Class 11 Chemistry Hydrogen Question and Answers

Hydrogen Question and Answers

Question 1. Explain why concentrated HCl is not used in the laboratory preparation of H₂ gas.
Answer:

Concentrated HCl is not used for the laboratory preparation of dihydrogen because HCl, being highly volatile, gets mixed with dihydrogen.

Question 2. Write down the name and formula of a compound that on electrolysis produces dihydrogen at the anode.
Answer:

Sodium hydride(NaH).

thermodynamics class11 chemistry

Question 3. What is syngas?
Answer:

syngas

All mixtures of CO and H₂ irrespective of their composition are called synthesis gas or syngas.

Question 4. Which isotope of hydrogen is used as a tracer in organic reactions?
Answer:

Deuterium D is usually used as a tracer in determining the mechanism of organic reactions.

Class 11 Chemistry Hydrogen

Question 5. Explain why dihydrogen is not suitable for balloons.
Answer:

As dihydrogen, the lightest substance known, is a highly inflammable gas, it is not suitable for balloons.

Question 6. Which bond between two atoms has the highest bond dissociation enthalpy?
Answer:

Bond dissociation enthalpy of the H —H bond is highest.

Question 7. Explain why H2 is more reactive than D2
Answer:

This is because the H —H bond dissociation enthalpy is less than the D —D bond dissociation enthalpy.

Isotopes of Hydrogen Class 11 Chemistry

Question 8. What change is expected to take place when vegetable oils are hydrogenated?
Answer:

Carbon-carbon double bonds are converted to carbon-carbon single bonds.

Question 9. Which isotope of hydrogen is used in nuclear reactors?
Answer:

Deuterium(H or D).

Question 10. Why are ionic hydrides used as solid fuels?
Answer:

When heated, ionic hydrides decompose to evolve dihydrogen gas which ignites readily.

chapter 6 chemistry class 11

Question 11. The densities of ionic hydrides are greater than that of the metal from which they are formed—why?
Answer:

The densities of ionic hydrides are greater than that of the metal from which they are formed

This is because hydride ions (H) occupy the holes in the lattice of the metal without distorting the metal lattice.

Question 12. Give examples of two interstitial hydrides.
Answer:

CuH and FeH.

Question 13. Which gaseous compound on treatment with dihydrogen produces methanol?
Answer:

Carbon monoxide(CO).

Uses of Hydrogen Class 11 Chemistry

Question 14. Give the chemical reaction that occurs when hydrogen is used as a rocket fuel.
Answer:

\(\mathrm{H}_2(g)+\frac{1}{2} \mathrm{O}_2(g) \rightarrow \mathrm{H}_2 \mathrm{O}(l)+286 \mathrm{~kJ}\)

hydrogen solutions

Question 15. A sample of water containing KCl does not behave as hard water, but a sample of water containing CaCI₂ or MgCl₂ behaves as hard water—why?
Answer:

A sample of water containing KCl does not behave as hard water, but a sample of water containing CaCI₂ or MgCl₂ behaves as hard water

The potassium salt of soap is soluble in water and forms a lather while calcium or magnesium salt of soap is insoluble in water and does not form a lather.

Question 16. What is EDTA, a compound used to determine the hardness of water?
Answer:

EDTA

EDTA is the disodium salt of ethylenediamine tetraacetic acid \(\left[\mathrm{NaO}_2 \mathrm{C}(\mathrm{COOH}) \mathrm{NCH}_2 \mathrm{CH}_2 \mathrm{~N}(\mathrm{COOH}) \mathrm{COONa}\right]\)

ch 6 chemistry class 11

Question 17. Can distilled water be called deionized water?
Answer:

Distilled water can be called deionized water because it does not contain any cations and anions.

Question 18. What is the difference between the water softened by the permit process and the water softened by the organic ion exchangers?
Answer:

The difference between the water softened by the permit process and the water softened by the organic ion exchangers

Although the water softened by the permit process contains no cation, it contains various anions (for example Cl-, SO|- etc.). However, the water softened by the organic ion exchangers contains no cations and anions.

chapter 9 Chemistry, class  11 

Question 19. What will be the hardness of a sample of water, 106 g of which contains| mol A12(SO₄)₃?
Answer:

50 ppm.

