RBSE Solutions Class 11 Chemistry Chapter 9 Hydrogen

Question 1. Pure hydrogen is obtained by electrolysis of
(a) Water containing Sulphuric acid
(b) Water containing NaOH
(c) Ba(OH)2
(d) Water containing KOH
Answer: (c) Ba(OH)2
In simple words: Pure hydrogen is made when electricity passes through barium hydroxide solution, splitting the water into hydrogen and oxygen.

🎯 Exam Tip: Remember that the purity of hydrogen depends on the electrolyte used; Ba(OH)2 is specifically chosen for pure hydrogen.

Question 3. Which of the following is not a property of hydrogen peroxide?
(a) Reducing agent
(b) Oxidising agent
(c) Dehydration
(d) Bleaching agent
Answer: (c) Dehydration
In simple words: Hydrogen peroxide can reduce, oxidize, and bleach, but it does not cause dehydration (removing water) in reactions.

🎯 Exam Tip: Hydrogen peroxide is known for its strong oxidizing and mild reducing properties, often acting as a bleaching agent due to oxygen release.

Question 4. How ortho and para hydrogen are different?
(a) Number of protons
(b) Molecular mass
(c) Direction of rotation of electron
(d) Direction of rotation of proton
Answer: (d) Direction of rotation of proton
In simple words: Ortho and para hydrogen differ in how their protons (the nucleus of the hydrogen atom) spin. In ortho-hydrogen, the protons spin in the same direction, while in para-hydrogen, they spin in opposite directions.

🎯 Exam Tip: Focus on the spin direction of the nucleus (proton) when distinguishing between ortho and para hydrogen, as this is the key difference.

Question 5. Heavy water is
(a) D2O
(b) D2O2
(c) H2O
(d) H2O2
Answer: (a) D2O
In simple words: Heavy water is water where the usual hydrogen atoms are replaced by deuterium, which is a heavier form of hydrogen, making its chemical formula \( D_{2}O \).

🎯 Exam Tip: Remember that heavy water replaces hydrogen with its heavier isotope, deuterium (D), leading to the formula \( D_{2}O \).

Question 7. In which group hydrogen is placed on the basis of electronic configuration?
Answer: Hydrogen has an electronic configuration of \( 1s^{1} \). Because of this configuration, it is placed in group 1 of the periodic table, similar to alkali metals.
In simple words: Hydrogen is in group 1 because its electron setup, \( 1s^{1} \), is like the alkali metals.

🎯 Exam Tip: When discussing hydrogen's position, always refer to its unique electronic configuration, which is the primary reason for its placement dilemmas.

Question 8. Write the name of isotopes of hydrogen and write the number of proton, electron and neutron in these.
Answer: The isotopes of hydrogen are:
(a) Hydrogen (protium): \( _{1}^{1} \mathrm{H} \). It has 1 proton, 1 electron, and 0 neutrons (1 - 1 = 0).
(b) Deuterium: \( _{1}^{2} \mathrm{H} \). It has 1 proton, 1 electron, and 1 neutron (2 - 1 = 1).
(c) Tritium: \( _{1}^{3} \mathrm{H} \). It has 1 proton, 1 electron, and 2 neutrons (3 - 1 = 2).
In simple words: Hydrogen has three forms: protium (no neutrons), deuterium (one neutron), and tritium (two neutrons). All have one proton and one electron.

🎯 Exam Tip: For isotopes, the atomic number (protons and electrons) stays the same, but the mass number (and thus neutrons) changes.

Question 9. What is ortho-hydrogen?
Answer: Ortho-hydrogen is a form of hydrogen where the rotation of the protons (or nucleus) in the two hydrogen atoms within the molecule is in the same direction.
In simple words: Ortho-hydrogen is when the two tiny centers (protons) inside a hydrogen molecule spin the same way.

🎯 Exam Tip: The distinction between ortho and para hydrogen lies solely in the relative spin direction of their nuclei.

Question 11. Helium is filled in balloons in place of hydrogen. Why?
Answer: Helium is used in balloons instead of hydrogen because helium is a noble gas, which means it is very stable and does not react easily, so it is not explosive. Hydrogen, on the other hand, is highly flammable and explosive.
In simple words: Helium is safe because it's not explosive, but hydrogen is dangerous because it can explode.

