Competency based question -Hydrogen

 1)    If same mass of liquid water and a piece of ice are taken, then why is the density of ice less than that of liquid water?

Sol: The mass per unit volume (i.e., mass/volume) is called density. Since water expands on freezing, therefore, volume of ice for the same mass of water is more than liquid water. In other words, density of ice is lower than liquid water and hence ice floats on water.

2)        Give reasons:

(i)   Lakes freeze from top towards bottom.

(ii)   Ice floats on water.

Sol: (i) During severe winter, the temperature of water in the lake keeps on decreasing. Since cold water is heavier, it keeps on going into the interior of the lake while warm water keeps on coming to the surface of the lake. This process continues till the temperature of entire water of lakes becomes 4°C. Since density of water is maximum at 277 K, any further decrease in the temperature will decrease its density. As a result, the temperature of the surface water keeps on decreasing and it ultimately freezes. Now, any further decrease in the temperature will decrease the temperature of water below 4°C. This process continues and as a result, the lakes keep on freezing from top to bottom.

(ii) Density of ice is less than water due to presence of empty spaces created because of H- bonding between H₂0 molecules. Hence, ice floats on water.

3)    When the first element of the periodic table is treated with dioxygen, it gives a compound whose solid state floats on its liquid state. This compound has an ability to act as an acid as well as a base. What products will be formed when this compound undergoes autoionisation?

Sol: The first element is hydrogen and its molecular form is dihydrogen (H₂). It reacts with oxygen to form water whose solid state is ice which is lighter than water and floats over water.

Water is amphoteric in nature, i.e., it acts as an acid in the presence of strong bases and acts as a base in the presence of strong acids.

4)    Rohan heard that instructions were given to the laboratory attendant to store a particular chemical, i.e., keep it in the dark room, add some urea in it, and keep it away from dust. This chemical acts as an oxidizing as well as a reducing agent in both acidic and alkaline media. This chemical is important for use in the pollution control treatment of domestic and industrial effluents.

(i)   Write the name of this compound.

(ii)   Explain why such precautions are taken for storing this chemical.

Sol: (i) The name of the compound is H₂0₂. It acts as an oxidizing as well as reducing agent in both acidic and basic media.

(ii) H₂0₂ is decomposed by light and dust particles. Urea is added as a negative catalyst, i.e., to check its decomposition.                             

2H₂0₂(1)→2H₂0(1) + 0₂(g)

Because of the oxidizing properties, H₂0₂ is widely used to control pollution by oxidation of harmful cyanides and obnoxious smelling sulphides present in domestic and industrial effluents. It also helps in sewage disposal by supplying 0₂ for oxidation of organic matter present – in sewage waters.

5)    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 property.

Sol: (i) The electrical conductance of a molecule depends upon its ionic or covalent nature. Ionic compounds conduct, whereas covalent compounds do not.

BeH₂ is a covalent hydride. Hence, it does not conduct. CaH₂ is an ionic hydride, which conducts electricity in the molten state. Titanium hydride, TiH₂ is metallic in nature and conducts electricity at room temperature. Hence, the increasing order of electrical conductance is as follows:

   BeH₂ < CaH₂ < TiH₂

(ii)   The ionic character of a bond is dependent on the electronegativities of the atoms involved. The higher the difference between the electronegativities of atoms, the smaller is the ionic character.

Electronegativity decreases down the group from Lithium to Caesium. Hence, the ionic character of their hydrides will increase (as shown below).

    LiH < NaH < CsH

(iii)   Bond dissociation energy depends upon the bond strength of a molecule, which in turn depends upon the attractive and repulsive forces present in a molecule. 

The bond pair in D–D bond is more strongly attracted by the nucleus than the bond pair in H–H bond. This is because of the higher nuclear mass of D₂. The stronger the attraction, the greater will be the bond strength and the higher is the bond dissociation enthalpy. Hence, the bond dissociation enthalpy of D–D is higher than H–H.

However, bond dissociation enthalpy is the minimum in the case of F–F. The bond pair experiences strong repulsion from the lone pairs present on each F-centre.

Therefore, the increasing order of bond dissociation enthalpy is as follows:

 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.

6)  Why the solution H₂O₂ did not concentrated on heating? How it can be concentrated? Sol: Hydrogen peroxide obtained by any method is dilute. It cannot be concentrated by boiling because it decomposes at a temperature below its boiling point. Therefore it is concentrated by following step:

a.   Evaporation:

Evaporation of dilute solution of H₂O₂ on water bath at 70°C gives 45 – 50% H₂O₂ solution.

b.   Vacuum evaporation:

The evaporation is further carried on, in a vacuum desiccator over concentrated sulphuric acid. In this way 66% solution of hydrogen peroxide is obtained.

 

c.   Distillation under reduced pressure:

66% solution of hydrogen peroxide on distillation under reduced pressure, yields hydrogen peroxide of 99% concentration.

 d.   Crystallization:

H₂O₂ solution obtained in the above step is placed in a freezing mixture of solid CO₂ and ether. Crystals of H₂O₂ formed are separated and melted to obtain pure H₂O₂.

 7)    For the manufacture of hydrogen peroxide from peroxides, phosphoric acid is more useful than sulphuric acid why?

Sol: H₂SO₄ acts as catalyst in the decomposition of H₂O₂. So for the manufacture of H₂O₂ from peroxides instead of H₂SO₄, weak acids like H₃PO₄, H₂CO₃ etc. are more useful.

3BaO₂ + 2H₃PO₄ → Ba₃(PO₄)₂ + 3H₂O₂

8)    An ionic crystal of an alkali metal has significant covalent character and is almost unreactive towards oxygen and chlorine. This is used in the synthesis of other useful hydrides. Write the formula of this hydride? Write its reaction with Al₂Cl₆?

