Q1. Hydrogen is regarded as a rogue element in the periodic table. Explain.
Ans: Although hydrogen is placed at the top of the alkali metal family of group 1, its position is not fully justified. While all the elements in group 1 are typical metals, hydrogen is a non-metal. Moreover, the same criteria is not used in case of helium (Z = 2). On the basis of electronic configuration (1s2), it should have been placed at the top of alkaline earth metals (group2) but it finds a place at the top of noble gases (group 18) as it resembles them in characteristics. In case hydrogen is placed separately in the periodic table, it would have been the best.
Q2. Elemental hydrogen reacts with other substances only slowly at room temperature. Why?
Ans: In the elementary state, hydrogen exists in diatomic form i.e., as and the two hydrogen atoms are linked to each other by single covalent bond (H?H). Since the bond dissociation energy is very high therefore, bond cleavage is extremely difficult. As a result, elemental hydrogen is little reactive at room temperature.
Q3. Give one example of a reaction in which dihydrogen acts as a reducing agent and an oxidising agent.
Ans: Dihydrogen acts as a reducing agent as it removes oxygen from the oxides of metal upon heating.
Dihydrogen can also act as oxidising agent while reacting with highly electropositive metals belonging to group 1 and 2. For example, in the ionic hydride, Na has transferred an electron to the hydrogen atom. Therefore, it is a reducing agent while diydrogen is an oxidising agent.
Q4. Why is dihydrogen gas not preferred in balloons?
Ans: Dihydrogen is the lightest gas and should have been used in balloons. But it is not preferred due to its highly combustible nature.
Q5. Account for the following in the preparation of H2O2 from barium peroxide BaO2 and dilute H2SO4.
(a) A thin paste of hydrated barium peroxide is used instead of anhydrous peroxide.
(b) The temperature of the reaction mixture is kept at O°C.
(c) The final solution must be kept slightly acidic.
(a) Anhydrous BaO2 is not used because the reaction with dilute H2SO4 is highly exothermic and will accelerate the decomposition of H2O2 into H2O and O2. Moreover, in this case a layer of BaSO4 initially formed in the reaction gets deposited over barium peroxide and any further reaction with acid does not take place. Keeping this in mind, hydrated barium peroxide (BaO2.8H2O) is used.
(b) The temperature of the reaction mixture is kept at O°C because higher temperature will cause the decomposition of hydrogen peroxide.
(c) The traces of ions present in the final solution will act as inhibitor and will retard the decomposition of H2O2.
Q6. Presence of water is avoided in the preparation of H2O2 from Na2O2. Assign reason.
Ans: Na2O2 reacts with water to form NaOH. The reaction is exothermic and the heat evolved causes the decomposition of hydrogen peroxide.
Q7. Explain the following:
(i) Temporary hard water becomes soft on boiling.
(ii) Water can extinguish most fires but not petrol fire.
(iii) Hard water is softened before being used in boilers.
(i) Upon boiling, the bicarbonates of calcium and magnesium decompose to form corresponding carbonates. They being water insoluble, get precipitated and can be removed by filtration. Thus, temporary hard water becomes soft on boiling
(ii) When water is added to petrol fire in order to extinguish it, petrol being lighter floats over water. As a result, the fire flares up rather than getting extinguished.
(iii) When hard water is used in boilers for quite sometime, the salts present in hard water slowly form an insoluble deposit all along inside the boiler. This is called boiler scale. As the scale is a poor conductor of heat, there is wastage of fuel. Moreover, it also reduces the life of the boiler. Therefore, hard water has to be softened before being used in boilers.
Q8. Hydrogen peroxide is used to restore the colour of old oil paintings containing lead oxide. Explain.
Ans: When the old oil paintings are kept in the atmosphere for a long period, the traces of H2S gas present in the atmosphere converts lead oxide (PbO) into lead sulphide (PbS) which is black in colour. Therefore, these paintings get tarnished. The original whiteness can be restored by keeping them in hydrogen peroxide solution for sometime as a result of which lead sulphide is oxidised to lead sulphate.
Q9. Why are the melting and boiling points of heavy water more than those of ordinary water?
Ans: The molecular mass of heavy water (D2O) is more than that of ordinary water (H2O). As a result, there is a greater degree of association of molecules in heavy water than in ordinary water. This results in greater melting and boiling points as compared to ordinary water.