🎯 Exam Tip: The non-flammable nature of helium makes it a much safer choice for balloons compared to explosive hydrogen.

Question 12. What is the oxidation number of oxygen in water?
Answer: In a water molecule, \( H_{2}O \):
Let the oxidation number of oxygen be \( x \).
We know that hydrogen usually has an oxidation number of \( +1 \).
So, for \( H_{2}O \):
\( 2(+1) + x = 0 \)
\( 2 + x = 0 \)
\( x = -2 \)
Therefore, the oxidation number of oxygen in water is \( -2 \).
In simple words: In water, oxygen's oxidation number is -2 because each hydrogen is +1, and the total charge must be zero.

🎯 Exam Tip: Remember that hydrogen's oxidation state is usually +1, and the sum of oxidation states in a neutral compound is zero.

Question 13. What is the value of bond angle in water molecule?
Answer: The bond angle in a water molecule is \( 104.5^{\circ} \). This specific angle is due to the presence of two lone pairs of electrons on the oxygen atom, which push the hydrogen atoms closer together.
In simple words: The H-O-H angle in a water molecule is \( 104.5^{\circ} \).

🎯 Exam Tip: Recall that the bond angle in water is slightly less than the ideal tetrahedral angle (\( 109.5^{\circ} \)) due to the repulsion from lone pairs of electrons on oxygen.

Question 14. What is the reason of high boiling point of water?
Answer: Water has a high boiling point because of strong intermolecular hydrogen bonding present between its molecules. A lot of energy is needed to break these strong hydrogen bonds, which is why water boils at a higher temperature.
In simple words: Water boils at a high temperature because its molecules are strongly linked by hydrogen bonds, requiring much energy to break them apart.

🎯 Exam Tip: Always associate high boiling points in compounds like water with strong intermolecular forces, especially hydrogen bonding.

Question 15. At what temperature, density of water is maximum.
Answer: The density of water is maximum at \( 4^{\circ}C \) temperature. Below this temperature, water molecules start forming an open, cage-like structure as ice, which makes it less dense.
In simple words: Water is heaviest at \( 4^{\circ}C \).

🎯 Exam Tip: Remember the anomalous expansion of water, where its maximum density occurs at \( 4^{\circ}C \), not \( 0^{\circ}C \).

Question 17. What is the reason of temporary hardness of water?
Answer: Temporary hardness in water is caused by the presence of soluble bicarbonates of calcium and magnesium. These bicarbonates can be removed by boiling.
In simple words: Temporary hardness in water comes from calcium and magnesium bicarbonates dissolved in it.

🎯 Exam Tip: Distinguish temporary hardness (due to bicarbonates) from permanent hardness (due to chlorides and sulfates).

Question 18. Write the chemical formula of calgon.
Answer: The chemical formula of calgon is \( Na_{2}[Na_{4}(PO_{3})_{6}] \). It is also known as sodium hexametaphosphate.
In simple words: Calgon's chemical name is sodium hexametaphosphate, and its formula is \( Na_{2}[Na_{4}(PO_{3})_{6}] \).

🎯 Exam Tip: Memorize the chemical formula of common water softeners like Calgon, as it is often asked directly.

Question 19. What is the oxidation number of oxygen in hydrogen peroxide?
Answer: In a hydrogen peroxide molecule, \( H_{2}O_{2} \):
Let the oxidation number of oxygen be \( x \).
Hydrogen usually has an oxidation number of \( +1 \).
For \( H_{2}O_{2} \):
\( 2(+1) + 2x = 0 \)
\( 2 + 2x = 0 \)
\( 2x = -2 \)
\( x = \frac{-2}{2} \)
\( x = -1 \)
So, the oxidation number of oxygen in \( H_{2}O_{2} \) is \( -1 \).
In simple words: In hydrogen peroxide, oxygen has an oxidation number of -1, which is different from its usual -2 in most compounds.

🎯 Exam Tip: Remember that in peroxides (like \( H_{2}O_{2} \)), oxygen has an oxidation state of -1, an exception to its common -2 state.