Properties of Hydrogen Class 11

Question 20. What is Calgon?
Answer:

Calgon

Sodium hexametaphosphate, Na₂[Na₄(PO₃)₆]

Question 21. Give the chemical formula of permtit.
Answer:

Na₂Al₂Si₂O₈ xH₈O.

Question 22. What is the main source of heavy water?
Answer:

The main source of heavy water

Ordinary water is the main source of heavy water.

Question 23. Can sea animals survive in distilled water?
Answer:

Sea animals cannot survive in distilled water because distilled water contains no salt or dissolved oxygen.

Question 24. Although D20 resembles H20 chemically, it is a toxic substance—why?
Answer:

D₂O is toxic because D⁺ ions react at a much slower rate than H⁺ ions in enzyme-catalyzed reactions.

Question 25. Which compound is used to color hair golden?
Answer:

Dilute solution of H₂O₂.

Question 26. What is the trade name of hydrogen peroxide used as an antiseptic?
Answer:

Perhydrol.

Question 27. What is the strength in the normality of an ‘11.2 volume’ H₂O₂ solution?
Answer:

2(N) H₂O₂ solution.

Question 28. Name a compound that suppresses the decomposition of H₂O₂.
Answer:

Acetanilide (PhNHCOCH₃).

Question 29. H₂O₂ molecule has an open-book-like structure. What is the angle between the two pages of the book in the gas phase?
Answer:

111.5°.

Question 30. Name an organic compound without peroxo bond which is used to manufacture H₂O₂.
Answer:

2-ethylanthraquinol.

Hydrogen Class 11 Notes

Question 31. Why do most of the reactions of H2 occur at much higher temperatures?
Answer:

Due to the much higher bond dissociation enthalpy of the H —H bond, dihydrogen is quite stable and relatively inert at temperature. It dissociates into atoms at about 5000K. For this reason, most of the reactions of dihydrogen occur at much higher temperatures.

ch 6 chemistry class 11

Question 32. What characteristics do you expect from electron-deficient hydrides with respect to their structure and chemical reactivity?
Answer:

Electron-deficient hydrides have less number of electrons in the valence shell of the central atom and so, their mononuclear units do not satisfy the usual Lewis octet rule. Due to a deficiency of electrons, these hydrides act as Lewis acids and form complex entities with Lewis bases such as NH₃, H- ion, etc.

⇒ \(\mathrm{B}_2 \mathrm{H}_6+2 \ddot{\mathrm{N}}_3 \rightarrow\left[\mathrm{BH}_2\left(\mathrm{NH}_3\right)_2\right]^{+}\left[\mathrm{BH}_4\right]^{-}\)

⇒ \(\mathrm{B}_2 \mathrm{H}_6+2 \mathrm{LiH} \rightarrow 2 \mathrm{Li}^{+}\left[\mathrm{BH}_4\right]^{-} \text {(Lithium borohydride) }\)

Question 33. Explain whyit is harmful to bathe in heavy water and use it for drinking purposes.
Answer:

Being a very hygroscopic substance, heavy water (D₂O) absorbs water from the body and thereby damages body cells. Also, it retards some cellular processes such as mitosis, cell division, and various enzyme-catalyzed reactions. For these reasons, it is harmful to bathe in heavy water and use it for drinking purposes.

Question 34. Explain why the thermal stability of H₂O₂ is very low.
Answer:

The bond dissociation enthalpy of the O —O bond presenting H₂O₂ molecule is very low (35kcal . mol^-1) i.e., the bond is very weak. For this reason, the thermal stability of H₂O₂ is extremely low.

Question 35. How the presence of H- ions be confirmed in ionic hydrides?
Answer:

In the molten state, ionic hydrides conduct electricity with the liberation of dihydrogen at the anode. This confirms the presence of hydride (H^– ) ions in them.

Question 36. How do you separate 2 allotropic forms of hydrogen?
Answer:

Ordinary hydrogen is a mixture of 75% of ortho and 25% of para-isomer at room temperature. On passing through activated charcoal kept at 20K, the para-isomer is adsorbed leaving behind the ortho-isomer. From the charcoal surface, para-hydrogen can be released by reducing pressure.

thermodynamics exercises

Question 37. Mention the difference in chemical characteristics of the two hydrides obtained when hydrogen combines with two elements having atomic numbers 17 and 20.
Answer:

The highly electronegative Cl having atomic number 17 combines with hydrogen to form the covalent hydride H —Cl. On the other hand, the highly electropositive Ca atom having atomic number 20 combines with hydrogen to form the ionic hydride CaH₂.

