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Chemistry 🧪 > S block NCERT > Flashcards

S block NCERT Flashcards

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1
Q

Group 1 of the periodic table consists of which elements?

A 
Lithium - Li (3)
Sodium - Na (11) 
Potassium - K (19)
Rubidium - Rb (37)  
Ceasium - Cs (55) 
Francium - Fr (87)
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2
Q

Why are group 1 elements also called alkali metals?

A

They form hydroxides on reaction with water which are strongly alkaline in nature.

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3
Q

Group 2 of the periodic table consists of which elements?

A 
Beryllium - Be(4) 
Magnesium - Mg(12) 
Calcium - Ca(20) 
Strontium - Sr(38) 
Barium - Ba(56) 
Radium - Ra(88)
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4
Q

Which elements are called alkaline earth metals and why?

A

All group 2 elements except Beryllium(Be) are called alkaline earth metals because their oxides and hydroxides are basic in nature and their oxides are found in the Earth’s crust.

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5
Q

Relative abundance of alkali metals

A

Sodium and potassium are abundant.
Li, Rb and Cs have lower abundances.
Fr is highly radioactive and it’s longest lived isotope is Fr 223 with a half life of 21 minutes.

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6
Q

What is the half life of Fr?

A

21 minutes.

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7
Q

Relative abundance of alkaline earth metals

A

Calcium - 5th most abundant in the Earth’s crust.
Magnesium - 6th most abundant in the Earth’s crust.
Barium and Strontium have lower abundances.
Beryllium is rare and Radium is rarest of all with only 10^-10 % found in igneous rocks.

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8
Q

5th most abundant in the Earth’s crust

A

Calcium

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9
Q

6th most abundant in the Earth’s crust

A

Magnesium

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10
Q

Radium is a rare element, where is it found and how much of it is found?

A

Radium comprises 10^-10% of igneous rocks.

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11
Q

Lithium shows diagonal relationship with?

A

Mg

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12
Q

Beryllium shows diagonal relationship with?

A

Al

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13
Q

What is diagonal relationship

A

Li and Be show anomalous properties and resemble the second element of their following group. This is called diagonal relationship. Li shows it with Mg and Be with Al. Diagonal relationship occurs to similarity of ionic size and charge/ radius ratio of the elements.

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14
Q

Why does diagonal relationship occur?

A

Diagonal relationship occurs due to similarity in ionic size and charge/radius ratio of the elements.

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15
Q

Which elements are most electropositive and why?

A

Group 1 elements are the most electropositive elements because of the loosely held s electron in the valance shell of these atoms. They lose an electron to form monovalent atom that has noble gas configuration. Therefore they are not found in the free state.

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16
Q

Why are alkali metals never found in the free state in nature?

A

They lose an electron to form monovalent M+ ions which have noble gas configuration.

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17
Q

Which elements have the largest size in a particular period?

A

Alkali metals have the largest size in a particular period.

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18
Q

Atomic and ionic radii of alkali metals.

A

Atomic radii - largest in a particular period.
Monovalent ion is smaller than parent metal.
Increases down the group, Li is smallest and Cs is largest.

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19
Q

Ionization enthalpy of alkali metals.

A

Considerably low. Decreases down the group from Li to Cs. Max - Li Min -Cs

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20
Q

Hydration enthalpy of alkali metals.

A

Li+> Na+>K+>Rb+>Cs+

Maximum of Li which is why Li has hydrated salts eg LiCl.2H2O

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21
Q

Density of alkali metals

A

Li

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22
Q

Why is the melting and boiling point of alkali metals low?

A

Due to weak metallic bonding due to only a single s electron. MP and BP decreases down the group. therefore max for Li and min for Cs.

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23
Q

Why do alkali metals impart characteristic colour to an oxidizing flame?

A

Heat excites the outermost orbital electron to an higher energy level, when it returns to ground state, there is emission of radiation in the visible region.

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24
Q

Colours of alkali and alkali earth metals - oxidizing flame.

A 
Lithium - Crimson red 
Sodium - Golden yellow
Potassium - Pale violet 
Rubidium - Red violet 
Ceasium - Sky blue
Beryllium - no colour 
Magnesium - no colour 
Calcium - Brick red
Strontium - Crimson red 
Barium - Apple green
25
Q

Colour of oxidizing flame of Lithium

A

Crimson red

26
Q

Colour of oxidizing flame of sodium

A

golden yellow

27
Q

Colour of oxidizing flame of potassium

A

pale violet/ lilac

28
Q

Colour of oxidizing flame of rubidium

A

red violet

29
Q

Colour of oxidizing flame of ceasium

A

sky blue

30
Q

Colour of oxidizing flame of beryllium

A

no colour (electron is tightly bound)

31
Q

Colour of oxidizing flame of magnesium

A

no colour (electron is tightly bound)

32
Q

Colour of oxidizing flame of calcium

A

brick red

33
Q

Colour of oxidizing flame of strontium

A

crimson red

34
Q

Colour of oxidizing flame of barium

A

apple green

35
Q

Which alkali metals are used as electrodes in photoelectric cells?

A

potassium and ceasium

36
Q

Reducing nature of alkali metals

A

Lithium is maximum and sodium is minimum.

Li>Rb>Cs>K>Na

37
Q

Reactivity towards air - alkali metals

A

Tarnish in air and burn vigorously with oxygen.
Lithium forms oxide. (plus some peroxide)
4Li + O2 –>2 Li2O (oxide - M2O with oxidation no. of Li is +1 and that of oxygen is -2)
Sodium forms peroxide. (plus some superoxide)
2Na + O2 –> Na2O2(peroxide - M2O2 with oxidation number of sodium +1 and oxygen -1)
K, Rb and Cs form superoxide.
K + O2 –> KO2 (superoxide - MO2 with oxidation number of K +1 and oxygen -1/2).
Because alkali metals tarnish in air they are stored in kerosene oil, lithium is lighter so it is stored in paraffin wax.
Li also reacts with nitrogen in the air forming Li3N.

