Hydrogen - NCERT Flashcards
1
Q
How does hydrogen exist in the atomic and elemental forms?
A
Atomic form : one proton and one electron
Elemental form : Diatomic (H2) molecule
Hydrogen forms more compounds than any other elements
2
Q
Similarities of hydrogen with Alkali metals
A
- Hydrogen has a electronic configuration of 1s1 which is similar to that of alkali metals ns1
- Hydrogen can lose one electron to become a unipositive ion, like alkali metals
- Hydrogen forms oxides, halides and sulphides like alkali metals.
3
Q
Dissimilarities of hydrogen with Alkali metals
A
- Ionization enthalpy of H is very high and resembles halogens more than alkali metals.
(H = 1312, Li= 520 and F = 1680) - It forms many covalent compounds, unlike alkali metals.
4
Q
Similarities between H and halogens
A
- H is one electron short of the noble gas configuration, like the halogens.
- It can gain an electron to form a uninegative ion, like halogens.
- It has high ionization enthalpy, like halogens.
- It forms many covalent compounds and combines with many compounds to form hydrides.
5
Q
Dissimilarities between H and halogens.
A
- Reactivity of H is very less as compared to halogens
6
Q
Why was H given a separate place in the periodic table?
A
H has some properties similar to alkali metals and some similar to halogens. It differs from both because loss of electron produces a nucleus which is 1.5 * 10^-3 pm, which is very small as compared to normal atomic and ionic sizes of 50 to 200pm. Therefore, H+ does not exist independently and is unique in its behavior so it is placed separately in the periodic table.
7
Q
Why is hydrogen the most abundant element in the universe? (3)
A
- It forms 70% of the total mass of the universe.
- It is the principal element in the solar atmosphere.
- Giant planets, Jupiter and Saturn consist mostly of hydrogen.
8
Q
Abundance of Hydrogen on Earth
A
- 0.15% of the Earth’s atmosphere. (due to it’s light nature it is less abundant in Earth’s atmosphere)
- 15.4% of the Earth’s crust in the combined form. (including oceans)
- Occurs in plant and animal tissues, proteins, carbohydrates, hydrides. hydrocarbons and other compounds.
9
Q
What are the three isotopes of Hydrogen?
A
The three isotopes of H are protium, deuterium (also called heavy hydrogen) and tritium. They differ in the number of neutrons. Ordinary H, or protium has no neutrons. (Mass no. 1 and atomic no. 1 ). Deuterium has 1 neutron. (Mass no. 2 and atomic no. 1 ). Tritium has 2 neutrons (Mass no. 3 and atomic no. 1 ). All chemical properties are same for the isotopes but differ in reaction rates.
10
Q
In 1934 who was awarded the Nobel prize for separating hydrogen isotopes with mass number 2 by physical means?
A
Harold C Urey, an American scientist.
11
Q
Why was Harold C Urey awarded the Nobel Prize in 1934?
A
Separating hydrogen isotope with mass number 2, by physical methods.
12
Q
What year was Harold C Urey awarded the Nobel Prize for separating a hydrogen isotope with mass no. 2 by physical methods?
A
1934
13
Q
Terrestrial hydrogen contains what % of deuterium?
A
0.0156% - mostly in the form of HD.
14
Q
Concentration of tritium is?
A
One atom per 10^8 atoms of protium.
15
Q
Which isotope of H is radioactive?
A
Tritium is radioactive. It emits low energy Beta particles. It’s half life is 12.33 years.
16
Q
How is tritium radioactive?
A
It emits low energy Beta particles.
17
Q
What is the half life of tritium?
A
12.33 years.
18
Q
Relative abundance of protium in %
A
99.985
19
Q
Relative abundance of deuterium in %
A
0.0156
20
Q
Relative abundance of tritium in %
A
10^-15
21
Q
Melting point order of the isotopes of H.
A
Tritium > Deuterium > Protium
22
Q
Boiling point order of the isotopes of H.
A
Tritium > Deuterium > Protium
23
Q
In general what is the trend of physical properties for the isotopes of H?
A
Tritium > Deuterium > Protium, or Deuterium > Protium.
But exception is Internuclear distance in pm, which is same for D and H
24
Q
Internuclear distance of H and D.
A
is the same. (74.14 pm)
25
Enthalpy of bond dissociation of H and D.
D > H. This causes difference in the rates of reaction.
26
Physical Properties, Chemical Properties and Rates of Reactions of the isotopes of H?
Physical properties - differ because of large diff in masses. T> D> P. (Internuclear distance is same for D and P.) Bond dissociation enthalpies differ therefore they have diff rates of reaction.
All chemical properties are the same.
27
What are the allotropes of H?
Ortho and para hydrogen are the allotropes of Hydrogen. (Allotropes is not the same as Isotopes).
28
Stability of ortho and para forms of H?