Question 38. Two samples of hard water contain the same cations, Ca²⁺ & Mg²⁺. One is marked as temporary and the other as permanent. In which respect do they differ?
Answer:

The two samples of water differ with respect to the anions of calcium and magnesium salts present. the sample watermarked as temporary hard water contains bicarbonates of Ca²⁺ and Mg²⁺ ions while the sample watermarked as permanent hard water contains chlorides and sulfates of Ca²⁺ and ²⁺ ions.

Question 39. Tube-well water, if left for some time, assumes brownish turbidity—explain.
Answer:

Tube-well water, if left for some time, assumes brownish turbidity

Tube-well water sometimes contains soluble ferrous bicarbonate [Fe(HCO₃)₂]. This compound, on aerial oxidation, is converted into brown ferric hydroxide, Fe(OH)₃, which remains in water as colloidal suspension, and because of this, water assumes a brownish turbidity.

⇒ \(4 \mathrm{Fe}\left(\mathrm{HCO}_3\right)_2+2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2 \rightarrow \underset{\text { (brown) }}{4 \mathrm{Fe}(\mathrm{OH})_3}+8 \mathrm{CO}_2\)

Question 40. Write the reactions for:

1. Preparation of H₂O₂ from two sodium salts and
2. Preparation of D₂O₂ from potassium persulphate.

Answer:

⇒ \(\begin{aligned} & \mathrm{K}_2 \mathrm{~S}_2 \mathrm{O}_8+2 \mathrm{D}_2 \mathrm{O} \rightarrow \mathrm{D}_2 \mathrm{O}_2+2 \mathrm{KDSO}_4 \\ & \text { Potassium } \\ & \text { persulphate } \end{aligned}\)

Question 41. Between deionised water and distilled water which one is more pure and why?
Answer:

Both deionized and distilled water are free from ions. Yet distilled water is superior to deionized water in terms of purity. This is because of the fact that deionized water contains a small amount of dissolved silica and CO₂ along with some germs and organic impurities. However, distilled water prepared in a glass apparatus does not contain any impurities other than trace amounts of silica and CO₂.

Question 42. Why is Na₂O₂ used for purifying air in submarines and in crowded places?
Answer:

Na₂O₂ reacts with CO₂ of air to evolve O₂ (2Na₂O₂ + 2CO₂ —> 2Na₂CO₃ + O₂t). For this reason, Na₂O₂ is used for the purification of air in submarines and in crowded places.

solutions thermodynamics

43. Comment on the reactions of dihydrogen with

1. Chlorine
2. Sodium And
3. Copper
4. Oxide.

Answer:

1. Dihydrogen reduces chlorine (Cl) to chloride ion (Cl-) and itself gets oxidized to form H⁺ ions. These two ions (H⁺ and Cl-) share an electron pair between themselves to form a covalent molecule of hydrogen chloride (HCl)
   ⇒ \(\mathrm{H}_2(g)+\mathrm{Cl}_2(g)\rightarrow 2 \mathrm{HCl}(g)\)
2. Sodium reduces dihydrogen to hydride ion (H~) and itself gets oxidized to sodium ion (Na⁺). In this reaction, an electron gets completely transferred from Na thereby forming ionic sodium hydride (NaH).
   \(2\mathrm{Na}(s)+\mathrm{H}_2(s)\stackrel{\Delta}{\longrightarrow} 2\mathrm{Na}^{+} \mathrm{H}^{-}(s)\)
3. Dihydrogen reduces copper(II) oxide to metallic copper while it gets oxidized to form a covalent molecule of H₂O.
   ⇒ \(\stackrel{+2}{\mathrm{CuO}}\stackrel{-2}{\mathrm{O}}(s)+\stackrel{0}{\mathrm{H}_2}(s)\stackrel{\Delta}{\longrightarrow}\stackrel{0}{\mathrm{Cu}}(s)+\stackrel{+1}{\mathrm{H}_2}\stackrel{-2}{\mathrm{O}}(l)\)

Question 45. An ionic alkali metal hydride has a covalent character to some extent and it does not react with oxygen and chlorine. This hydride is used in the synthesis of another hydride. Write the formula of the hydride and what happens when it reacts with AI₂CI₆.
Answer:

Since the alkali metal hydride possesses sufficient covalent character, the hydride is of the smallest alkali metal, Li, i.e. the hydride is LiH. As LiH is quite stable, it does not react with oxygen and chlorine. Lithium hydride reacts with AI₂CI₆ to form lithium aluminum hydride (LiAlH₄) which is extensively used as a reagent in the synthesis of different organic compounds.

