This article is on the Hydrogen Notes Class 11 of Chemistry. The notes on hydrogen of class 11 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the chapter Hydrogen of Class 11 Chemistry for the students so that they can get a quick glance of the chapter.
The most abundant element of the universe (≈76%), the fifteenth most abundant in the earth, and the tenth most abundant in earth crust Hydrogen is the lightest, smallest and first element of the periodic table with atomic number one.
In nature, hydrogen is found in the free as well as in the combined state. In the atmosphere, the presence of hydrogen is found to a very minute (only one part in a million by volume) owing to its high chemical reactivity. Low abundance in the atmosphere is also due to the earth’s gravitational field which is too small to hold on to so light an element.
POSITION OF HYDROGEN IN PERIODIC TABLE
Hydrogen is a unique element, it has only one electron in the 1s-orbital and no electrons in the other shell. It may donate one electron forming H+ (like alkali metals) or may accept one electron forming H– (like halogens). It ranges in character from being a strong Lewis base (H–) to being a strong Lewis acid (H+). It is this nature which makes it difficult for us to assign hydrogen its proper place in the periodic table because of its resemblance with halogens as well as with alkali metals.
Resemblance with Alkali Metals
Hydrogen resembles alkali metals in the following respects:
(a) Electronic Configuration – Like alkali metals, hydrogen also has one valence electron.
(b) Electropositive Character – Like other alkali metals, Hydrogen readily forms the Hydronium ion, which is very stable.
(c) Reaction with Non-Metals – Forms sulphides, oxides and halides same as alkali metals.
(d) Reducing action – Like alkali metals, hydrogen also acts as a strong reducing agent.
E.g., F2 + H2 → 2HF
Resemblance with Halogens
(a) Electronic configuration: Like halogens, hydrogen is short of one electron for stability in the outer orbit as explained below
H + e– → H– F + e– → F–
(b) Atomicity: All halogens exist as diatomic covalent molecules F2, Cl2, etc. In a similar way, hydrogen is a diatomic covalent molecule. E.g. H2, Cl2, Br2.
(c) Physical state: Hydrogen is a gas like F2 and Cl2.
(d) Ionization potential: I.P of hydrogen is very high, same as in the case of halogens.
H → 13.12 eV, F → 16.8 eV, Cl → 12.55 eV.
(e) Reaction with non-metals: Both, halogens and hydrogen combine with other non-metals by forming single bonds to give compounds like CCl4, CH4, and SiCl4.
Isotopes of Hydrogen
Isotopes are the different forms of the same element, which have the same atomic number but different mass numbers.
Methods of Preparation of Dihydrogen
- Laboratory Preparation
(i) From Acids
Metals which lie above hydrogen in electrochemical series i.e. metals with positive E°OP. do not displace H2 from dilute mineral acid like HCl or H2SO4. Metals having negative E°OP (i.e. metals lying below hydrogen in ECS) displace H2 from acids, e.g., Zn, Mg, Fe etc. displace H2 from acids.
Zn + H2SO4 → ZnSO4 + H2↑
(ii) From Alkali
Certain metals like Zn, Sn, Al, Pb, Si etc. (amphoteric metals) react with boiling NaOH to liberate H2.
Zn + 2NaOH → Na2ZnO2 (Sodium zincate) + H2 ↑
- Manufacture of H2 on Commercial Scale
(iii) By Electrolysis of Water
The water containing a small amount of an acid or alkali (about 15-20%) is electrolysed in an electrolytic cell. The anode and cathode are separated by an asbestos diaphragm. The cathode is usually made up of iron and anode usually of nickel.
At anode: OH– ions are discharged and oxygen is evolved.
4OH– → 2H2O + O2 + 4e–
At cathode: H+ ions are discharged and hydrogen is evolved.
2H+ + 2e– → H2↑
(iv) Bosch Process
This process is most common, and in this process, first steam is passed over hot coke at 1000°C to obtain water gas or Synthesis gas or Syn gas.
C (s) + H2O (v) + –Ni/10000C→ CO + H2 (Syn gas)
This process of obtaining ‘Synthesis gas’ from coal or coke is called “Coal gasification”
(v) From Hydrocarbons
(a) Thermal Cracking: CH4 –Catalyst/∆ → C + 2H2 ↑
(b) By partial oxidation: CH4 (s) + H2O (v) + –Ni/10000C→ CO + 3H2 (Natural gas) (Syn gas)
Physical Properties of Dihydrogen
(a) Colourless, odourless, tasteless gas
(b) Sparingly soluble in water due to non-polar nature
(c) Density is 0.09 gm/lit
(d) B.P is 20.4 K
(e) Pd metal can adsorb H2 gas
Chemical Properties of Dihydrogen
(a) Nature: H2 is neutral and so it does not aﬀect litmus.
