Chapter 10 The sBlock Elements Solutions
Question - 1 : - What are the common physical and chemical features of alkali metals?
Answer - 1 : -
Physical properties of alkali metals are as follows.
(1) They are quite soft and can be cut easily. Sodium metal can be easily cut using a knife.
(2) They are light coloured and are mostly silvery white in appearance.
(3) They have low density because of the large atomic sizes. The density increases down the group from Li to Cs. The only exceptionto this isK, which has lower density than Na.
(4) The metallic bonding present in alkali metals is quite weak. Therefore, they have low melting and boiling points.
(5) Alkali metals and their salts impart a characteristic colour to flames. This is because the heat from the flame excites the electron present in the outermost orbital to a high energy level. When this excited electron reverts back to the ground state, it emits excess energy as radiation that falls in the visible region.
(6) They also display photoelectric effect. When metals such as Cs and K are irradiated with light, they lose electrons.
Chemical properties of alkali metals
Alkali metals are highly reactive due to their low ionization enthalpy. As we move down the group, the reactivity increases.
(1) They react with water to form respective oxides or hydroxides. As we move down the group, the reaction becomes more and more spontaneous.
(2) They react with water to form their respective hydroxides and dihydrogens. The general reaction for the same is given as
(3) They react with dihydrogen to form metal hydrides. These hydrides are ionic solids and have high melting points.
(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides.
Since Li+ ion is very small in size, it can easily distort the electron cloud around the negative halide ion. Therefore, lithium halides are covalent in nature.
(5) They are strong reducing agents. The reducing power of alkali metals increases on moving down the group. However, lithium is an exception. It is the strongest reducing agent among the alkali metals. It is because of its high hydration energy.
(6) They dissolve in liquid ammonia to form deep blue coloured solutions. These solutions are conducting in nature.
The ammoniated electrons cause the blue colour of the solution. These solutions are paramagnetic and if allowed to stand for some time, then they liberate hydrogen. This results in the formation of amides.
In a highly concentrated solution, the blue colour changes to bronze and the solution becomes diamagnetic.
Discuss the general characteristics and gradation in properties of alkaline earth metals.
Answer - 2 : -
Generalcharacteristics of alkaline earth metals are as follows.
(i) The general electronic configurationof alkaline earth metals is [noble gas] ns2.
(ii) These metals lose two electrons to acquirethe nearest noble gas configuration. Therefore, their oxidation state is +2.
(iii)These metals have atomic and ionic radii smallerthan that of alkali metals. Also, when moved down the group, the effectivenuclear charge decreases and this causes an increase in their atomic radii andionic radii.
(iv) Since the alkaline earth metals have largesize, their ionization enthalpies are found to be fairly low. However, theirfirst ionization enthalpies are higher than the corresponding group 1 metals.
(v) These metals are lustrous and silvery whitein appearance. They are relatively less soft as compared to alkali metals.
(vi) Atoms of alkaline earth metals are smallerthan that of alkali metals. Also, they have two valence electrons formingstronger metallic bonds. These two factors cause alkaline earth metals to havehigh melting and boiling points as compared to alkali metals.
(vii) They are highly electropositive innature. This is due to their low ionization enthalpies. Also, theelectropositive character increases on moving down the group from Be to Ba.
(viii) Ca, Sr, and Ba impartcharacteristic colours to flames.
Ca– Brick red
Sr– Crimson red
Ba– Apple green
InBe and Mg, the electrons are too strongly bound to be excited. Hence, these donot impart any colour to the flame.
Thealkaline earth metals are less reactive than alkali metals and their reactivityincreases on moving down the group. Chemical properties of alkaline earthmetals are as follows.
(i) Reaction with air and water: Be and Mgare almost inert to air and water because of the formation of oxide layer ontheir surface.
(a) Powdered Be burns in air to form BeOand Be3N2.
(b) Mg, being more electropositive, burnsin air with a dazzling sparkle to form MgO and Mg3N2.
(c) Ca, Sr, and Ba react readily with airto form respective oxides and nitrides.
(d) Ca, Ba, and Sr react vigorously even withcold water.
(ii) Alkaline earth metals react with halogens athigh temperatures to form halides.
(iii) All the alkaline earthmetals, except Be, react with hydrogen to form hydrides.
(iv) They react readily with acids to form saltsand liberate hydrogen gas.
(v) They are strong reducing agents. However,their reducing power is less than that of alkali metals. As we move down thegroup, the reducing power increases.
(vi) Similar to alkali metals, the alkaline earthmetals also dissolve in liquid ammonia to give deep blue coloured solutions.
Question - 3 : - Whyare alkali metals not found in nature?