Question 20. How is the structure of hydrogen peroxide?
Answer: Hydrogen peroxide has a non-planar structure. It looks like an "open book" where the two hydrogen atoms and two oxygen atoms are not all in the same flat plane.
In simple words: Hydrogen peroxide has a twisted shape, like an open book, not a flat one.

🎯 Exam Tip: Visualize the non-planar, open-book structure of \( H_{2}O_{2} \) to understand its unique properties and bond angles.

RBSE Class 11 Chemistry Chapter 9 Short Answer Type Questions

Question 21. Write the similarities of hydrogen with alkali metals.
Answer: Hydrogen shares the following similarities with alkali metals:
1. Electronic Configuration: Hydrogen's electronic configuration is \( 1s^{1} \), which is similar to the outermost electron configuration of alkali metals (e.g., \( ns^{1} \)). This is why it is often placed in group 1.
2. Electropositive Character: Like alkali metals, hydrogen can form a positive ion (\( H^{+} \)) by losing one electron. For example, \( H \rightarrow H^{+} + e^{-} \) and \( Na \rightarrow Na^{+} + e^{-} \).
3. Oxidation State: Hydrogen can show an oxidation state of \( +1 \) in its compounds, just like alkali metals.
4. Valency: Hydrogen has a valency of 1, similar to alkali metals.
5. Reaction with Non-metals: Both hydrogen and alkali metals form binary compounds with non-metals.
6. Reducing Nature: Hydrogen acts as a good reducing agent, similar to alkali metals.
In simple words: Hydrogen is like alkali metals because both have one outer electron, can lose it to become positive, have a valency of 1, react with non-metals, and act as reducing agents.

🎯 Exam Tip: When listing similarities, focus on electronic configuration, ion formation, and common chemical behaviors like oxidation state and reducing properties.

Question 22. Write the similarities of hydrogen with halogens.
Answer: Hydrogen shares the following similarities with halogens:
(i) Electronic Configuration: Hydrogen has an electronic configuration of \( 1s^{1} \). Halogens have a general configuration of \( ns^{2}np^{5} \). Both need just one more electron to achieve a stable noble gas configuration. Hydrogen can gain an electron to become \( H^{-} \) like halogens gain an electron to become \( X^{-} \).
(ii) Oxidation State: Hydrogen can exhibit an oxidation state of \( -1 \) in some compounds (like metal hydrides, e.g., \( NaH \)), similar to halogens (e.g., \( NaCl \), where \( Cl = -1 \)).
(iii) Valency: Hydrogen has a valency of 1, just like halogens.
(iv) Reaction with Metals: Hydrogen combines with metals to form binary compounds (e.g., \( 2Na + H_{2} \rightarrow 2NaH \)), similar to how halogens react with metals (e.g., \( 2Na + Cl_{2} \rightarrow 2NaCl \)).
(v) Atomicity: Hydrogen exists as a diatomic molecule (\( H_{2} \)) under normal conditions, just like halogens (e.g., \( Cl_{2} \)).
In simple words: Hydrogen is like halogens because both need one electron to be stable, can have a -1 charge, have a valency of 1, react with metals, and exist as two-atom molecules.

🎯 Exam Tip: Highlight hydrogen's ability to gain an electron and form diatomic molecules as key similarities with halogens.

Question 23. Why ice floats on water?
Answer: Ice floats on water because it is less dense than liquid water. Ice is the crystalline form of water and has a highly organized three-dimensional structure held together by hydrogen bonds. X-ray studies show that each oxygen atom in ice is surrounded by four other oxygen atoms in a tetrahedral shape, at a distance of 276 pm. Each hydrogen atom forms a covalent bond with one oxygen and a hydrogen bond with another oxygen. This specific arrangement creates large open spaces within the ice structure, making it less compact and therefore less dense than liquid water.
In simple words: Ice floats because its structure has empty spaces, making it lighter than the same amount of liquid water.

🎯 Exam Tip: The open, cage-like structure formed by hydrogen bonding in ice is the fundamental reason for its lower density compared to liquid water.

