In this post, you will find the questions and answers of chapter Hydrogen including NCERT questions.
1. Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.
Answer: Hydrogen has been placed at the top of the alkali metal in group, but it is not a member of the group.
Its position is not justified properly because of its electronic configuration as (1s1). It can be placed with alkali metals because it also has similar configuration (ns1) as alkali metals.
However, it can also be placed along with halogen in group 17 since just like halogen it can acquire inert gas configuration by accepting one electron.
2. Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?
Answer: There are three isotopes of hydrogen, namely:
(a) Protium (1H1) represented as H
(b) Deuterium (1H2) represented as D
(c) Tritium (1H3) represented as T
Mass ratio of isotopes, H:D:T = 1:2:3
3. Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?
Answer: In diatomic form, the K-shell of hydrogen is complete (1s2) and so it is quite stable.
4. How can the production of dihydrogen obtained from ‘Coal gasification’ be increased?
Answer: The production of dihydrogen in coal gasification can be increased by reacting CO(y) present in syngas with steam in the presence of iron chromate catalysts.
With the removal of CO2 the reaction shifts in the forward direction and thus, the production of dihydrogen will be increased.
5. Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process?
Answer: In bulk, hydrogen can be produced by electrolysis of acidified water using Pt electrodes.
Electrolyte is added to increase the dissociation of water.
6. Complete the following reactions:
Answer: The balanced chemical equations are:
7. Discuss the consequences of high enthalpy of H-H bond, in terms of chemical reactivity of dihydrogen.
Answer: This is due to its small atomic size and also small bond length (74 pm) of H-H bond.
8. What do you understand by (i) Electron-deficient (ii) Electron-precise (iii) Electron-rich compounds of hydrogen? Provide justification with suitable examples.
Answer: (i) Electron deficient hydrides: Compounds in which central atom has incomplete octet, are called electron deficient hydrides. For example, BeH2, BH3 are electron deficient hydrides.
(ii) Electron precise hydrides: Those compounds in which exact number of electrons are present in central atom or the central atom contains complete octet are called precise hydrides e.g., CH4, SiH4, GeH4 etc. are precise hydrides.
(iii) Electron rich hydrides: Those compounds in which central atom has one or more lone pair of excess electrons are called electron rich hydrides, e.g.,NH3, H2O.
9. What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reaction?
Answer: It is expected to be a Lewis acid. They are likely to accept electrons to become stable. They can form coordinate bond with electron rich compound.
10. Do you expect the carbon hydride of type Cn H2n+2 to act as ‘Lewis’ acid or base? Justify your answer.
Answer: Carbon hydrides of the type Cn H2n+2 are electron precise hydrides. Because they have atom with exact number of electrons to form covalent bonds. Thus, they do not behave as Lewis acid or base. Since they have no tendency to accept or lose electrons.
11. What do you understand by the term ‘non-stoichiometric hydrides’? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.
Answer: Those hydrides which do not have fix composition are called non-stoichiometric hydrides, and the composition varies with temperature and pressure. This type of hydrides are formed by d and /block elements. They cannot be formed by alkali metals because alkali metal hydrides form ionic hydrides.
12. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.
Answer: In metallic hydrides, hydrogen is adsorbed as H-atoms. Due to the adsorption of H atoms the metal Lattice expands and become unstable. Thus, when metallic hydride is heated, it decomposes to form hydrogen and finely divided metal. The hydrogen evolved can be used as fuel.
13. How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes ? Explain.
Answer: When hydrogen is burnt in oxygen the reaction is highly exothermic, it produces very high temperature nearly 4000°C which is used for cutting and welding purposes.
14. Among NH3 H2O and HE, which would you expect to have highest magnitude of hydrogen bonding and why?
Answer: HF is expected to have highest magnitude of hydrogen bonding since, ‘F’ is most electronegative. Therefore, HF is the most polar.
