s & p Block Elements Chemistry

Chemistry of 's' & 'p' block elements

• Focus on comparing the characteristics between the 1st and 2nd-row elements in the periodic table.

First Congeners

• Hydrogen (H) loses an electron (e-) to become a hydrogen ion (H+).
• H+ then takes an electron to become a neutral hydrogen atom (H•).
• Finally, H• captures another electron to become a hydride ion (H-).

Reaction Examples

• Oxygen gas (O2) reacts with two molecules of hydrogen gas (2H2) to produce two water molecules (2H2O), indicating oxidation of hydrogen or reduction of oxygen.
• Nickel (Ni) and hydrogen gas (H2) react to produce nickel hydride (NiH2), showing nickel oxidation or hydrogen reduction.
• Carbon (C) reacts with two molecules of hydrogen gas (2H2) to produce methane (CH4).

Binary Hydride Properties

• Binary hydrides can act as acids if attached to electronegative elements.
• Acidity decreases when moving left to right across a period and decreases down a group.
• Basicity decreases from right to left across a period.
• Ionic character relies on the atom attached to the hydrogen atom.
  • Ionic character decreases down the group for covalent hydrides.
  • Ionic strength increases for ionic hydrides.
• Bond strength decreases down a column in the periodic table.

Isotope Properties

• The bond strength increases upon isotopic exchange ('D' vs. 'H').
• ν = 2π√K/μ, μ = reduced mass = (m1*m2) / (m1 + m2)
  • H-H: μ = 0.5
  • H-D: μ = 0.66
  • ν value decreases.
• If ν is low, the bond is more stable.

Dissociation Rate

• If higher energy is required to reach the threshold vibrational frequency for breaking, then the bond is more stable.
• A bond with isotopic substitution requires greater energy than its lower isotope analog.
• An X-H bond is weaker than an X-D bond, causing the kinetic isotope effect.

Reactivity of Hydrogen Molecules

• Hydrogen gas (H2) reacts with oxygen gas (O2) to produce water (2H2O) but does not undergo the reaction spontaneously.
• The reaction requires a catalyst or electrical spark.
• Hydrogen gas is diamagnetic, while oxygen gas is paramagnetic.
• Oxygen is converted from paramagnetic to diamagnetic during reaction.

Polar X-H Bonds

• When an X-H bond is ionic and X is highly electronegative:
  • The X-H bond furnishes a δ+ charge on the H atom.
  • This interacts with another X-H molecule through hydrogen bonding.
• A very polar Xδ-Hδ+ bond easily releases Hδ+, acting as an acid.

Hydrogen with Electropositive Elements

• When hydrogen is attached to an electropositive element:
  • The X-H bond's polarity changes, gaining hydridic character.
  • It can reduce substrates easily.
  • The higher the electropositivity of X, the more hydridic the H becomes.
    • X-H + H2O → X+ + OH- + H2↑
    • X-H + RCHO → RCH2O-+X+
    • BH3 + O(Et) → H3B(OEt)

Hydridic H reacts with Lewis Acids

• Lithium Hydride (LiH) reacts with Aluminium Hydride (AlH3) to produce Lithium Aluminium Hydride (LiAlH4).
• Lithium Hydride (LiH) reacts with Boron Hydride (BH3) to produce Lithium Boron Hydride (LiBH4).

Structure of Water and Hydrogen Fluoride in Solid State

• Structure of H2O and HF in solid-state is a Zig-Zag structure.
• There is no specific structure and 'n' numbers of possible structures exits

Acid-Base Reaction

• 4LiH + SiCl4 → SiH4 + 4LiCl is an acid-base reaction.
  • Lewis Acid + base (Lewis)
  • Salt + neutral product
  • Silane

Superhydride

• NaH + B(Et2) → NaB(Et2)H3 is an example of superhydride

Metallic Hydrides

• d-orf-block metal reacted with H2→ M(H2)
• Sometimes @ elevated temperature to hightemp
• In several cases, it has been fond that clusters of metal hydrides are very stable and have been utilized for some practical use.
• For example Cex Tlz-x is a metallic conductor. Date is formed @550°C and once formed then it remains as roch solid.
• Few hydrides are recently employed for energy device construction e.g LiH compound for battery application
• Few metal hydrides e.g. - Ni, Pd, Pt reacts with H₂ to activate H₂ but still any metal hydride yet to report.
• Several molecular metal complexes undergo Homo and hetero lytic cleavage of H₂ at the transition metal centers, called TMH (Transition metal hydrides)

Reactivity of Largely Covalent Hydrides

• R3Sn-H, RC+H, and R3Si-H are very covalent hydrides.
• MR3, M3, and Al3 are more covalent and less ionic hydrides.
• R3Sn-H + R'X → R'H + R3SnX
• R3CH + R'X → No reaction
• R3Si-H + R'X → R3SiX + CH3-CH3 (less favorable and needs a cat)

Basicity of Hydrides

• Compare the basicity of the following hydrides:
  • NH3, PH3, AsH3, SbH3
  • NH3, H2S, H2O, SeH2

Reaction Types

• The H2 molecule generally exhibits three reaction pathways:
  • H2 → 2H•(radical)
  • H2 → X+ + H- (heterolytic)
  • H2 → X- + H+ (heterolytic)

Group 1 Metals (Alkali Metals)

• General formula: M + H2O → M-OH + 1/2H2↑
• Electronic Configuration: ns1 (29np is vacant)
  • Always shows metallic characters and forms cationic species
• Atomic radii increase and ionization energy decreases down the group.
• Redox potentials are very negative, making them ready to oxidize and good reducing agents.
• Corresponding cations have large hydration energy which decreases down the group due to increasing size.