Question 24. Explain the washing soda method for removal of permanent hardness of water.
Answer: The washing soda method removes permanent hardness from water. Washing soda (\( Na_{2}CO_{3} \)) reacts with the soluble chlorides and sulphates of calcium and magnesium, which cause permanent hardness. These reactions form insoluble carbonates of calcium and magnesium, which can then be easily filtered out of the water. This process makes the water soft.
The reactions are:
\( CaCl_{2} + Na_{2}CO_{3} \rightarrow CaCO_{3} \downarrow + 2NaCl \)
\( MgCl_{2} + Na_{2}CO_{3} \rightarrow MgCO_{3} \downarrow + 2NaCl \)
\( CaSO_{4} + Na_{2}CO_{3} \rightarrow CaCO_{3} \downarrow + Na_{2}SO_{4} \)
\( MgSO_{4} + Na_{2}CO_{3} \rightarrow MgCO_{3} \downarrow + Na_{2}SO_{4} \)
In simple words: Washing soda removes hard water minerals like calcium and magnesium by turning them into solid bits that can be filtered out.

🎯 Exam Tip: Ensure you write the correct chemical formulas and balanced equations for the precipitation reactions involved in the washing soda method.

Question 25. Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?
Answer: Hydrogen has an electronic configuration of \( 1s^{1} \). Its outermost shell is not complete; it needs one more electron to achieve the stable configuration of helium (duplet). To achieve this stability, a hydrogen atom forms a covalent bond with another hydrogen atom. This results in a diatomic molecule (\( H_{2} \)) where both hydrogen atoms share electrons to complete their shells, making the \( H_{2} \) molecule much more stable than individual hydrogen atoms. This is why hydrogen exists as a diatomic molecule rather than a single atom under normal conditions.
In simple words: Hydrogen atoms join together to form \( H_{2} \) molecules to become stable. Each atom shares an electron to fill its outer shell, which is much more stable than being alone.

🎯 Exam Tip: Emphasize the concept of achieving a stable duplet configuration through covalent bonding as the main reason for hydrogen's diatomic nature.

Question 26. Why heavy water are slow?
Answer: Heavy water (\( D_{2}O \)) is used as a moderator in nuclear reactors to slow down fast-moving neutrons. Deuterium, being heavier than protium (normal hydrogen), forms weaker bonds with oxygen in \( D_{2}O \) compared to \( H_{2}O \). When neutrons collide with the deuterium atoms in heavy water, they lose energy more effectively and slow down without being absorbed, which is crucial for controlling the nuclear chain reaction. This "slowing down" capability is why heavy water is referred to as "slow."
In simple words: Heavy water is called "slow" because it helps slow down fast neutrons in nuclear reactors more effectively than regular water, controlling the reaction without absorbing the neutrons.

🎯 Exam Tip: Connect the "slow" property of heavy water to its role as a neutron moderator in nuclear fission, leveraging the mass of deuterium.

Question 28. Explain the laboratory method for preparation of dihydrogen.
Answer: In the laboratory, dihydrogen is typically prepared by reacting granulated zinc with dilute sulphuric acid. The process involves taking granulated zinc in a Woulf's bottle, and dilute sulphuric acid is added slowly through a thistle funnel. The hydrogen gas produced is collected in a gas jar by the downward displacement of water, as hydrogen is very light and insoluble in water.
The chemical reaction is:
\( Zn + H_{2}SO_{4} \rightarrow ZnSO_{4} + H_{2} \uparrow \)
In simple words: To make hydrogen in the lab, mix zinc bits with dilute sulfuric acid. The gas that forms is collected by pushing water out of a jar.

🎯 Exam Tip: Remember the reactants (granulated zinc and dilute sulphuric acid) and the collection method (downward displacement of water) for the laboratory preparation of hydrogen.

Question 30. What is heavy water?
Answer: Heavy water is deuterium oxide (\( D_{2}O \)). It is a form of water in which the hydrogen atoms are replaced by deuterium, which is a heavier isotope of hydrogen. It was discovered by the American scientist Harold C. Urey in 1932.
In simple words: Heavy water is like regular water, but its hydrogen atoms are replaced by a heavier type called deuterium, making it \( D_{2}O \).

