Hydrogen Chemistry Mind Map

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Hydrogen (H) - Basic Properties Symbol: H Atomic No.: 1 Atomic Mass: 1.008 Electronic Configuration: $1s^1$ Lightest element Discovery: Cavendish (1766), named by Lavoisier (1783) Position of Hydrogen in PT Not fully justified due to resemblance with: Alkali metals: $1s^1$ configuration Forms unipositive ion ($H^+$) Shows $+1$ OS Affinity for electronegative elements (e.g., $H_2O, Na_2O, NaCl, HCl$) Acts as reducing agents Halogens: Requires $1e^-$ to complete inert gas configuration Similar I.E. Diatomic gas in nature ($H_2$) Forms covalent compounds (e.g., $CH_4, SiH_4, CCl_4, SiCl_4$) Occurrence of Hydrogen Most abundant element in universe Present in combined state: water, coal, animal & vegetable matter Essential constituent of all organic compounds Isotopes of Hydrogen Isotope Atomic No. Mass No. Neutron Protium ($^1H_1$) 1 1 0 Deuterium ($^2H_1$ or D) 1 2 1 Tritium ($^3H_1$ or T) 1 3 2 Tritium is radioactive with a half-life of approx. 12.33 years, emits low energy $\beta$-particles. Hydrides Classified into three types: Ionic or Saline or Salt-like Hydrides: Formed between H and electropositive G-1 & G-2 elements. Also known as stoichiometric hydrides. Properties: Lighter element hydrides (Li, Be, Mg) have significant covalent character. Crystalline, non-volatile, non-conducting in solid state. Conduct electricity in molten state, liberate $H_2$ at anode. Covalent or Molecular Hydrides: Binary compounds of H with non-metals from p-block. Mostly volatile with low B.P. Classification: Electron Deficient: Central atom lacks complete octet (e.g., $BH_3, MgH_2, BeH_2$). Electron Precise: Central atom has complete octet (e.g., $CH_4, SiH_4$ - G-14 elements). Electron Rich: Central atom has lone pairs of electrons (e.g., $NH_3, PH_3, H_2O, H_2S$). Metallic or Non-stoichiometric Hydrides: Also known as interstitial hydrides. Formed by transition metals of G-3, 4, 5. G-6: Only Chromium forms CrH. G-7, 8, 9 do not form hydrides (hydride gap). Latest study: Ni, Pd, Ce, Ac are interstitial. Generally non-stoichiometric; composition varies with temperature and pressure. Examples: $TiH_{1.73}, CeH_{2.7}, LaH_{2.8}$ etc. Properties closely related to parent metal. Strong reducing agents due to free hydrogen in metal lattice. Water ($H_2O$) Human body: ~65% water; Plants: ~95% water. Physical Properties of Water Solid: Ice, Liquid: Water, Gas: Water vapour F.P.: $273.15 K$ B.P.: $373.15 K$ Max. Density: $1 gm/cm^3$ at $4^\circ C$ Due to hydrogen bonding, even covalent compounds like alcohol and carbohydrates dissolve in water. Structure of Water Gas phase: Bent molecule, H-O-H bond angle $104.5^\circ$, O-H bond length $95.7 pm$. Highly polar in nature. Oxygen atom has two lone pairs. Water in Crystalline Form (Ice) Ice is the crystalline form of water. At atmospheric pressure, ice is hexagonal. At low temp., condenses to cubic form. Density of ice is less than water, so ice floats. Chemical Properties of Water Amphoteric Nature: Behaves as both acid and base (Bronsted-Lowry concept). Acidic reaction: $H_2O(l) + NH_3(aq) \rightleftharpoons OH^-(aq) + NH_4^+(aq)$ Basic reaction: $H_2O(l) + H_2S(aq) \rightleftharpoons H_3O^+(aq) + HS^-(aq)$ Autoprotolysis: $H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$ pH of water is 7 (neutral). Oxidising and Reducing Nature: Can act as both: As oxidising agent: $2H_2O + 2Na \to 2NaOH + H_2$; $2H_2O + 2e^- \to 2OH^- + H_2$ As reducing agent: $2F_2 + 2H_2O \to 4HF + O_2$ Hydrolysis Reaction: Strong hydrating tendency. Hydrolyses many compounds (oxides, halides, carbides etc.). Examples: $SiCl_4 + 2H_2O \to SiO_2 + 4HCl$ $P_4O_{10}(s) + 6H_2O(l) \to 4H_3PO_4(aq.)$ $CaC_2 + 2H_2O \to Ca(OH)_2 + C_2H_2$ $SO_2 + H_2O \to H_2SO_3$ Hydrates Many salts crystallise as hydrated salts from aqueous solutions. Three types: Coordinated water: e.g., $[Ni(H_2O)_6]^{2+}$, $[Cr(H_2O)_6]^{3+}$ Interstitial water: e.g., $BaCl_2 \cdot 2H_2O$ Hydrogen bonded water: e.g., $[Cu(H_2O)_4]^{2+}SO_4 + H_2O$ in $CuSO_4 \cdot 5H_2O$, $FeSO_4 \cdot 7H_2O$ Hard Water and Soft Water Hard Water: Does not lather easily with soap. Presence of calcium and magnesium salts (hydrogen carbonates, chlorides, sulphates). Soft Water: Lathers readily with soap. Examples: Rain water, Distilled water. Types of Hardness of Water Temporary Hardness: Due to bicarbonates of calcium and magnesium. Removed by simple boiling. Permanent Hardness: