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Hydrogen for XI std chemistry

Cbse. Hydrogen for XI std chemistry. INTRODUCTION. Hydrogen, chemical element that exists as a gas at room temperature. When hydrogen gas burns in air, it forms water. French chemist Antoine Lavoisier named hydrogen from the Greek words for “water former.”

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Hydrogen for XI std chemistry

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  1. Cbse Hydrogen for XI std chemistry

  2. INTRODUCTION • Hydrogen, chemical element that exists as a gas at room temperature. When hydrogen gas burns in air, it forms water. French chemist Antoine Lavoisier named hydrogen from the Greek words for “water former.” • Hydrogen has the smallest atoms of any element. A hydrogen atom contains one proton, and only one electron . The proton is the center, or nucleus, of the hydrogen atom, and the electron travels around the nucleus. • Pure hydrogen exists as hydrogen gas, in which pairs of hydrogen atoms bond together to make molecules.

  3. The hydrogen atomconsisting the protonin the centre or the nucleus of the hydro-gen atom and the ele-ctron travelling aroun-d the nucleus.

  4. POSITION IN THE PERIODIC TABLE • Hydrogen is the first element in the periodic table of the elements and is represented by the symbol H. • Hydrogen, with only one proton, is the simplest element. It is usually placed in Period 1 and Group 1 of the periodic table. • Hydrogen can combine chemically with almost every other element and forms more compounds than does any other element. These compounds include water, minerals, and hydrocarbons—compounds made of hydrogen and carbon—such as petroleum and natural gas.

  5. Position of hydrogen in the periodic table

  6. OCCURRENCE OF DIHYDROGEN • Hydrogen is the tenth most common element on Earth. Because it is so light, though, hydrogen accounts for less than 1 percent of Earth's total mass. It is usually found in compounds. Pure hydrogen gas rarely occurs in nature, although volcanoes and some oil wells release small amounts of hydrogen gas. • Hydrogen is in nearly every compound in the human body. For example, it is in keratin, the main protein that forms our hair and skin, and in the enzymes that digest food in our intestines. Hydrogen is in the molecules in food that provide energy: fats, proteins, and carbohydrates.

  7. Presence of hydrogen in volcanoes and in our food particles

  8. Cont…. • Hydrogen accounts for about 73 percent of the observed mass of the universe and is the most common element in the universe. • Hydrogen atoms were the first atoms to form in the early universe and that the atoms of the other elements formed later from the hydrogen atoms. • About 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent.

  9. Cont…. • Common Molecules: Many common molecules contain hydrogen. In these molecules, butane contains ten hydrogen atoms, ammonia contains three hydrogen atoms, and water contains two hydrogen atoms.

  10. INTERIOR OF THE SUN • The Sun’s energy is produced in the core through nuclear fusion of hydrogen atoms into helium. Gases in the core are about 150 times as dense as water and reach temperatures as high as 16 million degrees C (29 million degrees F).

  11. ISOTOPES OF HYDROGEN • Atoms of an element that have different numbers of neutrons in their nuclei are called isotopes of that element. • Isotopes all usually share the same chemical behavior, but have different masses. • The isotopes of hydrogen are protium, deuterium, and tritium. Hydrogen always has one proton in its nucleus.

  12. PREPARATION OF DIHYDROGEN • Laboratory preparation of dihydrogen: 1.It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid. The chemical equation for this reaction is the following: Zn + 2HCl → ZnCl2 + H2 2.It can also be prepared by the reaction of zinc with aqueous alkali. The chemical equation for this reaction is the following: Zn +2NaOH Na2ZnO2 + H2 (Sodium zincate)

  13. Dihydrogen in 3D

  14. Cont…. • Commercial production of dihydrogen: 1. Electrolysis of acidified water using platinum electrodes gives hydrogen. 2 H2O   electrolysis 2H2 + O2 This chemical equation shows that two water molecules (with electricity), form two molecules of hydrogen gas and one molecule of oxygen gas. 2.High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

  15. Commercial production of dihydrogen

  16. Cont…. 3. It is obtained as a byproduct in the manufacture of sodium hydroxide & chlorine by the electrolysis of brine solutions . The reactions that takes place are: At anode : 2Cl- Cl2 +2e- At cathode: 2H2O + 2e- H2 + 2OH- The overall reaction is 2Na+ + 2Cl- +2H2O Cl2 + H2 + 2Na+ + 2OH-

  17. Cont…. 4. Reaction of steam on hydrocarbons at high temperature in the presence of catalyst yields hydrogen. e.g., CH4 + H2O 1270KCO + 3H2 Ni • The mixture of CO & H2 is called water gas. • It is used for synthesis of methanol & a number of hydrocarbons, therefore it is called synthesis gas or ‘syngas’. • The production of dihydrogen can be increased by reacting carbon monoxide with steam in the presence iron chromate as catalyst. CO + H2O 673K CO2 + H2 Catalyst • This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.