🎯 Exam Tip: Define heavy water by its chemical formula (\( D_{2}O \)) and its composition (deuterium instead of protium).

Question 31. Why ancient lead paintings are washed with dilute solution of hydrogen peroxide?
Answer: Ancient lead paintings often turn black over time because the lead-based pigments react with hydrogen sulfide gas present in the atmosphere to form black lead sulfide (\( PbS \)). To restore these paintings, they are washed with a dilute solution of hydrogen peroxide (\( H_{2}O_{2} \)). Hydrogen peroxide oxidizes the black lead sulfide (\( PbS \)) into white lead sulfate (\( PbSO_{4} \)), thereby removing the black discoloration and restoring the original colors.
The reaction is:
\( PbS + 4H_{2}O_{2} \rightarrow PbSO_{4} + 4H_{2}O \)
In simple words: Old lead paintings turn black due to lead sulfide. Hydrogen peroxide changes this black lead sulfide back into white lead sulfate, cleaning the painting.

🎯 Exam Tip: Understand the chemical transformation: black lead sulfide is oxidized to white lead sulfate by hydrogen peroxide, restoring the painting's colors.

Question 32. Explain the bleaching action of hydrogen peroxide.
Answer: Hydrogen peroxide (\( H_{2}O_{2} \)) acts as a mild but effective bleaching agent. Its bleaching action occurs due to its ability to decompose and release nascent oxygen (a single oxygen atom, \( [O] \)), which is a powerful oxidizing agent. This nascent oxygen reacts with colored substances, oxidizing them into colorless substances.
The decomposition reaction is:
\( H_{2}O_{2} \rightarrow H_{2}O + [O] \)
And the bleaching action is:
\( \text{Colouring matter} + [O] \rightarrow \text{Colourless matter} \)
In simple words: Hydrogen peroxide bleaches by breaking down and releasing single oxygen atoms. These oxygen atoms then react with colors, making them disappear.

🎯 Exam Tip: The key to hydrogen peroxide's bleaching action is the release of nascent oxygen, which then oxidizes and decolorize colored compounds.

Question 33. What is coal gasification?
Answer: Coal gasification is a process that converts coal into a gaseous mixture known as 'syngas' or synthetic gas. Syngas is primarily a mixture of carbon monoxide (\( CO \)) and hydrogen (\( H_{2} \)). This syngas is highly valuable as it can be used to synthesize methanol and various other hydrocarbons, serving as an important chemical feedstock.
The reaction for coal gasification is:
\( C + H_{2}O \rightarrow CO + H_{2} \)
(Coal + Steam \( \rightarrow \) Carbon Monoxide + Hydrogen)
In simple words: Coal gasification is a process that turns coal into a useful gas mixture called syngas (carbon monoxide and hydrogen), which is then used to make other chemicals.

🎯 Exam Tip: Remember that coal gasification produces syngas (\( CO + H_{2} \)), which is a crucial industrial raw material for synthesizing various chemicals.

Question 35. What is known as storage of hydrogen and why?
Answer: The storage of hydrogen refers to the methods and technologies used to safely and efficiently store hydrogen for various applications. It is a critical area of research because efficient hydrogen storage is essential for advancing hydrogen and fuel cell technologies, which can be used in stationary power, portable power, and transportation. Hydrogen is favored for storage because its calorific value (energy released per unit mass) is much higher than that of petrol and other fuels, making it a powerful energy carrier.
In simple words: Storing hydrogen means finding ways to keep it safely for future use. This is important for new energy technologies because hydrogen has more energy than petrol.

🎯 Exam Tip: Focus on hydrogen's high calorific value and its importance for fuel cell and energy storage applications as key reasons for its study.