Due to chlorides and sulphates of calcium and magnesium. Cannot be removed by simple boiling. Removed by Permutit or Calgon method. Heavy Water ($D_2O$) Oxide of deuterium ($^2H_1$). Discovered in 1932 by Urey and Washburn. Colourless, odourless, tasteless liquid. Used in preparation of Deuterium Compounds: $CaC_2 + D_2O \to CD_2 + Ca(OD)_2$ $SO_3 + D_2O \to D_2SO_4$ $Al_4C_3 + 12D_2O \to 3CD_4 + 4Al(OD)_3$ Used as moderator in nuclear reactors. Properties of $H_2O$ vs $D_2O$ Property $H_2O$ $D_2O$ F.P. $0^\circ C$ $3.8^\circ C$ B.P. $100^\circ C$ $101.42^\circ C$ Solubility of substances High Low Dihydrogen ($H_2$) Colourless, odourless, tasteless gas. Sparing soluble in water (non-polar). Lightest gas, density compared to air is $0.0695$. Melting point: $13.8 K$; Boiling point: $20.4 K$. Flammable, but does not help burn. Can be liquefied at low temperature and high pressure. Preparation of Dihydrogen (Lab Method) Reaction of Acid on Zinc Metal: Dilute HCl on Zn produces $H_2$ and $ZnCl_2$. $Zn + 2HCl \to ZnCl_2 + H_2$ Reaction of a Base on Zinc Metal: Aqueous NaOH on Zn produces $H_2$ and sodium zincate ($Na_2ZnO_2$). $Zn + 2NaOH \to Na_2ZnO_2 + H_2$ Preparation of Dihydrogen (Industrial Method) Electrolysis of Water: Acidified water using Pt electrodes. $2H_2O \to H_2 + O_2$ Electrolysis of Conc. Brine Solution: Produces $H_2$ with dichlorine and NaOH. $2NaCl + 2H_2O \to 2NaOH + H_2 + Cl_2$ Reaction of Steam over Coke or Hydrocarbon: Hydrocarbon or coke with steam at high temp. and catalyst. $C_nH_{2n+2} + nH_2O \xrightarrow{1270 K, Ni} nCO + (3n+1)H_2$ Mixture of $CO + H_2$ is called "Water Gas". Chemical Reactions of Dihydrogen Reaction with Halogens: Forms hydrogen halides. Order of reactivity: $F_2 > Cl_2 > Br_2 > I_2$ $H_2 + X_2 \to 2HX$ (where X = F, Cl, Br) Example: $H_2 + Cl_2 \to 2HCl$ Reaction with Dioxygen: Forms water (highly exothermic). $2H_2 + O_2 \xrightarrow{\text{Catalyst or Temp.}} 2H_2O \quad \Delta H = -285.9 KJ mol^{-1}$ Reaction with Dinitrogen: Forms ammonia (Haber Process). $N_2 + 3H_2 \xrightarrow{673K \text{ or } 200atm} 2NH_3 \quad \Delta H = -92.6 KJ mol^{-1}$ Reaction with Metals: Forms metallic hydrides at high temp. $2M + H_2 \to 2MH$ Reaction with Metal Ions and Metal Oxides: Reduces some metal ions and metal oxides (less than iron) to corresponding metal. $H_2(g) + Pd^{2+}(aq) \to Pd(s) + 2H^+(aq)$ $M_xO_y(s) + yH_2(g) \to xM(s) + yH_2O(l)$ Reaction with Unsaturated Hydrocarbons: Adds to form saturated hydrocarbons (hydrogenation). $H_2C=CH_2 + H_2 \xrightarrow{Ni/Pd/Pt} H_3C - CH_3$ $HC \equiv CH + H_2 \to H_2C=CH_2 + H_2 \to H_3C - CH_3$ Hydrogen Peroxide ($H_2O_2$) Discovered by French Chemist Thenard. Structure of $H_2O_2$ Non-linear, non-planar molecule with an open book structure. -O-O- linkage is called peroxide linkage. Solid Phase: H-O-H angle $101.9^\circ$, O-H length $0.98 \AA$, O-O length $1.46 \AA$. Gas Phase: H-O-H angle $111.5^\circ$, O-H length $0.95 \AA$, O-O length $1.47 \AA$. Preparation of Hydrogen Peroxide Action of sulphuric acid on sodium peroxide or barium peroxide. Electrolysis of $50\% H_2SO_4$: Produces $H_2$ at cathode and $H_2S_2O_8$ at anode. Distillation of $H_2S_2O_8$ at low pressure yields $30\% H_2O_2$. Physical Properties of $H_2O_2$ Colourless, odourless liquid. Density $1.46$ at $0^\circ C$ (higher than water due to H-bonding). Boiling point: $85^\circ C$. Dipole moment: $2.1 D$. Diamagnetic. Concentration of $H_2O_2$ $10$ volume $H_2O_2$: $1 ml$ of $H_2O_2$ solution releases $10 ml$ of $O_2$ at NTP. $10$ volume $H_2O_2$ solution contains $3.04\% H_2O_2$. Conversion of volume concentration to percentage concentration. Normality of $20$ volume $H_2O_2$. Chemical Properties of $H_2O_2$ Unstable liquid, decomposes into water and oxygen upon standing or heating. Powerful oxidising agent. Powerful reducing agent. Acts as a bleaching agent due to release of nascent oxygen. $H_2O_2 \to H_2O + [O]$ Bleaching action is due to oxidation of colouring matter to a colourless product. Not stored in glass bottles (alkali metal oxides in glass catalyse decomposition). Stored in paraffin wax coated glass, plastic, or Teflon bottles. Uses of $H_2O_2$ Bleaching delicate articles (wool, hair, feathers, ivory). Antiseptic and germicide for washing wounds, teeth, ears. Manufacture of sodium perborate, sodium percarbonate. Used in high quality detergents. Oxidant for rocket fuel. Detection of $Ti, V, Cr$ ions (forms peroxides with characteristic colours).