Exams Prep Master
Hydrogen, in chemistry, is the lightest atom on the planet. It just contains one electron. In 1766, Henry Cavendish was the first to discover hydrogen. It was originally known as 'Inflammable Air.' It was later given the name 'Hydrogen' by Lavoisier. Hydrogen may be found in both free and mixed forms. The three stable isotopes of hydrogen are protium, deuterium, and tritium. Here we will be discussing some of the important questions on the above-mentioned topic.
Very Short Answer Questions [1 Marks Question]
Ques- Name the isotopes of hydrogen.
Ans- Protium (11H), deuterium (12H), and tritium (13H) are the three isotopes of hydrogen found in nature.
Ques- What is coal gasification?
Ans- Coal gasification is a method of producing syn-gas from coal by heating it to a high temperature.
C (s) + H2O (g) 1270k CO (g) + H2(g)
Ques- Why is dihydrogen gas not preferred in balloons?
Ans- Dihydrogen should have been used in balloons because it is the lightest gas. It is not advised, however, due to its high combustibility.
Ques- Name the categories into which hydrides are categorized.
Ans- Hydrides are classified into the following groups:
- Hydrides that are ionic, saline, or salt-like.
- Hydrides that are covalent or molecular.
- Hydrides that are metallic or non-stoichiometric.
Ques- Write one chemical reaction for the preparation of D2O2.
Ans- The reaction of D2SO4 dissolved in water over BaO2 produces D2O2.
BaO2 + D2SO4 → BaSO4 + D2O
Ques- Explain why HCI is a gas and HF is a liquid.
Ans- HF molecules are held together by intermolecular hydrogen bonding. As a result, HF is a liquid at room temperature. Because the HCl molecules lack intermolecular hydrogen bonding, it exists as a gas at ambient temperature.
Ques- Give an example of electrons – deficient hydride.
Ans- An example of electron-deficient hydride is diborane.
Ques- What is the importance of heavy water in nuclear power generation?
Ans- It slows the speed of fast-moving neutrons in nuclear processes by acting as a moderator.
Ques- What is the mass of 1 mole of deuterium oxide and tritium oxide?
Ans- D2O = 20g, T2O = 22g.
Ques- What is the significance of H2O2 labelled as 30 volumes’?
Ans- When hydrogen peroxide is labelled "30 volume," it signifies that 1 mL of the solution produces 30 mL of oxygen gas at STP.
Short Answer Questions [2 Marks Question]
Ques- How can the production of hydrogen from water gas be increased by using a water gas shift reaction?
Ans- In the presence of a catalyst, carbon monoxide reacts with steam to generate carbon dioxide and pure hydrogen, known as the water-gas shift reaction.
The combination (CO(g) + H2(g)) is a water gas.
The process for creating pure hydrogen from water gas is as follows:
CO2 + 2H2 → CO + H2 + H2O
Ques- Name the classes of hydrides to which H2O, B2H6, and NaH belong.
Ans- H2O is an electron-rich covalent hydride/molecular hydride, also known as a covalent hydride.
B2H6 is an electron-deficient molecular hydride, often known as a covalent hydride.
The ionic hydride class includes NaH.
Ques- If the same mass of liquid water and a piece of ice is taken, then why is the density of ice less than that of liquid water?
Ans- The mass per unit volume (i.e. mass/volume) is the definition of density. Because water expands when it freezes, the volume of ice is more than the volume of liquid water for the same amount of water. In other words, ice floats over water because it has a lower density than liquid water.
Ques- Discuss briefly the de-mineralisation of water by ion exchange resin.
Ans- Water demineralization is the process of using cation and anion exchange to remove all soluble salts from the water. In the cation exchange process, sodium, calcium, and magnesium cations replace hydrogen cations. During the anion exchange process, OH is exchanged. Both of these variables work together to produce water.
H+ + OH– → H2O
Ques- Why does hydrogen occupy a unique position in the periodic table?
Ans- Despite the fact that hydrogen resembles both alkali metals (ns1) and halogens (ns2) , (np5) to some extent, it also varies from both. Because hydrogen has such a small size, it does not exist in isolation and is always coupled with other atoms or molecules. That is why it occupies such a unique position in the periodic table.
Ques- Give the main characteristics of isotopes.
Ans- Isotopes have the following primary characteristics:
- Isotopes are different versions of the same element's atoms.
- They have the same atomic number but have distinct mass values.
- The quantity of neutrons present varies.
Ques- How can the production of dihydrogen obtained from ‘coal gasification be increased’?