⇒ \(8\mathrm{LiH}+\mathrm{Al}_2\mathrm{Cl}_6\longrightarrow2\mathrm{LiAlH}_4+6\mathrm{LiCl}\)

Question 46. Sodium reacts with dlliydrogcn to form a crystalline ionic solid. It is non-volatile and a non-conductor of electricity. It also reacts vigorously with water to liberate II2 gas. Write the formula of the Ionic solid and give a reaction between the lids solid & water. What happens when the ionic solid in its molten stater Is electrolysed?
Answer:

Sodium reacts with dihydrogen to form sodium hydride which is a crystalline ionic solid.

⇒ \(2\mathrm{Na}+\mathrm{H}_2\stackrel{\Delta}{\longrightarrow}2\mathrm{Na}^{+}\mathrm{H}^{-}\)

Sodium hydride reacts with water as follows:

⇒  \(2 \mathrm{NaH}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow 2 \mathrm{NaOH}+2 \mathrm{H}_2\)

Sodium hydride, in its solid state, does not undergo electrolysis. However, in its molten state, undergoes electrolysis. However, in its molten state, NaH undergoes electrolysis to liberate dihydrogen (H₂) at the anode and metallic sodium at the cathode.

⇒ \(\mathrm{Na}^{+} \mathrm{H}^{-}(\text {molten }) \stackrel{\text { Electrolysis }}{\longrightarrow} \underset{\text { Cathode }}{2 \mathrm{Na}(\text { molten })}+\underset{\text { anode }}{\mathrm{H}_2(g)}\)

Question 47. Why is seawater not used in boilers?
Answer:

1. Sea water is hard water. It contains Mg(HCO₃)₂ and Ca(HCO₃)2, which on boiling, form a hard heat-insulating thick layer or scale of MgCO₃ and CaCO₃ on the inner surface of the boiler. Consequently, much heat is required to raise the temperature of the boiler, and thus, fuel economy is adversely affected.
2. Again at much higher temperatures, the boiler scales and the metal of the boiler expand unequally. Due to such uneven expansion, cracks are formed on the scales. When hot water comes in contact with the hot metal surface of the boiler through these cracks, it is suddenly converted into steam. Due to the high pressure, thus developed, the boiler may burst leading to accidents. Again, MgCl₂ and MgSO₄ present in seawater undergo hydrolysis to form HCl and H₂SO₄ which degrade the metallic component of the boiler. So, seawater is not used in boilers.

Question 48. The values of melting point, enthalpy of vaporization, and viscosity of H₂O and D₂O are given below:

1. Melting point(K)     373.0             374.4
2. Enthalpy of
   vaporisation
   (kJ • mol^-1, 373K)     40.66         41.61
3. Viscosity(centipoise) 0.8903      1.107

From the given data, determine which liquid has a greater magnitude of intermolecular forces of attraction.

Answer:

The magnitude of intermolecular forces of attraction depends on the magnitudes of melting point, enthalpy of vaporization, and viscosity of liquid. As, these parameters have higher values in the case of D₂O, the magnitude of intermolecular forces of attraction is greater for D₂O than for H₂O.

Question 49. How will you prepare heavy water from ordinary water? Explain its principle.
Answer:

Heavy water (D₂O) is prepared by prolonged electrolysis of ordinary water. As water is not a good conductor of electricity, an alkaline solution of water [~0.5(N) NaOH] is used for electrolysis. The bond dissociation energy of the O —H bond is less than that of the O —D bond. So, electrolysis of H₂O occurs at a faster rate and more easily than D₂O. Consequently, the amount of D₂O in ordinary water increases. Pure D₂O is obtained when the amount of residual liquid decreases.