15. Saline hydrides are known to react with water violently producing fire. Can C02, a well known fire extinguisher, be used in this case? Explain.
Answer: No. Because if saline hydrides react with water the reaction will be highly exothermic thus the hydrogen evolved in this case can catch fire. C02 cannot be used as fire extinguisher because C02 will get absorbed in alkali metal hydroxides.
16. 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.
Answer: (i) BeH2< TiH2 < CaH2
(ii) LiH < NaH < CsH
(iii) F—F < H—H < D—D
(iv) H2O < MgH2 < NaH
17. Compare the structures of H2O and H2O2.
Answer: In water, O is sp3 hybridized. Due to stronger lone pair-lone pair repulsions than bond pair-bond pair repulsions, the HOH bond angle decreases from 109.5° to 104.5°. Thus water molecule has a bent structure.
H2O2 has a non-planar structure. The O—H bonds are in different planes. Thus, the structure of H2O2 is like an open book.
18. What do you understand by the term ‘auto-protolysis’ of water? What is its significance?
Answer: Auto-protolysis means self-ionisation of water. It may be represented as:
H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH– (aq)
Due to auto-protolysis water is amphoteric in nature, i.e., it can act as an acid as well as base.
19. Consider the reaction of water with F2 and suggest, in terms of oxidation and reduction, which species are oxidised/reduced?
Answer: 2F2 (g) + 2H2O (l) → O2 (g) + 4H+ (aq) + 4F– (aq)
In this reaction water acts as a reducing agent and itself gets oxidised to O2 while F2 acts as an oxidising agent and hence itself reduced to F– ions.
20. Complete the following chemical reactions:
(i) PbS (s) + H2O2 (aq) →
(ii) MnO4– (aq) + H2O2 (aq) →
(iii) CaO (s) + H2O (g) →
(iv) AlCl3 (s) + H2O (l) →
(v) Ca3N2 (S) + H2O (l) →
Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions.
Answer: (i) PbS (s) +4H2O2 (aq) → PbSO4 (s) + 4H2O (l)
H2O2 is acting as an oxidizing agent in the reaction. Hence, it is a redox reaction.
(ii) 2MnO4– (aq) +H2O2 (aq) + 6H+ (aq) → 2Mn2+ (aq) + 8H2O (l) + 5O2 (g)
H2O2 is acting as a reducing agent in the acidic medium. Hence, it is a redox reaction.
(iii) CaO (s) + H2O (g) → Ca(OH)2 (aq)
The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.
(iv) AlCl3 (aq) + 3H2O (l) → Al(OH)3 (s) + 3HCl (aq)
The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.
(v) Ca3N2 (s) + H2O (l) → 3Ca(OH)2 (aq) + 2NH3 (aq)
The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.
21. Describe the structure of common form of ice.
Answer: Ice is the crystalline form of water. It takes a hexagonal form if crystallized at atmospheric pressure, but condenses to
cubic form if the temperature is very low. The three-dimensional structure of ice is represented as:
The structure is highly ordered and has hydrogen bonding. Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. The structure also contains wide holes that can hold molecules of appropriate sizes interstitially.
22. What causes the temporary and permanent hardness of water?
Answer: Temporary hardness of water is due to the presence of bicarbonates of calcium and magnesium in water i.e., Ca(HCO3)2 and Mg(HCO3) in water.
Permanent hardness of water is due to the presence of soluble chlorides and sulphates of calcium and magnesium i.e., CaCl2, CaSO4, MgCl2 and MgSO4.
23. Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.
Answer: The process of treating permanent hardness of water using synthetic resins is based on the exchange of cations, e.g., Na+, Ca2+, Mg2+, etc and anions, e.g., Cl–, SO42-, HCO3– etc, present in water by H+ and OH– ions respectively.
Synthetic resins are of two types: 1) Cation exchange resins 2) Anion exchange resins
Cation exchange resins have large organic molecule with SO3H group which are insoluble in water. Ion exchange resin (RSO3H) is changed to R-Na on treatment with NaCl. The resin exchange Na+ ions with Ca2+ and Mg2+ ions present in hard water and make it soft.
