Answer the following questions: Hydrogen (Chemistry)

Hydrogen (Chemistry)

Answer the following questions

22. Explain why hydrogen is not placed with the halogen in the periodic table.

● The hydrogen has the electronic configuration of 1s¹ which resembles with ns¹ general valence shell configuration of alkali metals.

● Like the formation of halides (X⁻) from halogens, hydrogen also has a tendency to gain one electron to form hydride ion (H⁻) whose electronic configuration is similar to the noble gas, helium.

● However, the electron affinity of hydrogen is much less than that of halogen atoms.

● Hence, the tendency of hydrogen to form hydride ion is low compared to that of halogens to form the halide ions.

1/2 H₂ + e⁻ → H⁻         ΔH = +36 kcalmol⁻¹

1/2 Br₂ + e⁻ → Br⁻          ΔH = -55 kcalmol⁻¹

● Since, hydrogen has similarities with alkali metals as well as the halogens; it is difficult to find the right position in the periodic table.

● However, in most of its compounds hydrogen exists in + 1 oxidation state. Therefore, it is reasonable to place the hydrogen in group 1 along with alkali metals.

Q. An the cube at 0° C is placed in some liquid water at 0° C, the ice cube sinks - Why ? 

23. Discuss the three types of Covalent hydrides.

1. Electron precise (CH₄, C₂H₆, SiH₄, GeH₄)

2. Electron - deficient (B₂H₆) and

3. Electron - rich hydrides (NH₃, H₂O)

24. Predict which of the following hydrides is a gas on a solid (a) HCl (b) NaH. Give your reason.

HCl. This is a covalent hydride in which hydrogen is attached to another element by sharing of electrons. Since HCl consist of discrete, small molecules that have relatively weak intermolecular forces, they are generally gases or volatile liquids.

25. Write the expected formulas for the hydrides of 4th period elements. What is the trend in the formulas? In what way the first two numbers of the series different from the others ?

● The expected formula is MH or MH₂ (M = Ti, Zr, Hf, V, Zn)

● Trend in formulas : Most of the hydrides are non - stoichiometric with variable composition (TiH_{1.5 – 1.8} and PdH_{0.6 – 0.8})

● The first two members of the series form ionic hydrides whereas others form covalent or metallic hydrides.

26. Write chemical equation for the following reactions.

i. reaction of hydrogen with tungsten (VI) oxide NO3 on heating.

ii. hydrogen gas and chlorine gas.

i) WO₃ + 3H₂ → W + 3H₂O

ii) Hydrogen reacts with chlorine to give hydrogen chloride at room temperature under light H₂ + Cl₂ → 2HCl 

27. Complete the following chemical reactions and classify them in to (a) hydrolysis (b) redox (c) hydration reactions.

i. KMnO4 + H2O2 →

ii. CrCl3 + H2O →

iii. CaO + H2O →

i) 2KMnO₄ (aq) + 3H₂O₂ (aq) → 2MnO₂ + 2KOH + 2H₂O + 3O₂(g) − Redox reaction

ii) CrCl₃ + 6H₂O → [Cr(H₂O)₆]Cl₃.- Hydration reaction

iii) CaO + H₂O → Ca(OH)₂ - Hydrolysis reaction

28. Hydrogen peroxide can function as an oxidising agent as well as reducing agent. substantiate this statement with suitable examples.

● Hydrogen peroxide can act both as an oxidizing agent and a reducing agent. Oxidation is usually performed in acidic medium while the reduction reactions are performed in basic medium

In acidic conditions :

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O (E⁰ = + 1.77V)

For example,

2FeSO₄ + H₂SO₄ + H₂O₂ → Fe₂(SO₄)₃ + 2H₂O

In basic conditions :

H₂O₂⁻ + OH⁻ → O₂+ H₂O + 2e⁻              (E° = + 0.08V)

For example, 2KMnO_{4 (aq)} + 3H₂O_{2 (aq)} → 2MnO₂ + 2KOH + 2H₂O + 3O_{2 (g)}

29. Do you think that heavy water can be used for drinking purposes ?

Heavy water is not used for drinking, because it reduces the activity of plants, animals and human beings. It is not the basic for life as ordinary water.

30. What is water-gas shift reaction ?

The carbon monoxide of the water gas can be converted to carbon dioxide by mixing the gas mixture with more steam at 400°C and passed over a shift converter containing iron/copper catalyst. This reaction is called as water gas shift reaction.

CO + H₂O → CO₂ + H₂

31. Justify the position of hydrogen in the periodic table ?

● The hydrogen has the electronic configuration of 1s¹ which resembles ns¹ general valence shell configuration of alkali metals.

● Like the formation of halides (X⁻) from halogens, hydrogen also has a tendency to gain one electron to form hydride ion (H⁻) whose electronic configuration is similar to the noble gas, helium,

● However, the electron affinity of hydrogen is much less than that of halogen atoms.

● Hence, the tendency of hydrogen to form hydride ion is low compared to that of halogens to form the halide ions.

1/2 H₂ + e⁻ → H⁻           ΔH = +36 kcalmol^−l

1/2 Br₂ + e⁻ → Br ⁻           ΔH = −55 kcalmol^−l

● Since, hydrogen has similarities with alkali metals as well as the halogens, it is difficult to find the right position in the periodic table.

● However in most of its compounds hydrogen exists in +1 oxidation state.

● Therefore it is reasonable to place the hydrogen in group 1 along with alkali metals. 

32. What are isotopes? Write the names of isotopes of hydrogen.

Elements with same atomic number but with different mass number are called isotopes.