Q10. A mixture of hydrazine, hydrogen peroxide and Cu(II) catalyst is used as rocket fuel. Why?
Ans: In the presence of Cu(II) catalyst, hydrogen peroxide oxidises hydrazine (NH2 - NH2) to nitrogen. The reaction is highly exothermic and is accompanied by large increase in volume. Therefore, the mixture provides an upward thrust and is used as a rocket fuel.
Q11. Is it possible to remove completely the temporary hardness due to Mg(HCO3)2 by boiling?
Ans: Yes, it is possible because on boiling, will readily decompose to form a precipitate of MgCO which can be removed by filtration
As a result, water will become free from temporary hardness.
Q12. Is it correct to say that hydrogen can behave as metal? State the condition under which such behaviour is possible.
Ans: Metals are electropositive elements and when their compounds with non-metals (e.g. NaCI) are electrolysed, these are discharged at cathode. Hydrogen can also behave similarly when combined with a highly electronegative element like chlorine (e.g. HCl). When the electrolysis of the molten acid is carried, hydrogen is also evolved at the cathode.
Q13. Elements with atomic numbers 17 and 20 form compounds with hydrogen. Write the formulae of the two compounds and compare their chemical behaviour in water.
Ans: The elements with atomic numbers 17 and 20 are chlorine and calcium. Their corresponding hydrides are HCl and CaH2. While HCl dissociates in water to give acidic solution, CaH2 reacts with water to evolve hydrogen gas and the resulting solution is basic in nature.
Q14. Compare the chemical properties of H2O and H2O2.
Ans: Both H2O and H2O2 are the covalent compounds of hydrogen and oxygen. These are liquids at room temperature. They differ widely in their chemical characteristics. While hydrogen peroxide in dilute solution is weakly acidic and also acts as an oxidising, reducing as well as bleaching agent; water shows none of these characteristics. It is rather neutral according to Arrhenius theory. For the details of their chemical characteristics, please consult textual knowledge.
Q15. What are the advantages of using hydrogen as a fuel?
Ans: Hydrogen has the following advantages over conventional fuels such as coal, kerosene, petrol etc.
(i) Its combustion is always complete and there are no unburnt particles. Thus, it can solve all pollution problems which are major threat to modem civilisation.
(ii) Hydrogen has more calorific value than petrol and other fuels. For example, 9.5 kg of liquid hydrogen can evolve as much energy on combustion as 25 kg of petrol.
(iii) In space rockets, liquid hydrogen gives higher upward thrust as compared to other fuels.
Q16. Why is ice less dense than water? What types of attractive forces must be overcome to melt ice?
Ans: Ice is less dense or lighter than water because of its porous or cage like structure on account of the presence of intermolecular hydrogen bonding in H2O molecules. When ice is melted to form water, energy has to be supplied to overcome attractive forces on account of hydrogen bonding.
Q17. Explain why phosphoric acid is preferred to sulphuric acid in the preparation of H2O2 from hydrated barium peroxide.
Ans: The reaction with sulphuric acid is given as:
formed is slightly soluble in water.
The ions present in the solution accelerate or promote the decomposition of hydrogen peroxide to water and oxygen. In case phosphoric acid is used, then barium phosphate gets completely precipitated and the solution does not contain any ions.
Q18. Hydrogen peroxide acts both as an oxidising agent and as reducing agent in alkaline solution towards certain first row transition metal ions. Illustrate both these properties of H2O2 using chemical equation.
Ans: H2O2 as oxidising agent in alkaline medium. It oxidises manganese sulphate to manganese dioxide in the presence of an alkali
(Ionic equation) H2O2 as reducing agent in alkaline medium. It reduces potassium ferricyanide to potassium ferrocyanide in the presence of an alkali
Q19. Why cannot water be used to extinguish petrol fire?
Ans: Petrol lighter than water and the two are immiscible. When water is sprayed over petrol, the latter floats over water and its vapours still burn.
Q20. Which isotope of hydrogen is used as a tracer in organic reactions.
Ans: Hydrogen has three isotopes, viz., H, D and T. Due to difference in masses, the rate constants of these isotopes with the same substrate are different. In other words, both D and T show isotope effect. But since T is not only radioactive but is also least abundant hydrogen isotope, therefore, D is used as a tracer to study the mechanism of organic reactions.
Q21. Name one example of a reaction in which dihydrogen acts
(i) as an oxidising agent and
(ii) as a reducing agent.