2R-Na (s) + M2+ (aq) → R2M (s) + 2Na+ (aq), where, M = Mg, Ca.
The resins can be regenerated by adding aqueous NaCl solution.
There are cation exchange resins in H+ form. The resins exchange H+ ions for Na+, Ca2+ and Mg2+ ions.
2RH (s) + M2+ (aq) ⇌ R2M (s) + 2H+ (aq)
Anion exchange resins exchange OH– ions for anions like Cl–, and SO42- present in water.
RNH2 (s) + H2O (l) ⇌ RNH3+.OH– (s)
RNH3+.OH– (s) + X- (aq) → RNH3+.X– (s) + OH– (aq)
During the complete process, water first passes through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature. This acidic water is then passed through the anion exchange process where OH– ions neutralize the H+ ions and deionize the water obtained.
24. Write chemical reaction to show the amphoteric nature of water.
Answer: Water is amphoteric in nature because it acts as an acid as well as a base. Reaction representing the amphoteric nature of water are:
(a) Reaction with H2S: H2O (l) + H2S (aq) → H3O+ (aq) + HS– (aq)
In the forward reaction, H2O accepts a proton from H2S . Hence, H2O acts as a base.
(b) Reaction with NH3: H2O (l) + NH3 (aq) → NH4+ (aq) + OH– (aq)
In the forward reaction, H2O donates a proton to NH3 . Hence, H2O acts as an acid.
(c) Self ionisation of water: H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH– (aq)
In the forward reaction, one molecule of water which donates a proton acts as an acid and the other which accepts a proton acts as a base.
25. Write chemical reactions to justify that hydrogen peroxide can function as an oxidising as well as reducing agent.
Answer: As an oxidising agent: 2Fe2+ (aq) + 2H+ (aq) + H2O2 (aq) → 2Fe3+ (aq) + 2 H2O (l)
H2O2 oxidises Fe2+ into Fe3+, so it acts as oxidising agent.
As a reducing agent: I2(s) + H2O2 (aq) + 2OH– (aq) → 2I– (aq) + 2 H2O(l) + O2(g)
H2O2 reduces I2 to I–, so it acts as reducing agent.
26. What is meant by ‘demineralised water’ and how can it be obtained?
Answer:
Demineralised water is free from all soluble mineral salts. It does not contain any anions or cations. Demineralised water is obtained by passing water successively through a cation exchange (in the H+ form) and an anion exchange (in the OH– form) resin.
During the cation exchange resin, H+ exchanges for Na+, Ca2+, Mg2+ and other cations present in water. This process results in release of proton which makes the water acidic.
2RH (s) + M2+ (aq) ⇌ R2M (s) + 2H+ (aq)
During the anion exchange resin, OH– exchanges, for anions like Cl–, HCO3–,SO42-etc.
RNH2 (s) + H2O (l) ⇌ RNH3+.OH– (s)
RNH3+.OH– (s) + X- (aq) → RNH3+.X– (s) + OH– (aq)
OH– ions liberated in anion exchange process will neutralize the H+ ions liberated in the cation exchange process.
H+(aq) + OH–(aq) → H2O(l)
27. Is demineralised or distilled water useful for drinking purposes? If not, how can it be made useful?
Answer: No, demineralised water is not fit for drinking purposes. It can be made useful by adding required amount of ions which are useful for our body.
28. Describe the usefulness of water in biosphere and biological systems.
Answer: The usefulness of water in biosphere and biological system is:
(a) Major part of all living system is made of water.
(b) It constitutes about 65 – 70% of body weights of animals and plants.
(c) Some properties of water like high specific heat, thermal conductivity, surface tension, high polarity allow water to play a major role in biosphere.
(d) Because of high heat of vaporisation it is responsible ro regulate temperature of living beings.