(b) Combustion: H2 is highly combustible and in presence of air, it burns with pale blue ﬂame. It does not support combustion.
2H2 + O2 → 2H2O(l) ∆H = –285.9 kJmol-1
The reaction is highly exothermic and its calorific value is higher than other fuels.
(c) Reducing action: Dihydrogen reduces oxides of certain less electropositive metals (generally less electropositive than Zn and placed above H in ECS) like Fe, Pb, Cu etc. The product is metal, e.g.,
Fe2O3 +3H2 (g) –∆ → 2Fe + 3H2O
CuO (s) + H2 (g) –∆→ Cu + H2O
(d) Reaction with non-metals: With non-metals, hydrogen forms covalent hydrides, e.g., H2 + ½ O2 → H2O
3H2 + N2 –750K/Fe, Mo → 2NH3 (Haber’s process)
(e) Reaction with Metals: Under suitable conditions, H2 reacts with metals to form hydrides which are chieﬂy ionic. Alkali metals and alkaline earth metals (except Be) react with H2 directly.
2M+ H2 → 2MH (M = Li, Na, K, Rb etc.)
M+H2 → MH2 (M = Mg, Ca, Ba etc.)
Binary compounds of hydrogen with other elements are generally called hydrides, but strictly speaking, this term should be applied to compounds of hydrogen with elements, which are less electronegative than hydrogen. They may be of MHn or MmHn type.
1. Ionic Hydrides
(a) They are hydrides of elements having low electronegativity (ranging from 0.9 to 1.2 usually). They are formed by alkali metals, alkaline earth metals (except Be and Mg) and some lanthanides.
(b) In these hydrides, hydrogen accepts electrons from combining elements and exists as H–.
(c) These hydrides can be prepared by heating (≈ 1000 – 1100 K) metals with hydrogen directly.
E.g. 2Na + H2 → 2NaH
Some Important Chemical Properties
(a) Stability: Ionic hydrides have quite high heats of formation and consequently high stability. On moving down in the group, the stability decreases which may be attributed to a poor overlap between the relatively smaller 1s orbital of hydrogen and larger s-orbital of heavier metals. LiH > NaH > KH > RbH > CsH
(b) Gets hydrolyzed by giving alkali and H2 gas.
(c) At high temperatures, act as strong reducing agents.
2. Covalent Hydrides
(a) These hydrides are formed mainly by p-block elements (except noble gases) and by some s-block elements like Be and Mg.
(b) These hydrides usually consist of discrete covalent molecules held together by weak ‘Vander Waals’ forces and so these hydrides are usually volatile. The melting points and boiling points of covalent hydrides are low. Some of them are liquids or even solids in a few cases.
(c) They do not conduct electricity and their general formula may be MHX (for s-block metals) or MH(8–n) (for s-block elements), where n is the number of valence electrons. This generalization is not valid for the boron family.
Covalent hydrides can be electron deficient also such as DIBORANE B2H6 and other such bridge bond hydrides. They can be electron precise as well as electron-rich such as CH4 and NH3 respectively.
3. Metallic / Interstitial Hydrides
(a) These hydrides are formed by d- and f-block metals, with electronegativity ranging from 1.2 to 1.4.
(b) These hydrides are usually non-stoichiometric (e.g. TiH 1.73, LaH 2.8 etc.) and these hydrides have properties similar to those of parent metals i.e. why they are called metallic hydrides. In these hydrides, hydrogen occupies some interstitial sites in the metallic lattice i.e. why they are called interstitial hydrides. However, it is not certain whether the interstitial hydrogen is present as H or H+ with delocalised electrons.
Structure of Water
The molecule of water is V-shaped and its structure is given below: In water, the central oxygen atom is sp3 hybridised. Out of four sp3 hybrid orbitals, two form σ bond with s-orbital of H-atom while rest two are occupied by lone pairs of electrons. Due to lp-bp repulsion, the H—O—H bond angle is contracted to 104.5°. O—H bond length is 95.7pm. Due to its V-shaped structure, H2O is a polar molecule ( µ =1.84 D).
Structure of Ice
In ice, each oxygen atom is bonded to 4 other oxygen atoms by hydrogen bonds. This gives ice, an open framework. When ice melts, some hydrogen bonds are broken. The ‘bridges’ and the open framework collapse, causing water molecules to fall into the empty spaces. This close packing of molecules allows the liquid to occupy a smaller volume than the ice and the density increases.