Ans- The carbon monoxide from the syngas combination is reacted with steam at high temperatures in the presence of an iron chromate catalyst. This procedure, referred to as coal gasification, boosts the generation of dihydrogen gas.
CO(g) + H2O (g) 673k catalyst KCO2(g) + H2(g)
Ques- What is understood by hydrogenation?
Ans- Hydrogenation is the reaction between molecular hydrogen and an organic or inorganic substrate. As a result, when hydrogen molecules are added to an alkene, a hydrogenation process, also known as a reduction reaction, happens.
Ques- Why is ice less dense than water and what kind of attractive forces must be overcome to melt ice?
Ans- The ice is made up of an open structure with a lot of empty gaps. Ice has a lower density than water as a result of this. Hydrogen bonds are broken as ice melts, enabling water molecules to fill in the gaps. As a result, ice does not have the same open structure as liquid water.
Ques- Why is water an excellent solvent for ionic or polar substances?
Ans- Water is a polar solvent because of its high dielectric constant. The high dielectric constant of water reduces the attraction between cation and anion. As a result, using dipole forces, water molecules may easily remove ions from the lattice location.
Ques- Why is the Ionisation enthalpy of hydrogen higher than that of sodium?
Ans- Ionisation enthalpy of hydrogen is higher than that of sodium because by Taking a valence electron from the H atom takes more energy than removing one from the Na atom due to the greater nuclear attraction. As a result, the ionization enthalpy of hydrogen (1312 kJ/mol) is greater than that of sodium (496 kL/mol).
Long Answer Questions [3 Marks Question]
Ques- Basic principle of the hydrogen economy is transportation and storage of energy in the form of liquid or gaseous hydrogen. Which property of hydrogen may be useful for this purpose? Support your answer with the chemical equation if required.
Ans- Hydrogen is a gas at normal temperature. It is difficult to transport it by rail or by road due to its size. By chilling and applying high pressure, gaseous H2 may be converted to liquid H2, which has a significantly smaller volume and can be carried more easily. One of the most essential qualities of hydrogen for hydrogen economy is its ability to be converted to a liquid by cooling it under high pressure.
Ques- What is the importance of heavy water?
Ans- In nuclear reactors, heavy water acts as a neutron moderator, delaying neutrons so that they are more likely to react with fissile uranium-235 rather than uranium-238, which accumulates neutrons but does not fission.
It's used as a tracer chemical in reaction mechanisms study and for the production of other deuterium compounds like CD4, D2SO4, and so on.
Ques- An acidic solution of hydrogen peroxide behaves as an oxidising as well as a reducing agent. Illustrate it with the help of a chemical equation.
Ans- The chemical equation for H2O2, which acts as both an oxidising and reducing agent, is shown below:
i) Acidified KI is oxidised by H2O2 to iodine.
2KI + H2O2 + H2SO4 → I2 + K2SO4 → 2H2O
ii) Reactions that justify the reduction of nature include:
H2O2 + Cl2 → 2HCl + O2
Ques- Why are water molecules polar?
Ans- The water molecule (H2O) is polar due to its twisted shape. It's also a polar solvent. On one side, it bears a positive and negative charges. This molecule is made up of two hydrogen atoms and one oxygen atom. When two water molecules are near enough together, polar forces operate to bring them together. The oxygen atom in a water molecule will create a bond with several hydrogen atoms from other water molecules.
Ques- Why does the water show a high boiling point as compared to hydrogen sulphide? Give reasons for your answer.
Ans- The strong electronegativity of oxygen (E.N.=3.5) causes significant H-bonding in water, resulting in water existing as an associated molecule. Water has a higher boiling point than hydrogen sulphide due to hydrogen bonding between O and H in its molecules. The strong hydrogen bond is responsible for the high boiling point.
Because hydrogen sulphide can not form hydrogen bonds and is a gas at room temperature, sulphur is less electronegative (E.N.=2.5). Molecules are allowed to move about in this space, suggesting that it is lightly packed.
Ques- Why can dilute solutions of hydrogen peroxide not be concentrated by heating? How can a concentrated solution of hydrogen peroxide be obtained?
Ans- It is because explosions are always a possibility, that heat cannot be utilized to concentrate H2O2 solution over 30% concentration. The solution is concentrated at a lower pressure of roughly 15 mm at 35-40°C. 90 per cent H2O2 is achieved after many distillations. Even higher concentrations can be achieved by cooling with solid CO2 and an ether bath until crystallisation occurs. Crystals are separated, melted, and then refrozen to achieve 99 per cent pure H2O2.
Ques- How will you account for 104.5° bond angle in water?