Saline Hydride Formation

• 2M + H2 → 2MH
• Reaction with Oxygen:
  • 4Li + O2 → 2Li2O
  • 2Na + O2 → Na2O2
  • K + O2 → KO2
• Reaction with Water:
  • 2Li + 2H2O → 2Li(OH) + H2
  • 2Na + 2H2O → 2NaOH + H2
• All hydroxides easily provide hydroxide in the lab.

Hydroxide Basicity

• Basicity of these hydroxides increases down the series.
• Ionic character also increases down the series, same for corresponding chlorides.
• Lithium reacts with C powder to produce corresponding Carbide.
  • Potassium, Rubidium and Caesium form intercalation compounds with graphite.
• Dissolving in Liquid Ammonia:
  • All react (especially Li, Na, K) to produce a free electron compound (golden yellow solution).

Anomalous Properties of Lithium

• Forms highly covalent molecules.
• Forms Lithium Nitride (Li3N) when reacting with N2, or graphitilize2 when heated.
• Lithium salts are generally less soluble in water than other alkali metal salts.
• Alkali metals also react with weakly hydrocarbon proton donors, e.g.:
  • 2Na + 2C6H6 → Na+C5H5 + H2↑ is an organometallic compound (ionic).

Electrolysis of Molten Salts

• A typical synthesis like, 2NaCl(molten) → 2Na + Cl2↑ (Down's process).

Uses of Alkali Metals

• Lithium:
  • Light metal for aircraft parts.
  • Used in Li-ion batteries (high energy density).
  • Lubricant industry.
• Sodium:
  • Flavoring food.
  • Road de-icing (NaCl, NaI).
  • Na+ in ion exchange membranes.
  • Strong base.
• Potassium:
  • Soap industries.
  • KCI & K2SO4 fertilizer.
  • KCN (strong poison) is used in metal extraction industries.

Reactivity of Alkali Metals

• Sodium Hydride (NaH) + NH3→ NaNt2 + H2 is (strong base).

Halides of Alkali Metals

• Most alkali metal halides are stable salts and demonstrate very little reactivity (salt metathesis reaction).

Oxides of Alkali Metals

• 2Nall + Call2→ Na2lO3 + Call2:
  • Na2O2 + H2O → Naest + H2O2
  • KO2 + H2O → KOH + H2O2 + O2
  • 2KO2 + CO2→ 2K2CO3 + 3O2
• Li2O3 → Li2O + CO2

Reactions

• 2NO2+ is a Reaction which leads to Li2O + MOS

Oxides

• Cesium and Rubidium form oxides of differing compositions depending on formation conditions.

Group II Metals (Alkaline Earth Metals)

• Composed of: Be, Mg, Ca, Sr (+2 cations), Ba
• Greater mechanical strength and higher melting point.
• Reactivity increases down the group due to decreased ionization energy.
• Reactivity decreases down the group and are less reactive compared to alkali metals.
• M + 2H2O → M(OH)2 + H2 (Magnesium reacts only with hot water).

Reactions with Air

• Beryllium, Berilium is stable in air and slowly forms a layer of Beryllium Oxide (BeO).
• Magnesium and Calcium, react with oxygen to form Magnesium Oxide (MgO) and Calcium Oxide (CaO) on the surface.

Reactions with Powder

• Strontium and Barium in powder form react with O2/λ to produce corresponding oxides.
• Hydride complexes are formed by all metals form MH2 type complexes except Beryllium.
  • Beryllium typically gives Beryllium Hydride (BeH2) compound.
  • Releases H2 upon reaction with water.
  • The solubility of these hydroxides is lower than corresponding alkali metals (alkali metal hydroxides)

Metals

• Metal Halides:
  • All metals form MX2 (X=halide) type complexes in solution except Beryllium Chloride.
  • This occurs using the corresponding hydroxides with acids:
    • MCl2 + H2O
    • Solubility order, MgCl2 > CaCl2 > SrCl2 > BaCl2

Oxides and Hydroxides

• MgO and BeO are are insoluble in water.
• The reaction rate of these oxides with water increases down the group following formula MO + H2O → M(OH)2
• Reaction Rate: MgO < CaO < SrO (bad)

Metal Sulfides

• Metals react with Sulfur to become Metal sulfides following formula, M + S → MS metal sulfides
• Key complexes in this group are Chlorophyll → Mg2+ porphyrninato complexes.
• Beryllium Magnesium are less active with each other forming organometallic groups.

Few Notes to Remember

• Few notes important to keep in mind e.g. Beryl: Be3A12SiO3
• As well as dolomite and calcium silicate.
  • Dolomite can be magnesium (Main component Of cement)

Group II Metals

• Magnesium Hydride is the products of Magnesium and water:
  • MgH2 + H2O → Mg(OH)2 + H2↑
  • Beryllium chloride (BeCl2) is a highly covalent molecule.
  • Other chlorides are soluble *in water.
  • All MCl2 type complexes are insoluble *in water except BeF2.
• Beryllium Oxide + C+ Cl2→Becl2 + Carbon Monoxide.
• Beryllium(I)Bromide(BeBr2) + Beryllium(II)Fluoride(Be F2)-→Beryllium
• Oppisite reaction to synthesis
• Beryllium Chloride is linear in dimer.

Drying

• Calcium Flouride(CaF2) is useds as a drying agent.

Carbides

• 3Sr+N2→Sr3N2 + Hydrogen2O→ Magnesium (I)Hydrogen2 + Nitrogen 3