Hydrogen has three naturally occurring isotopes, viz, protium (₁H¹ or H), deuterium (₁H² or D) and (₁H³ or T) tritium.

33. Give the uses of heavy water.

Heavy water is widely used as moderator in nuclear reactors as it can lower the energies of fast neutrons.

It is commonly used as a tracer to study organic reaction mechanisms and mechanisms of metabolic reactions.

It is also used as a coolant in nuclear reactors as it absorbs the heat generated.

34. Explain the exchange reactions of deuterium.

CH₄ + 2D₂ → CD₄ + 2H₂

2NH₃ + 3D₂ → 2ND₃ + 3H₂

35. How do you convert parahydrogen into ortho hydrogen ?

1. The para form can be catalytically transformed into ortho - form using platinum or iron.

2. It can also be converted by passing an electric discharge,

3. Heating above 800°C

4. Mixing with paramagnetic molecules such as O₂, NO, NO₂ or with nascent / atomic

36. Mention the uses of deuterium.

1. As moderator in nuclear reactor

2. To find the mechanism of a chemical reaction it is used as tracer element

37. Explain preparation of hydrogen using electrolysis.

High purity hydrogen (>99.9%) is obtained by the electrolysis of water containing traces of acid or alkali or the electrolysis of aqueous solution of sodium hydroxide or potassium hydroxide using a nickel anode and iron cathode. However, this process is not economical for large - scale production.

At anode : 2OH⁻ → H₂O + 1/2 O₂ + 2e⁻

At cathode : 2H₂O + 2e⁻ → 2OH⁻ + H₂

Overall reaction : H₂O → H₂ + 1/2 O₂

38. A groups metal (A) which is present in common salt reacts with (B) to give compound (C) in which hydrogen is present in –1 oxidation state. (B) on reaction with a gas (C) to give universal solvent (D). The compound (D) on reacts with (A) to give (B), a strong base. Identify A, B, C, D and E. Explain the reactions.

Common salt is NaCl

The metal in this salt is Na(A)

2Na + H₂ (B) → 2NaH (C)

H₂ + O₂ (D) → 2H₂O (E)

H₂O + 2Na → 2NaOH(F) + H₂

 Symbol : Name

(A) Na : (i) Sodium

(B) H₂ : (ii) Hydrogen

(C) NaH : (iii) Sodium Hydride

(D) O₂ : (iv) Oxygen

(E) H₂O : (v) Water

(F) NaOH : (vi) Sodium Hydroxide

39. An isotope of hydrogen (A) reacts with diatomic molecule of element which occupies group number 16 and period number 2 to give compound (B) is used as a modulator in nuclear reaction. (A) adds on to a compound (C), which has the molecular formula C3H6 to give (D). Identify A, B, C and D.

Diatomic molecule which occupies Group No.16 and Period number 2 is Oxygen.

2D₂ + O₂ → 2D₂O (heavy water)

 Formula : Name

(A) D₂ : Deuterium

(B) D₂O : Heavy water

(Q CH₃ − CH = CH₂ : Propene (or) propylene

(D) CH₃ − CHD = CH₂D : Propylene deuteride (or) deuterated propylene

40. NH3 has exceptionally high melting point and boiling point as compared to those of the hydrides of the remaining element of group 15 - Explain.

NH₃ is a covalent hydride which has a lone pair of electron on electro negative nitrogen atom.

Therefore it forms inter molecular hydrogen bond with other NH₃ molecule. It forms condensation product of the type (NH₃)_n. Thus the boiling point of ammonia is high compared to other elements of group −15.

41. Why interstitial hydrides have a lower density than the parent metal.

Metallic (interstitial) hydrides: Metallic hydrides are usually obtained by hydrogenation of metals and alloys in which hydrogen occupies the interstitial sites (voids). Hence, they are called interstitial hydrides; Most of the hydrides are non - stochiometric with variable composition (TiH₁._{5 - 1.8} and PdH_{0.6 - 0.8}). Hence they have lower density than the parent metal.

42. How do you expect the metallic hydrides to be useful for hydrogen storage ?

● Metallic hydrides are usually obtained by hydrogen of metals and alloys in which hydrogen occupies the interstitial sites (voids).

● Hence, they are called interstitial hydrides. Most of the hydrides are non - stoichiometric with variable composition (TiH₁._{5 - 1.8} and PdH _{0.6 - 0.8}). Some are relatively light, inexpensive and thermally unstable which make them useful for hydrogen storage applications.

43. Arrange NH3, H2O and HF in the order of increasing magnitude of hydrogen banding and explain the basis for your arrangement.

● When a hydrogen atom (H) is covalently bonded to a highly electronegative atom such as flurorine (F) or oxygen (O) or nitrogen (N), the bond is polarized. Order  of electronegativity : F>O>N Ascending order of hydrogen bond strength:

NH₃ < H₂O < HF

44. Compare the structures of H2O and H2O2.

Structure of H₂O:

It is polar molecule with 2 lone pairs electrons. Oxygen atom in water is sp³ hybridised lp - lp repulsion, is greater than bp − bp repulsion. The H − O − H bond angle is reduced from 109°28 to 104°. It has a bent shape.

structure of H₂O₂:

● H₂O₂ has a non - polar structure. The molecular dimensions in the gas phase and solid phase differ.

● Structurally H₂O₂ is represented by the dihydroxyl formula in which the two OH⁻ groups do not lie in the same plane.

● One way of explaining the shape of hydrogen peroxide is that the hydrogen atoms would lie on the pages of a partly opened book, and the oxygen atoms along the spine.

● In the solid phase of molecule, the dihedral angle reduces to 90.2° due to hydrogen bonding and the O - O-H angle expands from 94.8° to 101.9°