  18. PHYSICAL PROPERTIES OF H2 • Dihydrogen is a : • Colourless , • Odourless • Tasteless • Combustible gas • Lighter than air • Insoluble in water • It’s melting point – 18.73 K & boiling point – 23.67 K

  19. CHEMICAL PROPERTIES OF H2 • Hydrogen gas does not usually react with other chemicals at room temperature, because the bond between the hydrogen atoms is very strong and can only be broken with a large amount of energy. • Since its orbital is incomplete with 1s1 electronic configuration, it does combine with almost all the elements . It accomplishes reactions by: 1.loss of one e- to give H+ 2.gain of an e- to form H- 3.sharing electrons to form a single covalent bond.

  20. CHEMISTRY OF DIHYDROGEN • Reaction with halogens: It reacts with halogens, X2 to give hydrogen halides, HX, H2+X2 2HX (X= F, Cl, Br, I) While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst. • Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic. 2H2 + O2catalystorheating 2H2O ; H = -285.9 kJ mol-1

  21. Cont…. • Reaction with dinitrogen: With dintrogen it forms ammonia. 3H2 +N2673K,200atm 2NH3; H=-92.6 kJ mol-1 This is the method for the manufacture of ammonia by Haber process. Haber Process: German chemist and Nobel laureate Fritz Haber developed an economical method of producing ammonia from air and seawater. In his process, nitrogen is separated from the other components of air through distillization. Hydrogen is obtained from seawater by passing an electric current through the water. The nitrogen and hydrogen are combined to form ammonia (NH3).

  22. Cont…. • Reaction with metals: Hydrogen also forms ionic bonds with some metals, at a high temperature, creating a compound called a hydride. H2 +2M 2MH Where M is an alkali metal (e.g. lithium, sodium, potassium, rubidium, cesium, and francium.) • Reactions with metal ions & metal oxides: It reduces some metal ions in aqueous solution & oxides of metals (less active than iron ) into corresponding metals. H2+Pd 2+ Pd + 2H+ yH2 +MxOy xM + yH2O

  23. Cont…. • Reactions with organic compounds: 1.Hydrogenation of vegetable oils using nickel as catalyst gives edible fats. (margarine & vanaspati ghee). 2.Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols. H2+CO+RCH=CH2 RCH2CH2CHO H2 +RCH2CH2CHO RCH2CH2CH2OH

  24. USES OF DIHYDROGEN • The largest use of dihydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid & nitrogenous fertilizers. • Dihydrogen is used in the manufacture of vanaspati fat. • It is used in the manufacture of bulk organic chemicals, particularly methanol. CO + 2H2catalystcobalt CH3OH • It is widely used for the manufacture of metal hydrides. • It is used for the preparation of hydrogen chloride, a highly useful chemical.

  25. Cont…… • In metallurgical processes, it is widely used to reduce heavy metal oxides to metals. • Atomic hydrogen & oxy-hydrogen torches find use for cutting & welding purposes. • It is used as a rocket fuel in space research. • Dihydrogen is used in the fuel cells for generating electrical energy. It has many advantages over the conventional fossil fuels & electric power.

  26. Various uses of dihydrogen

  27. HYDRIDES • Dihydrogen also forms ionic bonds with some metals, at a high temperature, creating a compound called a hydride. • If E is the symbol of an element then hydride can be expressed as EHX (e.g. MgH2) or EmHn (e.g. B2H6). • The hydrides are classified into three categories: 1.Ionic or saline or saltlike hydrides. 2.Covalent or molecular hydrides. 3.Metallic or non-stoichiometric hydrides.

  28. Nearly all elements are able to form hydride compound

  29. IONIC OR SALINE HYDRIDES • These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character. • Covalent character is found in the lighter metal hydrides (e.g. LiH, BeH2 & MgH2). • The ionic hydrides are crystalline, non-volatile & non-conducting in solid state. • Their melts conduct electricity & on electrolysis liberate dihydrogen gas at anode, which confirms the existence of H-ion. 2H-(melt) anode H2+2e- • Saline hydrides react violently with water producing dihydrogen gas . NaH + H2O NaOH + H2

  30. COVALENT OR MOLECULAR HYDRIDE • Dihydrogen forms molecular compounds with most of the p-block elements. For e.g. CH4, NH3, H2O & HF. • Hydrogen compounds of non metals have also been considered as hydrides. Being covalent they are volatile compounds. • Molecular hydrides are further classified according to the relative number of electrons & bonds in their Lewis structure into: 1.Electron-deficient 2.Electron-precise 3.Electron-rich hydrides.