RBSE Class 11 Chemistry Chapter 9 Long Answer Type Questions

Question 36. It is justified to place hydrogen in a separate place in periodic table. Explain the statement.
Answer: The position of hydrogen in the periodic table has always been a topic of discussion due to its unique properties, which show similarities with both alkali metals (Group 1) and halogens (Group 17). While hydrogen has an electronic configuration of \( 1s^{1} \), suggesting placement in Group 1, it also needs only one electron to complete its shell, similar to halogens. Its behavior is abnormal compared to both groups. For instance, hydrogen can lose an electron to form \( H^{+} \) (like alkali metals) but also gain an electron to form \( H^{-} \) (like halogens). Given these dual characteristics and its distinct behavior, it is indeed justified to assign hydrogen a separate, unique position in the periodic table, rather than forcing it into either Group 1 or Group 17, as its unique properties cannot be fully accommodated by either group's characteristics.
In simple words: Hydrogen has properties like both alkali metals and halogens, but it doesn't perfectly fit with either. Because it's so special, it should have its own place in the periodic table.

🎯 Exam Tip: When justifying hydrogen's position, discuss its similarities to both Group 1 and Group 17 elements, but highlight its unique dual nature that prevents a clear fit into either.

Question 37. What is hydride? How many types of hydrides are there? Explain with example.
Answer: Hydrides are binary compounds formed when dihydrogen (\( H_{2} \)) combines with other elements, except noble gases. These compounds are categorized into three main types:
1. Covalent or Molecular Hydrides: Dihydrogen forms molecular hydrides with most p-block elements. These are discrete covalent molecules held together by weak Van der Waals forces. They have low melting and boiling points and are volatile. Examples include \( CH_{4} \), \( NH_{3} \), \( H_{2}O \), and \( HF \). These are further classified based on the number of electrons and bonds in their Lewis structure.
2. Ionic or Saline Hydrides: These are stoichiometric compounds formed by dihydrogen with most s-block elements (alkali and alkaline earth metals). They are crystalline solids, non-volatile, and non-conducting in the solid state. They possess high melting and boiling points. Examples include \( NaH \), \( BeH_{2} \), and \( MgH_{2} \).
3. Metallic or Interstitial Hydrides: These hydrides are formed when dihydrogen reacts with d-block and f-block elements. Due to the small size of hydrogen, it occupies the empty spaces (interstices) within the metal lattice without changing its type, but causing some distortion. They are often non-stoichiometric, meaning they are deficient in hydrogen. Examples include \( ScH_{2} \), \( LaH_{2} \), \( TiH_{2} \), and \( CrH_{3} \).
In simple words: Hydrides are compounds where hydrogen bonds with other elements. There are three types: covalent (like water), ionic (like \( NaH \)), and metallic (where hydrogen fits into metal gaps).

🎯 Exam Tip: Clearly define each type of hydride by the block of elements it forms with, its physical properties, and provide specific examples for each category.

Question 38. What do you understand by hardness of water ? Explain the permutit method for removal of permanent hardness of water with diagram.
Answer: Hardness of water is its property that prevents the formation of lather (foam) with soap. This is caused by the presence of dissolved bicarbonates, chlorides, and sulphates of calcium (\( Ca^{2+} \)) and magnesium (\( Mg^{2+} \)) ions.

Permutit Method (Zeolite Process): This method is used to remove permanent hardness. Permutit is hydrated sodium aluminum silicate, often written as \( NaZ \), where \( Z = (Al_{2}O_{3})_{y} (SiO_{2})_{z} (H_{2}O)_{n} \). When hard water passes through the permutit, the sodium ions in the zeolite are exchanged with the calcium and magnesium ions present in the hard water, softening it. The reactions are:
\( CaSO_{4} + Na_{2}Z \rightarrow CaZ + Na_{2}SO_{4} \)
\( MgCl_{2} + Na_{2}Z \rightarrow MgZ + 2NaCl \)
In general, if M represents a metal ion like \( Ca^{2+} \) or \( Mg^{2+} \):
\( Na_{2}Z(s) + M^{++}(aq) \rightarrow MZ(s) + 2Na^{+}(aq) \)
The hard water ions are trapped by the zeolite, and sodium ions are released into the water, making it soft. The exhausted zeolite can be regenerated by treating it with a concentrated brine (NaCl) solution.
In simple words: Water hardness means it doesn't foam well with soap due to calcium and magnesium. The permutit method uses a special substance called zeolite to swap out these hardness-causing minerals for sodium, making the water soft.

🎯 Exam Tip: Understand that the permutit method is an ion-exchange process, replacing calcium and magnesium ions with sodium ions to soften water. Know the regeneration process too.