Ans- Two of the six electrons on an oxygen atom are bonded to a hydrogen atom, leaving two lone electron pairs. Due to the presence of these lone pairs of electrons, the bond angle in H2O is 104.5°. This may be explained using the valence shell electron pair repulsion concept (VSEPR).
The sp3 hybridisation of oxygen in the H2O molecule results in a tetrahedral structure. By forming sigma bonds with two hybrid orbitals, H atoms occupy two places, while lone pairs occupy two positions. The estimated bond angle is 109.5°, whereas it is 104.5° in reality. The attraction between lone pairs is stronger than the attraction between bond pairs. As a result, water's bending angle lowers from 109.5 to 104.5 degrees.
Ques- What are the commercial methods of preparing hydrogen?
Ans- The commercial methods of preparing hydrogen are:
- By electrolysing acidified water using platinum electrodes.
- By electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
- By letting steam react with hydrocarbons at very high temperatures in the presence of some catalyst.
Ques- State the types of hydrogen bonding that exist in nature.
Ans- The various types of hydrogen bonding are:
- Intramolecular Hydrogen bonding: A hydrogen bond occurring within a molecule itself is known as an intramolecular hydrogen bond.
- Intermolecular hydrogen bonding: Hydrogen bonds between different molecules of either the same or different types of compounds is known as intermolecular hydrogen bonds.
- Symmetrical Hydrogen Bond: A special type of bond in which the proton is usually placed in between the two identical atoms is known as an asymmetrical hydrogen bond.
Ques- Mention some of the applications of hydrogen fuel.
Ans- Hydrogen fuel has multiple applications. Some of the most important ones are mentioned below.
- It proves to be the best alternative to conventional fuels. And has similar levels of efficiency as that of any natural gas.
- Hydrogen fuel can be utilised to provide energy to almost every kind of vehicle or electrical devices, such as generators, electric motors, cars, boats or aeroplanes.
- The conditions for storing hydrogen are found very rarely, such as, it can’t be stored in cryogenic tanks or high-pressure tanks, which creates a major problem for the same.
Very Long Answer Questions [5 Marks Question]
Ques- Can we use concentrated sulphuric acid and pure zinc in the preparation of dihydrogen? Write the chemical reactions to show the amphoteric nature of water. Why is hydrogen peroxide stored in wax-lined plastic coloured bottles?
Ans- Concentrated H2SO4 cannot be used because concentrated H2SO4 also functions as an oxidizing agent and is oxidized to SO2, it cannot be used.
Zn + 2H2SO4 (Cone.) ———> ZnSO4 + 2H2O + SO2
Because pure Zn is non-porous and has a slow reaction rate, it is not used. Impurities in Zn aid in the formation of an electrochemical pair, which speeds up the process.
Water is amphoteric in nature and it behaves both as an acid as well as the base. With acids stronger than themselves (e.g., H2S) it behaves as a base and with bases stronger than itself (e.g., NH3) it acts as an acid.
(i) As a base: H2O(I) + H2S(aq) ——-> H3O(aq) + HS–(aq)
(ii) As an acid: H2O(I) + NH3(aq) ———> OH–(aq) + NH4+(aq)
(c) When H2O2 is exposed to a rough surface, it decomposes quickly (acting as the catalyst). Exposure to light also decomposes it. As a result, the breakdown of H2O2 is slowed by waxed smooth surfaces and coloured bottles.
Ques- Complete the following chemical reactions.
(i) PbS(s) + H2O2 (aq) ————->
(ii) MnO4– (aq) + H2O2 (aq) ————->
(iii) CaO(s) + H2O(g) ————->
(iv) AlCl3(g) + H2O(l)————->
(v) Ca3N2(S) + H2O(l) ————->
Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions.
Ans- (i) PbS(s) + 4H2O2(aq) ————-> PbSO4(s) + 4H2O(l)
(ii) 2MnO4– (aq) + H2O2(aq) + 6H+(aq) ————-> 2Mn (aq) + 8H2O(l) + 5O2(g)
(iii) CaO(s) + H2O(g) ————->Ca(OH)2(aq)
(iv) AlCl3(aq) + 3H2O(l) ————-> Al(OH)3(S) + 3HCl (aq)
(v) Ca3N2(s) + H2O(l) ————->3Ca(OH)2(aq) + 2NH3(aq)
(a) Hydrolysis reactions, (iii) (iv) and (v)
(b) Redox reactions (i) and (ii)
Ques- Arrange the following
(i) CaH2, BeH2 and TiH2 in order of increasing electrical conductance.