Question 50. Can phosphorus form PHg with its outer electronic configuration of 3s²3p³?
Answer:

Phosphorus cannot form PH5 although it shows +3 and +5 oxidation states. Dihydrogen acts as a weak oxidizing agent due to the high bond dissociation enthalpy of H—Hbond (435.88 kJ-mo^-1) and slightly negative electron-gain enthalpy (-73 kJ mol^-1). So, dihydrogen can oxidize phosphorus to a +3 oxidation state but not to its highest oxidation state of +5. Therefore, phosphorous can form only PH₃ and not PH₅.

Question 51. How will you prepare dinitrogen from HNO₃?
Answer:

Magnesium and manganese react with a very dilute solution of HN03(2%) to form dihydrogen.

⇒ \(\mathrm{Mg}+2\mathrm{HNO}_3\rightarrow\mathrm{Mg}\left(\mathrm{NO}_3\right)_2+\mathrm{H}_2 \uparrow\)

⇒ \(\mathrm{Mn}+2\mathrm{HNO}_3\rightarrow\mathrm{Mn}\left(\mathrm{NO}_3\right)_2+\mathrm{H}_2 \uparrow\)

Question 52. Do you think the hydrides of N, O, and F will have lower boiling points than the hydrides of their corresponding group members? State reasons.
Answer:

The hydrides of the elements N, O, and F are NH₃, H₂O, and HF respectively. These hydrides are expected to have lower boiling points than that of their corresponding group members (PH₃, H₂S & HCl) when their masses are considered.
However, because of the high electronegativity of N, O, and F, their hydrides undergo extensive hydrogen bonding (intermolecular). As a result, boiling points of NH3, H₂O, and HF are much higher than the hydrides of their corresponding group members, i.e., PH₃, H₂S, and HCl respectively.

Question 53. KF reacts with HF to form the compound, KF-2HF. Discuss the probable structure of the compound.
Answer:

The h-bond in the HF molecule is very strong. When KF gets added to HF, one F^– ion forms an H — bond with two HF molecules to form the H₂F^–₃ ion [F—-H —F—H —F —].

Question 54. Calculate the amount of energy liberated due to combustion of 4g dihydrogen.
Answer:

Amount of energy liberated due to combustion of1 mol, i.e., 2g dihydrogen = 242 kJ • mol^-1

⇒ \(\left[\mathrm{H}_2(g)+\frac{1}{2} \mathrm{O}_2(g) \rightarrow \mathrm{H}_2 \mathrm{O}(g) ; \Delta \mathrm{H}=-242 \mathrm{~kJ}\cdot\mathrm{mol}^{-1}\right]\)

Amount of energy liberated due to 4g dihydrogen = 242×2 = 484 kJ-mol-1

Question 55. Under what conditions, water reacts with calcium cyanamide and what are the products formed due to this reaction?
Answer:

Superheated steam reacts with calcium cyanamide under high pressure to form ammonia gas and calcium carbonate as a result of hydrolysis.
\(\text { CaNCN }(\text{Calciumcyanamide })+3\mathrm{H}_2\mathrm{O}\rightarrow\mathrm{CaCO}_3+2\mathrm{NH}_3\)

Question 56. Why H₂O₂ is a better oxidant than water?
Answer:

H₂O₂ is a better oxidant than water because H₂O₂ being unstable readily dissociates to form stable water molecules along with the evolution of O₂ gas. \(2 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2+\text { Heat }\)

Solved Questions

Question 1. What is heavy water? Why is it so-called?
Answer:

Heavy water

Deuterium oxide is commonly known as heavy water because its density is higher than that of ordinary water, i.e., it is heavier than ordinary water.

Question 2. A water sample contains 1 millimole of Mg²⁺ ion per liter. Calculate the hardness of the water sample in ppm units.
Answer:

1 millimol of Mg2+=1 millimole of MgCl2 = 0.095 g of MgCL2.

1 1. or 1000 g or 103 g of water contains 0.1 g of MgCl2.

106g of water contains 0.095 x 103 = 95g of MgCl2

The degree of hardness of water is 95 ppm.

Question 3. BaO₂ is a peroxide but MnO₂ is not a peroxide explain.
Answer:

There is peroxide linkage in the Ba02 molecule ( —O —O—). But there is no such bonding present in MnO, molecule (O —Mn=0). Thus Mn02 is not a peroxide.