Physical Properties of Water
Pure water is a colourless, odourless and tasteless liquid. It gives a bluish tinge in thick layers. Water has an abnormally high freezing point, boiling point, heat of vaporization, heat of fusion. Water has high specific heat, surface tension and thermal conductivity than most other liquids. These properties are responsible for water to play a vital role in life processes. These unique properties of water are due to the presence of hydrogen bonding.
Chemical Properties of Water
(a) Acid-Base Nature: Water acts as both an acid and a base, and is said to be amphoteric. Water acts as a base towards acids stronger than itself and as an acid in the presence of a base stronger than itself, as shown below:
(b) Oxidation and Reduction: Water acts both as an oxidizing as well as a reducing agent.
e.g. 2Na + H2O → 2NaOH + H2
2NaH + H2O → 2NaOH + H2
(c) Hydrolytic Reaction: Compounds like calcium hydride, aluminium nitride, calcium phosphide, calcium carbide, silicon halide etc. undergo hydrolysis with water.
CaH2 +2H2O → Ca(OH)2 + 2H2
AlN + 3H2O → Al(OH)3 + NH3
(d) Hydrate Formation: Water reacts with certain metal salts to form hydrates. In hydrated salts water may remain in 5 types:
(i) Co-ordinated water, (ii) Hydrogen-bonded water, (iii) Lattice water, (iv) Zeolitic water, (v) Clathrate water.
Soft and Hard Water
A sample of water which easily produces lather with soap is known as “Soft Water” while “hard water” doesn’t produce lather with soap easily. The soluble Ca2+, Mg2+ or Fe2+ ions react with soap to give Ca or Mg soap which being insoluble in water get precipitated.
MgCl2 + 2C17H35COONa → (C17H35COO)2Mg ↓ + 2NaCl
CaCl2 + 2C17H35COONa → 2NaCl + (C17H35COO)2Ca ↓
Hardness of Water and Removal
(a) Temporary hardness: It is due to the dissolution of bicarbonates of Ca and Mg in water. It can be removed by boiling.
Ca(HCO3)2 —Boiling → CaCO3 + H2O + CO2
Mg(HCO3)2 — Boiling → MgCO3 + H2O + CO2
It can also be done by adding lime.
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Mg(HCO3)2 + Ca(OH)2 → CaCO3 + MgCO3 + 2H2O
(b) Permanent Hardness and its Removal: Permanent hardness happen due to the dissolution of chloride and sulphates of Ca, Mg, Fe. It cannot be removed by boiling of water.
It can be removed by following
(i) Adding washing soda: CaCl2 + Na2CO3 → CaCO3 + 2NaCl
(iii) Calgon Process: Calgon which is also known as Grahm’s salt is the trade name of sodium
hexametaphosphate (NaPO3)6. The Ca2+ and Mg2+ ions dissolved in hard water react with Calgon to produce complex anions, which are very inactive and don’t produce a precipitate with soap.
Na2[Na4(PO3)6] + 2CaCl2 → Na2[Ca2(PO3)6] + 4NaCl
(iv) By Ion Exchange Resin: Resins are giant organic molecules attached with acidic or basic groups. Cation exchange resin contains –COOH or –SO3H groups. These remove cations like Na+, Ca2+, Mg2+, Fe2+ etc by exchange with H+. Anion exchange resins contain –NH2 group they are represented by RNH3+ OH–. These resins remove negative ions such as Cl–, SO42-, NO3– etc.
Preparation of H2O2
(a) Lab method of preparation of H2O2 – H2O2 obtained by passing a current of CO2 through a cold pasty solution of BaO2 in water.
BaO2 + CO2 + H2O → H2O2 + BaCO3
(b) Industrial process: By electrolysis of 50% H2SO4 at 0°C using Pt electrode.
2H2SO4 → 2H+ + 2HSO4–
At cathode: (Cu) – 2H+ + 2e– → H2
At anode: (Pt) – 2HSO4– → H2S2O8 + 2e–
H2S2O8 + 2H2O → H2O2 + 2H2SO4
(c) Auto – oxidation of 2-ethyl anthraquinone: This is the most modern method and needs H2, atmospheric oxygen and water as the major raw materials.
First and foremost, 2-ethyl anthraquinol is catalytically reduced to 2-ethyl anthraquinone in an organic solvent by H2 and Pd (catalyst). Secondly, 2-ethyl anthraquinol is oxidised by air to 2-ethyl anthraquinone. H2O2 obtained in the process is extracted with water to give a 20% H2O2 solution. 2-ethyl anthraquinone is thus reused. The process is repeated, thus it is a cyclic process.
Physical Properties of H2O2
(a) Pure H2O2 is weakly acidic in nature and exists as an associated liquid due to hydrogen bonding.
(b) The smell of H2O2 resembles as nitric acid.