(e) It is an excellent fluid for the transportation of minerals and nutrients in plants.
(f) It is also required for photosynthesis in plants.
29. What properties of water make it useful as a solvent? What types of compound can it (i) dissolve (ii) hydrolyse?
Answer: Water is highly polar in nature that’s why it has high dielectric constant and high dipole moment. Because of these properties, water is a universal solvent.
It can hydrolyse many oxides metallic or non-metallic, hydrides, carbides, nitrides etc.
30. Knowing the properties of H2O and D2O, do you think D2O can be used for drinking purpose.
Answer: No, D2O is injurious to human beings, plants and animals.
31. What is the difference between the terms hydrolysis and hydration?
Answer: Hydrolysis is a chemical reaction in which a substance reacts with water under neutral, acidic or alkaline conditions.
Na2CO3 + 2H2O → 2NaOH + H2CO3
Hydration is the property of a chemical compound to take up molecules of water of crystallisation and get hydrated.
CuSO4 + 5H2O → CuSO4.5H2O
32. How can saline hydrides remove traces of water from organic compounds?
Answer: Saline hydrides (i.e, CaH2, NaH, etc.) react with water and form the corresponding metal hydroxide with the liberation of H2 gas. Thus, these hydrides can be used to remove traces of water from the organic compounds.
NaH(s) + H2O(l) → NaOH(aq) + H2(g)
CaH2(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
33. What do you expect the nature of hydrides is, if formed by elements of atomic numbers 15,19, 23 and 44 with dihydrogen? Compare their behaviour towards water.
Answer: Atomic No. 15 is of phosphorus. The hydride is PH3 and its nature is covalent.
Atomic No. (Z = 19) is of potassium. The hydride is KH and it is ionic in nature.
Atomic No. (Z = 23) is of vanadium. The hydride is VH. It is interstitial or metallic.
Atomic No. 44 is of ruthenium, its hydride is interstitial or metallic.
34. Do you expect different products in solution when aluminium (III) chloride and potassium chloride treated separately with (i) normal water (ii) acidified water (iii) alkaline water? Write equation wherever necessary.
Answer: (i) In normal water, aluminium chloride reacts with water to form aluminium hydroxide and HCl while potassium chloride will dissolve in water and ions will get hydrated.
(ii) In acidic water, potassium chloride will be unaffected while aluminium chloride reacts to form aluminium and chloride ions.
(iii) In alkaline water, potassium chloride will be unaffected while aluminium chloride form tetrahydroxoaluminate complex or meta-aluminate.
35. How does H2O2 behave as a bleaching agent?
Answer: Bleaching action of H2O2 is due to the oxidation of colouring matter by nascent oxygen.
H2O2 → H2O + [O]
36. What do you understand by the terms: (i) Hydrogen economy (ii) hydrogenation (iii) syngas (iv) water-gas shift reaction
(v) fuel-cell?
Answer: (i) Hydrogen economy: The basic principle of hydrogen economy is the storage and transportation of energy in the form of liquid or gaseous dihydrogen.
(ii) Hydrogenation: Hydrogenation means addition of hydrogen across double and triple bonds in presence of catalyst to form saturated compounds.
(iii) Syngas: The mixture of CO and H2 are called synthesis or syngas. It can be produced by the reaction of steam on hydrocarbon or coke at high temperature in the presence of nickel catalyst. The process of producing syngas from coal is called as coal gasification.
(iv) Water-gas shift reaction: The amount of hydrogen in the syngas can be increased by the action of CO of syngas mixture with steam in the presence of iron chromate as catalyst. This is called water-gas shift reaction.
(v) Fuel-Cell: It is a cell which converts chemical energy of fuel directly into electrical energy.
37. Which isotope of hydrogen is radioactive?
Answer: Tritium.
38. What is the importance of heavy water with regard to nuclear power generation?
Answer: It is used as a moderator and helps to control the nuclear reaction.