38
Q

Reactivity towards water - alkali metals

A

Alkali metals react with water to form hydroxide and dihydrogen. Li has the most negative E- value yet it’s reaction with water is less vigorous as compared to Na which has the least negative -E value. This is due to the small size and high hydration enthalpy of lithium.

39
Q

Alkali metals also react with proton donors such as

A

alcohols, gaseous ammonia and alkynes.

40
Q

Reactivity towards hydrogen - alkali metals

A

Alkali metals react with hydrogen at 673K to form ionic hydrides with high MP. LiH is covalent. (Li reacts at 1073K)

41
Q

Reactivity towards halogens - alkali metals

A

Alkali metals react vigorously with halogens to form ionic halides.
However due to polarization LiCl is covalent,
LiI is the most covalent.

42
Q

Order of reactivity of alkali metals

including with H and halogens

A

Cs> Rb> K > Na> Li

43
Q

Reducing nature - alkali metals

A

Reducing nature is dependent on ionization enthalpy, sublimation enthalpy and hydration enthalpy. In the aqueous (normal - given in NCERT state) - Li is the most and Na is the least. But in gaseous state, it increases down the group. Li has high reducing nature due to it’s high hydration enthalpy.

44
Q

Reducing nature of alkali metals is dependent on

A

Reducing nature is dependent on ionization enthalpy, sublimation enthalpy and hydration enthalpy.

45
Q

Solutions in liquid ammonia - alkali metals

A

Alkali metals dissolve in ammonia giving deep blue coloured conducting, paramagnetic solution. Colour is due to ammoniated electron but conductivity is due to both ammoniated electron and ammoniated cation. On standing, hydrogen gas is liberated and amide is produced. Colour changes from blue to bronze and solution changes from paramagnetic to diamagnetic.

46
Q

Uses of lithium

A

Lithium is used to make alloys.
With lead (Pb) it makes white metal which is used in bearings of motor engines.
With aluminum (Al) it is used to make aircraft parts.
With magnesium (Mg) it is used to make armor plates.
Li is used in thermonuclear reactions.
Li is also used in electrochemical cells.

47
Q

Alloy of Li and Pb is used for

A

making bearings of motor engines. It is called white metal.

48
Q

Alloy of Li and Al is used for

A

making aircraft parts.

49
Q

Alloy of Li and Mg is used for

A

armor plates.

50
Q

Uses of sodium

A

Na-Pb alloy is used to form organo- lead compounds PbEt4 and PbMe4 which were used as anti knock additives to petrol. Now we use lead free petrol.
Liquid Na is used as a coolant in fast breeder nuclear reactors.

51
Q

Uses of potassium

A

KCl is used as a fertilizer.
KOH is used in soft soaps and as an absorbent of CO2.
K is used in photoelectric cells.

52
Q

Use of ceasium

A

Photoelectric cells.

53
Q

Properties of oxides and hydroxides of alkali metals

A 
Peroxides and superoxides are stable with larger metals due lattice energy. Therefore order for oxide is
Li2O > Na2O>K2O
K2O2> Na2O2> Li2O2 
CsO2> RbO2> KO2
Super oxides are paramagnetic due to unpaired electron in the pi* 2p electron. Superoxides are also yellow orange in colour. Na2O2 - sodium peroxide is used as an oxidizing agent in inorganic chemistry. 
On reaction with water 
Oxides:
M2O + H2O --> MOH 
M2O2 + H2O --> MOH + H2O2 
MO2 + H2O --> MOH + H2O2 + O2
Hydroxides are white crystalline solids.
54
Q

How are alkali metal halides prepared?

A

On reaction of appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid.

55
Q

Properties of alkali metal halides.

A

Alkali metal hydrides can be prepared by reaction of appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid. The enthalpy of fluorides becomes less negative as we move down a group but the enthalpy of chlorides, bromides and iodides follow the reverse order. The enthalpy becomes less negative as we go from fluorides to iodides. The MP order is MF>MC>MBr>MI. All halides are soluble in water but LiF is less soluble in water due to it’s high lattice enthalpy and CsI is less soluble in water due to it’s low hydration enthalpy.
Lithium halides are covalent and are soluble in acetone, ethanol and ethyl acetate.
LiCl is soluble in pyridine.

56
Q

Stability of carbonates and hydrogen carbonates of alkali metals.

A

Increases down the group.

57
Q

Why does lithium carbonate not stable?

A

Lithium is very small and it polarizes the large carbonate ion, forming stable Li2O and CO2.

58
Q

Reason for anomalous properties of lithium.

A

Small size and high polarizing power, i.e. charge/radius ratio.

59
Q

Points of difference between lithium and other alkali metals - seven points.

A
  1. Li is hard and its MP and BP are higher than the rest of the alkali metals.
  2. It is the least reactive and the strongest reducing agent. It reacts with air to give oxide, Li2O and nitride, Li3N.
  3. It’s chloride is deliquescent - LiCl.2H2O.
  4. Lithium hydrogen carbonate does not exist in the solid state.
  5. On reaction with ethyne , Li does not form ethynide.
  6. LiNO3 on decomposition does not give nitrite but gives Li2O and NO2.
    LiNO3 –> Li2O + NO2
    NaNO3 –> NaNO2 + O2
  7. LiF and Li2O are not readily soluble in water.

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