Ortho is more stable than para.
29
Proton spin of ortho and para forms of H?
Ortho - parallel
| Para - opposite
30
Pure of ortho and para forms of H?
Ortho - pure form cannot be prepared.
| Para - cool at absolute zero.
31
At room temperature allotropes of Hydrogen are
At room temp, H is made up of 75% ortho and 25% para isomers.
32
Laboratory preparation of H2 is
Reaction of zinc with dil HCl
Zn + 2HCl -> ZnCl2 + H2
Reaction of zinc with aqueous NaOH (alkali) also gives Hydrogen
Zn + 2NaOH -> Na2ZnO2 +H2
33
When the same amount of zinc is treated separately with excess of sulphuric acid and excess amounts of sodium hydroxide solution, the ratio of volume of hydrogen evolved is?
(a) 1:1
(b) 1:2
(c) 2:1
(d) 9:4
Ans - 1:1
Same amount of H2 is produced in both reactions.
Zn + 2HCl -> ZnCl2 + H2
Zn + 2NaOH -> Na2ZnO2 +H2
34
Commercial method of preparation of H (i)
Electrolysis of acidified water using platinum electrodes.
H2 gas is obtained on the cathode.
2H2O = 2H2 + O2
35
Commercial method of preparation of H (ii)
High purity, 99.95% dihydrogen is obtained by electrolyzing warm aqueous barium hydroxide solution between nickel electrodes.
36
Which commercial method of producing H provides 99.95% pure H2?
Electrolysis of warm aqueous barium hydroxide solution with nickel electrodes provides 99.95% pure hydrogen.
37
Commercial method of preparation of H (iii)
Electrolysis of brine solution. Electrolysis of a soln of NaCl in water gives NaOH and also produces H2 at the cathode and Cl2 at the anode. Due to production of NaOH, the pH of the soln increases.
38
In electrolysis of brine solution, the pH
increases because basic NaOH is formed.
39
Commercial method of preparation of H (iv)
Reaction of hydrocarbons with steam at high temperatures in the presence of catalyst gives H2 gas.
40
Catalyst and temperature for reaction of hydrocarbons with steam - commercial production of H2 gas.
Catalyst - Ni
| Temperature - 1270K
41
Reaction of hydrocarbon and steam at 1270K in the presence of Ni gives
CO and H2
| CH4 + H2O ---> CO + 3H2
42
Mixture of CO and H2 is called
water gas / syngas / synthesis gas
43
CO + H2 is used for
production of methanol and a number of hydrocarbons. So it is known as syngas.
44
What is syngas?
A mixture of CO + H2 which is used to produce a number of hydrocarbons, methanol, etc
45
Nowadays syngas can be produced from?
Sewage, sawdust, newspapers and scrap wood.
46
Production of syngas from coal is called?
Coal gasification.
C + H2O --> CO + H2
T: 1270K
47
Coal gasification is
Production of syngas from coal
C + H2O --> CO + H2
T: 1270K
48
Water gas shift reaction is
Reaction of carbon monoxide of syngas mixture with steam in the presence of Iron chromate catalyst.
CO +H2O --> CO2 + H2
T: 630K
Catalyst: Iron chromate - FeCrO4
49
CO2 produced in water gas shift reaction is removed by?
Scrubbing with sodium arsenite solution
50
Industrial methods of preparation of dihydrogen distribution
77% - petrochemicals
18% - coal
4% - electrolysis
1% - other methods
51
77% of industrial H2 is prepared by?
petrochemicals
52
18% of industrial H2 is prepared by?
coal
53
4% of industrial H2 is prepared by?
electrolysis of aqueous solutions
54
1% of industrial H2 is prepared by?
other methods
55
Physical properties of H2 gas
It is colorless, odorless, and tasteless gas. It is lighter than air and it is combustible. It is insoluble in water.
56
Is hydrogen gas soluble in water?
No it is not. H2 i insoluble in water.
57
Chemical behavior of hydrogen (or any molecule) is determined by
Bond dissociation enthalpy
58
Bond dissociation enthalpy of H2 is
the highest for the single bond between two atoms of any element
59
Dissociation of dihydrogen at different temperatures is?
At 2000K - 0.081%
| At 5000K - 95.5%
60
Dissociation of dihydrogen at 2000K is
0.081%
61
Dissociation of dihydrogen at 5000K is
95.5%
62
Why is H2 relatively inert at room temperatures?
High H-H bond enthalpy
63
Atomic Hydrogen is produced by?
Atomic hydrogen is produced at high temperature in an electric arc or under ultraviolet radiations.
64
Why (and how?) does hydrogen combine with almost all the elements?
Because of it's incomplete orbital, 1s1 configuration H reacts with al the elements either by losing one electron to form H+, or gaining one to form H- or forming covalent bond.
65
Reaction of dihydrogen with halogens
Reacts with halogens to give hydrogen halides.