  31. METALLIC HYDRIDES • These are formed by many d-block & f-block elements. • The metals of group 7,8 & 9 do not form hydride. • These hydrides conduct heat & electricity though not as efficiently as their parent metals do. • Unlike saline hydrides, they are almost non-stoichiometric, being deficient in hydrogen. For e.g. LaH2.87 & YbH2.55. • Law of constant composition does not hold good. • The property of absorption of hydrogen on transition metal is widely used in catalytic reduction/hydrogenation reactions for the preparation of large number of compounds. • Some of the metals can accommodate a very large volume of hydrogen & can be used as its storage media.

  32. Water • A major part of all living organisms is made up of water. • Human body has about 65% & some plants have as much as 95% water. • It is a crucial compound for the survival of all life forms. • It is a solvent of great importance.

  33. Different uses of water

  34. Physical properties of water • It is a colourless & tasteless liquid. • The unusual properties of water in the condensed phase (liquid & solid) are due to the presence of extensive hydrogen bonding between water molecules. • Water has a higher specific heat, thermal conductivity, surface tension, dipole moment & dielectric constant when compared to other liquids. • It is an excellent solvent for transportation of ions & molecules required for plant & animal metabolism. • Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol & carbohydrates dissolve in water.

  35. STRUCTURE OF WATER • In the gas phase water is a bent molecule with a bond angle of 104.50 , and O-H bond length of 95.7 pm. • It is a highly polar molecule . • In the liquid phase water molecules are associated together by hydrogen bonds. • Density of water is more than that of ice.

  36. Hydrogen Bonding in Water: Hydrogen bonds are chemical bonds that form between molecules containing a hydrogen atom bonded to a strongly electronegative atom . Because the electronegative atom pulls the electron from the hydrogen atom, the atoms form a very polar molecule, meaning one end is negatively charged and the other end is positively charged. Hydrogen bonds form between these molecules because the negative ends of the molecules are attracted to the positive ends of other molecules, and vice versa. Hydrogen bonding makes water form a liquid at room temperature.

  37. STRUCTURE OF ICE: • Ice has a highly ordered three dimensional hydrogen bonded structure. • Examination of ice crystals with x-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms a distance of 276pm. • Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

  38. Structure of ice

  39. Chemical properties of water • Amphoteric nature: it has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the Bronsted sense it acts an acid with NH3 and a base with H2S. H2O+ NH3 OH- + NH4+ H2O+ H2S H3O++ HS- The auto-protolysis (self- ionization) of water takes place as follows: H2O +H2O H3O+ +OH- acid-1 base-2acid-2 base-1

  40. REDOX REACTIONS INVOLVING WATER • Water can be easily reduced to dihydrogen by highly electropositive metals. 2H2O +2Na 2NaOH +H2 Thus ,it is a great source of dihydrogen. • Water is oxidised to O2 during photosynthesis. 6CO2 +12H2O C6H12O6 + 6H2O +6O2 • With fluorine also it is oxidised toO2. 2F2 + 2H2O 4H+ + 4F- +O2

  41. Hydrolysis reaction • Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent& some ionic compounds are hydrolysed in water. P4O10 +6H2O 4H3PO4 SiCl4 +2H2O SiO2 + 4HCl N3- + 3H2O NH3 +3OH-

  42. A hydrolysis process generally involves water

  43. HYDRATES FORMATION • From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz., (i) coordinated water e.g., [Cr(H2O)6 ]3+ 3Cl- (ii) interstitial water e.g., BaCl2.2H2O (iii) hydrogen-bonded water e.g., [Cu(H2O)4]2+SO42-.H2O in CuSO4.5H2O

  44. HARD & SOFT WATER: • The presence of calcium & magnesium salts in the form of hydrogen carbonate, chloride & sulphate in water makes water ‘hard’. • Hard water does not give lather with soap. • Water free from soluble salts of calcium & magnesium is called soft water. • It gives lather with soap easily.

  45. Cont…. • Hard water forms scum/precipitate with soap. • Soap containing sodium stearate( C17H35COONa) reacts with hard water to precipitate out Ca/Mg stearate. 2C17H35COONa(aq)+M2+(aq) M(C17H35COO)2 +2Na+(aq); M is Ca/Mg • Hard water is harmful for boilers, because of deposition of salts in the form of scale. This reduces the efficiency of the boiler. • The hardness of water is of two types: • 1.Temporary hardness. • 2.Permanent hardness.

  46. TEMPORARY HARDNESS: • Temporary hardness is due to the presence magnesium & calcium hydrogen carbonates. It can be removed by: 1.Boiling: During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg (OH)2 & Ca(HCO3)2 is changed to insoluble CaCO3. It is because of high solubility product of Mg(OH)2 as compared to that of MgCO3, that Mg(OH)2 is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water. Mg(HCO3)2heating Mg(OH)2 + 2CO2 Ca(HCO3)2heating CaCO3 + H2O + CO2

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