39. What is zeolite?
Answer: It is hydrated sodium aluminium silicate, Na2Al2Si2O8.xH2O.
40. What is water gas? How is it prepared?
Answer: Mixture of CO and H2 in the ratio of 1 : 1 is called water gas. It is prepared by passing steam over red hot coke.
41. Write two uses of interstitial hydrides.
Answer: The two uses of interstitial hydrides are: (i) In the storage of H2 and (ii) Catalyst for hydrogenation reaction.
42. Give an example of an ionic hydride and a covalent hydride:
Answer: NaH is an ionic hydride and B2H6 is a covalent hydride.
43. Give two advantages of using hydrogen over gasoline as a fuel.
Answer: Advantage of using hydrogen over gasoline as a duel are:
(a) High heat of combustion
(b) It is pollution free.
44. What type of elements form interstitial hydrides?
Answer: d-block and f-block elements.
45. What is perhydrol?
Answer: Perhydrol is the trade name of H2O2. It is used as an antiseptic.
46. What is meant by hard water?
Answer: Water which does not produce lathers with soap is known as hard water. Hardness is due to the presence of bicarbonates, sulphates and chlorides of Ca2+ and Mg2+.
47. Which gas is evolved when Mg3N2 (Magnesium nitride) is treated with H2O? Give chemical reaction.
Answer: NH3 gas is evolved. The balanced chemical reaction is:
Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH3
48. Which compounds cause temporary hardness of water?
Answer: Ca(HCO3)2 and Mg(HCO3)2 causes temporary hardness of water.
49. Which isotope of hydrogen does not have neutron?
Answer: 1H1 does not have neutron. It is called protium.
50. Name a substance which can oxidise H2O2.
Answer: Acidified KMnO4 can oxidise H2O2.
51. Which type of hydrides are generally non-stoichiometric in nature?
Answer: Interstitial hydrides are non-stoichiometric in nature.
52. What is the cause of bleaching action of H2O2?
Answer: H2O2(l) → H2O(l) + O(g)
Nascent oxygen produced is responsible for bleaching action.
53. What is the use of hydrogen in the manufacture of Vanaspati Ghee?
Answer: H2 is used as reducing agent to convert vegetable oil into vegetable ghee.
54. Name the phenomenon of adsorption of hydrogen on metal surface.
Answer: Occlusion.
55. Show how H2O2 junctions both as a reducing and as an oxidising agent.
Answer: As oxidising agent: 2I– + H2O2 + 2H+ → I2 + 2H2O
As reducing agent: H2O2 + Ag2O → 2Ag + H2O+ O2
56. What are interstitial hydrides? Give two examples.
Answer: Many transition and inner-transition metals absorb hydrogen into the interstices of their lattices to yield metal like hydrides also called the interstitial hydrides. These hydrides are generally non stoichiometric and their composition vary with temperature and pressure, e.g., TiH1.73, CeH2.7
57. The aqueous solution of H2O2 is acidic in nature. Explain with the help of example. Name two substances which catalyse the decomposition reaction of H2O2.
Answer: The aqueous solution of H2O2 is weakly acidic in nature.
H2O2 + H2O ⇌ H3O+ + HO2–
It gives two types of salts with alkalies, peroxides and hydroperoxides.
2NaOH + H2O2 → Na2O2 + 2H2O
NaOH + H2O2 → NaHO2 + H2O
MnO2 and finely divided metals like Pt and Fe catalyse the decomposition of H2O2.
58. Complete the following reactions:
(i) SiCl4 + LiAlH4 →
(ii) Mg3N2 + H2O →
(iii) NaH + CO →
Answer: (i) SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3
(ii) Mg3N2+ 6H2O →3Mg(OH)2+ 2NH3
(iii) NaH + 2CO → HCOONa + C
59. Explain the following:
(i) Temporary hardness can remove by boiling
(ii) Soft water lathers with soap but hard water not.