(c) It causes blisters on the skin.
(d) Stored in plastic containers after the addition of stabilizers.
(e) A dilute solution of H2O2 is concentrated by vacuum distillation or by distillation under pressure.
Structure of H2O2
All four atoms in H2O2 are non-planar. Structure of H2O2 has an open book structure having two leaves at 90°, the H-atoms are placed one on each core, the H–O making an angle of 101.5° with O–O bond.
Bond Angle: Dipole moment value of H2O2 suggests that all the four atoms in H2O2 do not lie in a plane, and the structure can be compared with a book open at angle 94°. The H—O—O bond angle is 97°.
(b) Oxidising and reducing nature : H2O2 acts as a strong oxidising agent under acidic and alkaline conditions. Oxidation in the acidic medium is slow while rapid in an alkaline solution as H2O2 itself is a weak acid.
(i) Oxidising action in acidic medium
2Fe2+ (aq) + 2H+ + (aq) H2O2 (aq) → 2Fe3+ (aq) + 2H2O (l)
PbS (s) + 4H2O2 (aq) → PbSO4 (s) + 4H2O (l)
(ii) Reducing action in acidic medium
2MnO4– + 6H+ + 5H2O2 → 2Mn2+ + 8H2O + 5O2
HOCl + H2O2 → H3O+ + Cl– + O2
(iii) Oxidising action in basic medium
2Fe2+ + H2O2 → 2Fe3+ + 2OH–
Mn2+ + H2O2 → Mn4+ + 2OH–
(iv) Reducing action in basic medium
2MnO4– + 3H2O2 → 2MnO2 + 3O2 + 2H2O + 2OH–
I2 + H2O2 + 2OH– → 2I– + 2H2O + O2
(c) Bleaching action: H2O2 acts as a bleaching agent due to its oxidising nature e.g., bleached human hair (black to golden brown), ivory, silk, wool, feather etc.
Uses of H2O2
Its wide scale use has led to a tremendous increase in the industrial production of H2O2. Some of the uses are listed below:
(a) In daily life, it is used as hair bleach and as a mild disinfectant. As an antiseptic, it is sold in the market as perhydrol.
(b) It is used to manufacture chemicals like sodium perborate and percarbonate, which are used in high-quality detergents.
(c) It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc.
(d) It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils, fats, etc.
(e) Nowadays, it is also used in Environmental (Green) Chemistry. For example, in pollution control treatment of domestic and industrial effluents, oxidation of cyanides, restoration of aerobic conditions to sewage wastes, etc.
Strength of H2O2
(a) As a percentage: The strength of H2O2 is sometimes reported in terms of percentage. Bottles containing H2O2 are labelled as 40%, 60%… etc. This means that 100 ml solution of H2O2 contains 40 g H2O2.
(b) As number volume: The strength of hydrogen peroxide is often expressed in terms of the volume of oxygen evolved on heating. Bottles containing hydrogen peroxide are labelled as ‘10 Volume’, ‘20 Volume’, ‘100 Volume’ etc. This means that one volume of hydrogen peroxide solution gives so many volumes to oxygen (after complete decomposition) at N.T.P. Thus 1 mL of ‘10 Volume’ solution produces 10 mL of oxygen at N.T.P, 1,000 mL of a ‘10 Volume’ solution produce 10,000 mL of oxygen at N.T.P, 1000 mL of a ‘20 Volume’ solution produce 20,000 mL of oxygen at N.T.P. etc.
% strength = 17/56 × volume strength
• Volume strength = 5.6 × normality
• Volume strength = 11.2 × molarity
Heavy water is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.
It is used for the preparation of other deuterium compounds, for example:
CaC2 + 2D2O → C2D2 + Ca(OD)2
SO3 + D2O → D2SO4
Al4C3 +12D2O → 3CD4 + 4Al(OD)3
Hydrogen Economy – Use of Hydrogen as a Fuel
Advantage: (i) H2 is an environmentally clean fuel because it gives water on combustion. However, a small amount of nitrogen oxides may be formed due to high temperatures. (ii) H2 has the highest calorific value and it is a better fuel than any other fuel.
Disadvantages: No doubt, H2 is the best alternative to fossil fuels, but the use of hydrogen as a fuel is very dangerous and needs attention. The storage of the mixture of H2 and O2 is very risky. In 1986, the space shuttle Challenger, an explosion occurred in the fuel tank containing H2 and O2.
This article has tried to highlight the chemistry of hydrogen in the form of revision notes for class 11 students in order to understand the basic concepts of the chapter. The notes on hydrogen have not only been prepared for class 11 but also for the different competitive exams such as iit jee, neet, etc.