Sol:

The hydride is LiH. Due to Li it behaves as covalent compound. It is highly stable.

8LiH + Al₂Cl₆ → 2LiAlH₄ + 6LiCl

9)    Why hard water does not give fast lather with soap? Sol:

In hard water bicarbonate, chlorides and sulphates of Ca and Mg are present. Soaps are sodium salt of higher fatty acids e,g., sodium stearate(C₁₇H₃₅COONa). (MPBoardSolutions.com) Salts of calcium and magnesium react and form precipitates of calcium and magnesium stearate.

2C₁₇ H₃₅ COONa + M²⁺ → (CH₁₇ H₃₅ COO)₂ M + 2Na⁺

Until all the salts are precipitated, no lather is formed with soap and this soap is wasted.

 10)     What is conducting water? Write their uses? 

Sol:Kohlrausch distilled the water 42 times at low pressure in an aperture made up of quartz.

This water is called conducting water. This is used for conductivity.

11)     Explain why HCI is a gas and HF is a liquid? Sol:

F is smaller and more electronegative than Cl, so it forms stronger H – bonds as compared to Cl. As a consequence, more energy is needed to break the H – bonds in HF than HCI and hence the boiling point of HF is higher than that of HCI. That’s why HF is liquid and HCI is a gas.

12)     Why H2O2  is called Antichlor? Sol:

In neutral medium H₂0₂ reduce halogen to acid, metal oxides to metals and ozone to 02.

Cl2  + H2O2  → 2HCl + O2 Br2  + H2O2  → 2HBr + O2.

Due to its ability to reduce chlorine it acts as an Antichlor in bleaching by destroying the unreacted chlorine.

13)     Do you expect the carbon hydrides of the type (Cn H2n+2) to act as Lewis acid or base? Justify your answer?

Sol:

CnH2n+2  such as CH4, C2H6  etc. neither act as Lewis acid nor Lewis base. It is because octet of all the carbon atoms are completed.

14)     In which compound the oxidation number of hydrogen is negative? Sol:

When hydrogen reacts with higher metal or highly reactive metals like Na, K, Ca etc then electrovalent hydrides are formed. In this hydride the hydrogen has negative oxidation state.

15)     On the basis of electronic configuration justify the position of hydrogen in periodic table?

Sol:

Hydrogen is the first and the lightest element of the periodic table. It is not a metal but a non – metallic element. But on the basis of electronic configuration it is kept in first group of 5 – block. It is found in atomic form only at high temperature. In elemental form it is found as diatomic molecule i.e., as H2  and is also called as dihydrogen. One proton and one electron is found in hydrogen atom. Hydrogen forms large number of compounds and is an element of high industrial importance.

Hydrogen is the first element of periodic table with one proton in the nucleus and one electron in the first shell (K – shell). It is not possible to assign a to hydrogen in Mendeleev’s and modem periodic table because it shows similarities and dissimilarities with alkali metals (IA group), halogens (VIIA group) and carbon group (IV A group). This makes position of hydrogen very controversial. Because of this it is also known as Rogue element

16)     H2O2  is used for shining the old oil paintings? Sol:

Old oil paintings contain basic lead oxide, H2S present in atmosphere change lead oxide into lead sulphide. Thus, white paintings become black.

When the paintings are washed with hydrogen peroxide black lead sulphide oxidised into lead sulphate and painting shine again.

 

17)     Describe ortho and para hydrogen. How will you obtain para hydrogen? Sol:

Hydrogen atom consists of one proton in the nucleus and one electron in the extra nuclear part. Both electron and nucleus spin about their own axis. (MPBoardSolutions.com) When two atoms of hydrogen combine to form a hydrogen molecule, the spins of electron should be in opposite direction but the spins of nuclei may either be in the same direction or in opposite directions. When nuclear spins are in same direction, it is called ortho hydrogen. When nuclear spins are in opposite directions, it is called para hydrogen.

 

 

 

Preparation of para hydrogen:

Para hydrogen can be obtained from ordinary hydrogen by keeping it in a quartz vessel with active charcoal at 20K for about 3 to 4 hours.

  18)     Why the density of water is maximum at 4°C? Sol:

Maximum density of water:

Melting of ice decreases hydrogen bonds because cage – like structure is broken. With increase in temperature from 0°C to 4°C with the cleavage of H – bonds, water molecules starts moving closer to each other as a result volume decreases while density increases. When the temperature becomes greater than 4°C, kinetic energy of H2O molecule increases resulting in expansion of water. As a result of this volume increases while density decreases. That is at 4°C, density of water is maximum.

19)     Rohan heard that instructions were given to the laboratory attendent to store a particular chemical Le., keep it in the dark room, add some urea in it, and keep it away from dust This chemical acts as an oxidizing as well as a reducing agent in both acidic and alkaline media. This chemical is important for use in the pollution control treatment of domestic and industrial effluents?

  1.     Write the name of this compound.

       2.     Explain, why such precautions are taken for storing this chemical? Sol:

1.     H2O2  (Hydrogen peroxide).

2.     The following precautions must be taken while storing hydrogen peroxide:

3.     It must be kept in wax lined coloured bottles because the rough glass surface, light and dust particles are responsible for its decomposition.

4.     A small amount of negative catalyst such as urea, glycerol, phosphoric acid etc. is generally added which retards its decomposition.

20)     Teacher told the students tat water should be softened before using in broilers. Kamala one of the students, asked the reason. The teacher explained the reason in detail to her and then Kamala got satisfied.

a)   Predict the explanation given by the teacher.

 

b)   Temporary hard water becomes soft on boiling .Why?