Answer: (i) On boiling, the bicarbonates of calcium and magnesium decompose to insoluble carbonate which can be removed by filtration.
Ca(HCO3)2 + Heat → CaCO3 + H2O + CO2
Mg(HCO3)2 + Heat → MgCO3 + H2O + CO2
(ii) Because of the presence of Ca2+ and Mg2+ ions in hard water which exchange with Na+ ions of the soap to form corresponding calcium and magnesium salts that form insoluble ppt.
RCOONa + Ca2+ → (RCOO)2Ca + 2Na+
RCOONa + Mg2+ → (RCOO)2Mg + 2Na+
60. (a) How is dihydrogen prepared from water by using a reducing agent?
(b) Give the industrial use of dihydrogen which depends upon heat liberated when it bums.
Answer: (a) Dihydrogen is prepared from water by the action of alkali metals like Na and K which is a strong reducing agent.
2Na + 2H2O → 2NaOH + H2
2K + 2H2O → 2KOH + H2
(b) For welding purposes.
H2(g) + 1\2 O2(g) → H2O (g) + heat
61. Water molecule is bent, not linear. Explain?
Answer: In water molecule, O is SP3 hybridized. Due to stronger lone pair-lone pair repulsion than bond pair-bond pair repulsions, the HOH bond angle decreases from 109.5° to 104.5°. Thus water is bent molecule.
62. Account for the following:
(i) dihydrogen gas is not preferred in balloons.
(ii) Cone. H2SO4 cannot be used for drying H2.
Answer: (i) Dihydrogen is the lightest gas but due to its highly combustible nature it is not preferred in balloons.
(ii) Cone. H2SO4 on absorbing H2O forms moist H2 produces so much heat that hydrogen catches fires.
63. Calculate the volume strength of a 3% solution of H2O2.
Answer: 100 ml of H2O2 solution contain H2O2 = 3g.
1000 ml of H2O2 solution will contain = (3/100) x 1000 = 30g
H2O2 decomposes as: 2H2O2 → 2H2O + O2
68g of H2O2 give O2 at NTP = 22.4 L
30g of H2O2 will give O2 at NTP = (22.4/68) x 30 = 9.88 L = 9880 mL
But 30g of H2O2 is present in 1000 mL of H2O2
1000 mL of H2O2 solution gives O2 at NTP = 9880 mL
1 mL of H2O2 solution will give O2 at NTP = (9880/1000) = 9.88 mL
Hence volume strength of 3% H2O2 is 9.88
64. Complete the following reactions:
Answer: The balanced chemical reactions are:
65. Account for the following:
(a) Can phosphorus with electronic configuration 3s2 3p3 form PH5?
(b) Water is responsible for moderation of body temperature. How?
(c) Hard water is not suitable for boilers as well as for laundry.
Answer: (a) High ∆aH value of dihydrogen and less negative value of ∆eg H of hydrogen do not favour to exhibit highest oxidation state of P and consequently the formation of PH5, although P exhibit +3, +5 oxidation states.
(b) Because of high heat of vaporization and high heat capacity.
(c) Hard water form precipitate with soap and deposition of salts in the form of scales
66. 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?
Answer: (a) Cone. H2S04 cannot be used because it acts as an oxidizing agent also and gets reduced to SO2.
Zn + 2H2SO4 (Conc.) → ZnSO4+ 2H2O + SO2
Pure Zn is not used because it is non-porous and reaction will be slow. The impurities in Zn help in constitute of electrochemical couple and speed up reaction.
(b) Water is amphoteric in nature and it behaves both as an acid as well as base. With acids stronger than itself (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) The decomposition of H2O2 occurs readily in the presence of rough surface (acting as catalyst). It is also decomposed by exposure of light. Therefore, wax lined smooth surface and coloured bottles retard the decomposition of H2O2.
Read More: Chapter 2 Structure of Atom Q and A