Order of reactivity : F> Cl> Br> I (small size provides better overlapping)
Reaction with fluorine occurs in the dark, iodine requires catalyst.
66
Reaction of dihydrogen with oxygen
Reacts with oxygen to form water, the reaction is highly exothermic. Used as a fuel cell.
67
Reaction of dihydrogen with nitrogen
Habers process to form ammonia. Exothermic.
68
Reaction of dihydrogen with metals
Forms hydrides
69
Reaction of dihydrogen with metal ions and metal oxides
Dihydrogen acts as a reducing agent and reduces metal ions and metal oxides.
70
Reaction of dihydrogen with vegetable oils
Reaction of dihydrogen with vegetable oils is called hydrogenation of vegetable oils. Nickel acts as a catalyst and it produces vanaspati ghee and margarine.
(Saturation of unsaturated hydrocarbons)
71
Hydroformylation of olefins forms
Hydroformylation of olefins forms aldehydes which further undergo oxidation to give alcohols.
72
Single largest use of dihydrogen is
synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.
73
Manufacture of methanol by syngas catalyst
Cobalt
74
Atomic hydrogen and oxyhydrogen torches are used for
cutting and welding purposes.
75
Atomic hydrogen for atomic hydrogen torches are produced by
Dissociation of dihydrogen with the help of electric arc, it is then allowed to recombine on the surface to be welded and generate a temperature of 4000K.
76
Different types of hydrides are
1. Ionic or saline or salt like
2. Covalent or molecular hydrides
3. Metallic or non stoichiometric or interstitial hydrides
77
Ionic or Salt like hydrides characteristics
1. S block + hydrogen
2. Mostly ionic but LiH, BeH2 and MgH2 are covalent due to Fajan's rule.
3. BeH2 and MgH2 are polymeric.
4. Crystalline, non- volatile and non conducting in the solid state but conduct electricity in molten state.
5. Saline hydrides react violently react with water to produce hydrogen gas.
6. LiH does not react with O2 or Cl2 and generally unreactive at room temperatures.
78
Polymeric saline hydrides are
BeH2 and MgH2
79
LiH reacts with O2 and Cl2 to form?
LiH does not react with O2 and Cl2. It is unreactive and used in the synthesis of other useful halides like LiAlH4 and LiBH4.
80
LiH is used in the synthesis of
Useful halides like LiAlH4 and LiBH4.
8LiH + Al2Cl6 --> LiAlH4 + 6LiCl
LiH + B2H6 --> 2LiBH4
81
Characteristics of Covalent/ Molecular hydrides
1. Covalent and volatile compounds
| 2. Further classified into electron deficient, electron precise and electron rich hydrides.
82
Electron deficient covalent hydrides are
Group 13 hydrides are electron deficient, example BH3 has only 6 electrons. (so it exists as B2H6). They act as Lewis acids.
83
Electron precise covalent hydrides are
Group 14 halides have exactly 8 electrons which is why they are called electron precise. They all have tetrahedral structure. eg. CH4 - methane SiH4 - silicane
GeH4 - Germane
84
Electron rich covalent hydrides are
Group 15, 16 and 17 hydrides are electron rich hydrides, They have more than the required no. of electrons which are expressed in the form of lone pairs. NH3 - 1lp, H2O - 2lp and HF - 3lp. Because of excess of electrons they behave like Lewis bases. NH3, H2O and HF also show hydrogen bonding.
85
Metallic, interstitial or Non - stoichiometric hydrides
Formed by d and f block elements.
Group 7, 8 and 9 in d block do not form hydrides and in group 6 only Chromium forms hydride -CrH (Hydride gap).
They conduct heat and electricity but not as good as their parent metals.
Non stoichiometric and do not follow the law of constant composition.
Except for Ni, Ce. Pd and Ac all other hydrides have a lattice different from the parent metal.
Used as catalysts and to store H.
86
What is hydride gap?
Elements of group 7,8 and 9 in the periodic table do not form hydrides and in group 6 only chromium forms CrH. This is the hydride gap.
87
Hydrides of which metals have the lattice same as their parents?
Ni, Pd, Ac, Ce.
88
The property of absorption of hydrogen metals is widely used in
Catalytic reactions and hydrogenation.
89
Which metals can accommodate a large amount of hydrogen and used as its storage media?
Platinum and palladium.
90
Percentage of water in humans and plants
Humans - about 65%
| Some plants - 95%
91
Distribution of water over the earth's surface
Max - oceans
| Min - rivers
92
Properties of water due to extensive hydrogen bonding
High freezing point, high boiling point, high heat of vaporization and high heat of fusion as compared to H2Se and H2Te
93
Properties of water
High freezing point, high boiling point, high heat of vaporization and high heat of fusion, high specific heat, thermal conductivity, dipole moment and dielectric constant.
