is a qualitative analysis used to detect the presence of certain metal ions based on the characteristic emission spectrum produced when they are heated in a flame.
Sodium salts,when introduced into a Bunsen flame,impart a characteristic $Golden \ yellow$ colour due to the excitation of electrons.

(C) The Solvay process is an industrial method used for the large-scale production of $Na_2CO_3$ (Sodium carbonate).
In this process,$CO_2$ and $NH_3$ are passed through a concentrated solution of $NaCl$ (brine) to form $NaHCO_3$,which is then heated to yield $Na_2CO_3$.

(C) When sodium is heated in a current of dry ammonia gas at $300-400 \ ^\circ C$,it reacts to form sodium amide $(NaNH_2)$ and hydrogen gas is evolved.
The chemical equation for the reaction is:
$2Na + 2NH_3 \to 2NaNH_2 + H_2$

(A) Washing soda is the common name for sodium carbonate decahydrate.
Its chemical formula is $Na_2CO_3 \cdot 10H_2O$.

(B) Sea water contains various dissolved salts. The most abundant salt present in sea water is sodium chloride $(NaCl)$,which constitutes about $2.7\%$ to $2.9\%$ by mass of sea water.

(B) The metallic lustre exhibited by sodium is explained by the oscillation of loose electrons.
When light falls on the surface of sodium,the free electrons present in the metallic lattice start oscillating at their mean positions due to the interaction with the incident light.
These electrons get excited to higher energy levels and,upon returning to their lower energy levels,they emit light in all directions.
This phenomenon of light reflection and emission gives sodium its characteristic metallic lustre.

(D) The alkali metals,such as $Sodium$ $(Na)$,are highly reactive and react vigorously with water at room temperature to form metal hydroxides and hydrogen gas.
The chemical equation for the reaction is: $2Na + 2H_2O \to 2NaOH + H_2$.