Answer - 3 : -
Alkalimetals include lithium, sodium, potassium, rubidium, cesium, and francium.These metals have only one electron in their valence shell, which they loseeasily, owing to their low ionization energies. Therefore, alkali metals arehighly reactive and are not found in nature in their elemental state.
Question - 4 : - Find the oxidation state of sodium in Na2O2.
Answer - 4 : -
Let the oxidation state of Na be x.The oxidation state of oxygen, in case of peroxides, is –1.
Therefore,
Therefore,the oxidation sate of sodium is +1.
Question - 5 : - Explainwhy is sodium less reactive than potassium?
Answer - 5 : -
Inalkali metals, on moving down the group, the atomic size increases and theeffective nuclear charge decreases. Because of these factors, the outermostelectron in potassium can be lost easily as compared to sodium. Hence,potassium is more reactive than sodium.
Question - 6 : - Comparethe alkali metals and alkaline earth metals with respect to (i) ionizationenthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.
Answer - 6 : -
| Alkali metals | | Alkaline earth metals |
| (i) | Ionization enthalpy: These have lowest ionization enthalpies in respective periods. This is because of their large atomic sizes. Also, they lose their only valence electron easily as they attain stable noble gas configuration after losing it. | (i) | Ionization enthalpy: Alkaline earth metals have smaller atomic size and higher effective nuclear charge as compared to alkali metals. This causes their first ionization enthalpies to be higher than that of alkali metals. However, their second ionization enthalpy is less than the corresponding alkali metals. This is because alkali metals, after losing one electron, acquires noble gas configuration, which is very stable. |
| (ii) | Basicity of oxides: The oxides of alkali metals are very basic in nature. This happens due to the highly electropositive nature of alkali metals, which makes these oxides highly ionic. Hence, they readily dissociate in water to give hydroxide ions. | (ii) | Basicity of oxides: The oxides of alkaline earth metals are quite basic but not as basic as those of alkali metals. This is because alkaline earth metals are less electropositive than alkali metals. |
| (iii) | Solubility of hydroxides: The hydroxides of alkali metals are more soluble than those of alkaline earth metals. | (iii) | Solubility of hydroxides: The hydroxides of alkaline earth metals are less soluble than those of alkali metals. This is due to the high lattice energies of alkaline earth metals. Their higher charge densities (as compared to alkali metals) account for higher lattice energies. |
Question - 7 : - Inwhat ways lithium shows similarities to magnesium in its chemical behaviour?
Answer - 7 : -
Similaritiesbetween lithium and magnesium are as follows.
(i) Both Li and Mg reactslowly with cold water.
(ii) The oxides of bothLi and Mg are much less soluble in water and their hydroxides decompose at hightemperature.
(iii) Both Li and Mg react withN2 to form nitrides.
(iv) Neither Li nor Mg formperoxides or superoxides.
(v) The carbonates of both arecovalent in nature. Also, these decompose on heating.
(vi) Li and Mg do not formsolid bicarbonates.
(vii) Both LiCl and MgCl2 are soluble inethanol owing to their covalent nature.
(viii) Both LiCl and MgCl2 are deliquescent in nature. Theycrystallize from aqueous solutions as hydrates, for example, 
and
Question - 8 : - Explainwhy alkali and alkaline earth metals cannot be obtained by chemical reductionmethods?
Answer - 8 : -
Inthe process of chemical reduction, oxides of metals are reduced using astronger reducing agent. Alkali metals and alkaline earth metals are among thestrongest reducing agents and the reducing agents that are stronger than themare not available. Therefore, they cannot be obtained by chemical reduction oftheir oxides.
Question - 9 : - Whyare potassium and cesium, rather than lithium used in photoelectric cells?
Answer - 9 : -
Allthe three, lithium, potassium, and cesium, are alkali metals. Still, K and Csare used in the photoelectric cell and not Li.
Thisis because as compared to Cs and K, Li is smaller in size and therefore,requires high energy to lose an electron. While on the other hand, K and Cshave low ionization energy. Hence, they can easily lose electrons. Thisproperty of K and Cs is utilized in photoelectric cells.
Question - 10 : - Whenan alkali metal dissolves in liquid ammonia the solution can acquire differentcolours. Explain the reasons for this type of colour change.
Answer - 10 : -
Whenan alkali metal is dissolved in liquid ammonia, it results in the formation ofa deep blue coloured solution.
Theammoniated electrons absorb energy corresponding to red region of visiblelight. Therefore, the transmitted light is blue in colour.
Ata higher concentration (3 M), clusters of metal ions are formed. This causesthe solution to attain a copper–bronze colour and a characteristic metalliclustre.