(ii) LiH, NaH and CsH in order of increasing ionic character.
(iii) H–H, D–D and F–F in order of increasing bond dissociation enthalpy.
(iv) NaH, MgH2 and H2O in order of increasing reducing properties.
Ans- (i) A molecule's electrical conductivity is primarily determined by its covalent or ionic composition. In the molten form, CaH2 is an ionic hydride that conducts electricity. At room temperature, titanium hydride (TiH2) is a metallic compound that conducts electricity. Ionic chemicals conduct, but covalent molecules do not. BeH2 is a covalent hydride . As a result, it does not conduct.
Hence, the increasing order of electrical conductance is as follows:
BeH2 < CaH2 < TiH2
(ii) The electro negativities of the atoms involved determine the ionic nature of a connection. The ionic character is lesser the greater the difference between the electro negativities of atoms. From Lithium to Caesium, the electronegativity drops. As a result, their hydrides' ionic nature will improve (as shown below).
LiH < NaH < CsH
(iii) The nucleus attracts the bond pair in a D–D bond more strongly than the bond pair in a H–H bond. Because D2 has a greater nuclear mass, this is the case. The bond strength and bond dissociation enthalpy increase in proportion to the degree of the attraction.
Dissociation of bonds The energy of a molecule is determined by its bond strength, which is determined by the repulsive and attractive forces present in the molecule.
As a result, D–D has a greater bond dissociation enthalpy than H–H. In the case of F–F, however, bond dissociation enthalpy is the lowest. The bond pair is strongly repelled by the lone pairs that exist on each F-centre.
As a result, the following is the ascending order of bond dissociation enthalpy:
F–F < H–H < D–D
(iv) Ionic hydrides are strong reducing agents. NaH can easily donate its electrons. Hence, it is most reducing in nature. Both, MgH2 and H2O are covalent hydrides. H2O is less reducing than MgH2 since the bond dissociation energy of H2O is higher than MgH2. Hence, the increasing order of the reducing property is H2O < MgH2 < NaH.
Ques- Among NH3, H2O and HF, which would you expect to have the highest magnitude of hydrogen bonding and why?
Ans- The extent of hydrogen bonding mainly depends on
(i) Electronegativity
(ii) Number of hydrogen atoms available for bonding.
Among oxygen, fluorine and nitrogen, the increasing order of their electronegativities is N < O < F.
Therefore, the expected order of the extent of hydrogen bonding is HF > H2O > NH3.
But, the actual order is H2O > HF > NH3.
Despite the fact that fluorine is more electronegative than oxygen, water has a high level of hydrogen bonding.
In HF, there is a deficiency of hydrogens, whereas water has exactly the right number of hydrogens. As a result, straight-chain bonding is the only option.
By virtue of its great hydrogen bonding capacity, oxygen, on the other hand, creates a massive ring-like structure.
Hydrogen bonding has a finite amount of strength. Because nitrogen only has one lone pair in the case of ammonia. As a result, it is unable to fulfil all hydrogens.
Ques- What do you understand by the term “non-stoichiometric hydrides”? Do you expect this type of hydride to be formed by alkali metals? Justify your answer.
Ans- Non-stoichiometric hydrides are hydrogen-deficient compounds that are generated when dihydrogen reacts with d- and f-block elements. The law of constant composition does not apply to these hydrides.
Eg- LaH2.87, YbH2.55, TiH 1.5 – 1.8 etc.
Alkali metals create naturally ionic stoichiometric hydrides. Hydride ions are similar in size to alkali metal ions (208 pm). As a result, the forming metal and the hydride ion have a high binding force. Stoichiometric hydrides are generated as a result. Non-stoichiometric hydrides are unable to produce alkali metals.
Ques- Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.
Ans- Hydrogen has a 1s1 electrical design. In its valence shell, it just possesses one electron. It may either lose its single valence electron, causing it to act like alkali metals or acquire one, causing it to behave like halogens.
Alkali metals have hydrogen atoms that are similar to:
(1) It, like alkali metals, contains just one electron in the valence shell.
(2) It creates oxides, halides, and sulphides in the same way that alkali metals do.
The hydrogen atom is similar to halogens in the following ways:
(1) It just takes one electron to attain the closest noble gas structure, much like halogens.
(2) Hydrogen, like halogens, has a relatively high ionisation enthalpy value.
As a result of the aforesaid features, hydrogen belongs to the group-I alkali metals or the group-17 halogens. However, despite its resemblance to alkali metals and halogens, hydrogen has several distinct features that distinguish it from both alkali metals and halogens, leading to its placement in the Periodic Table as a separate element.
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