Question 39. What do you mean by molecular association in water. Explain the structure of water with diagram. Explain the normal structure of ice.
Answer: Molecular association in water refers to the tendency of water molecules to form clusters or networks through intermolecular hydrogen bonding. Because water molecules are polar, the slightly positive hydrogen atom of one molecule is attracted to the slightly negative oxygen atom of another molecule, forming a hydrogen bond. This leads to molecules 'associating' or sticking together, giving water its unique properties like high boiling point and surface tension.

Structure of Water: A water molecule (\( H_{2}O \)) consists of an oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom has six electrons in its outermost shell. In a water molecule, the oxygen atom undergoes \( sp^{3} \) hybridization, forming four \( sp^{3} \) hybrid orbitals. Two of these orbitals form covalent bonds with the hydrogen atoms, while the other two contain lone pairs of electrons. The presence of these two lone pairs on the oxygen atom causes repulsion, reducing the bond angle from the ideal tetrahedral angle of \( 109^{\circ}28' \) to approximately \( 104.5^{\circ} \). This gives water its bent or V-shape structure.

Normal Structure of Ice: Ice is the crystalline form of water. It has a highly ordered, three-dimensional structure stabilized by an extensive network of hydrogen bonds. X-ray studies reveal that each oxygen atom in ice is tetrahedrally surrounded by four other oxygen atoms at a distance of 276 pm. One hydrogen atom lies between each pair of oxygen atoms, forming a covalent bond with one oxygen atom and a hydrogen bond with another. This arrangement creates large, open, cage-like spaces within the ice lattice, leading to a lower density for ice compared to liquid water, which is why ice floats.
In simple words: Molecular association in water means molecules stick together with hydrogen bonds. A water molecule has a bent shape with a \( 104.5^{\circ} \) bond angle. Ice has an open, ordered structure due to hydrogen bonds, making it less dense than water.

🎯 Exam Tip: Focus on hydrogen bonding as the central concept for explaining molecular association, water's bent structure (due to lone pairs), and ice's open, low-density crystalline lattice.

Question 40. Explain the following reaction with respect to hydrogen peroxide
(a) Oxidising and reducing nature
(b) Addition reactions
(c) Formation of peroxide
(d) Dissociation.
Answer:
(a) Oxidizing and Reducing Nature: Hydrogen peroxide (\( H_{2}O_{2} \)) is a versatile compound that can act as both a strong oxidizing agent and a reducing agent, depending on the reaction conditions and the other reactants.
In acidic medium, as an oxidizing agent, for example:
\( K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4} + 3H_{2}O_{2} \rightarrow K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 7H_{2}O + 3O_{2} \)
In basic medium, as a reducing agent, for example:
\( I_{2} + H_{2}O_{2} + 2OH^{-} \rightarrow 2I^{-} + 2H_{2}O + O_{2} \)

(b) Addition Reactions: Hydrogen peroxide can participate in addition reactions, typically with unsaturated compounds. For instance, ethene reacts with hydrogen peroxide to form an addition product, ethylene glycol.
\[ \underset{\text{Ethene}}{CH_{2}=CH_{2}} + H_{2}O_{2} \rightarrow \underset{\text{Ethylene glycol}}{CH_{2}OH-CH_{2}OH} \]

(c) Formation of Peroxide: Hydrogen peroxide, especially in acidic conditions, can react with strong bases to form peroxide salts. For example, sodium carbonate reacts with hydrogen peroxide to form sodium peroxide.
\[ \underset{\text{base}}{Na_{2}CO_{3}} + \underset{\text{acid}}{H_{2}O_{2}} \rightarrow \underset{\text{Salt (Sodium peroxide)}}{Na_{2}O_{2}} + H_{2}O + CO_{2} \]

(d) Decomposition: Hydrogen peroxide is inherently unstable and readily decomposes, especially when exposed to light, heat, or certain catalysts. It breaks down into water and oxygen.
\( 2H_{2}O_{2} \rightarrow 2H_{2}O + O_{2} \)
This is an exothermic reaction. While it can be catalyzed by substances like platinum (Pt), cobalt (Co), gold (Au), or copper (Cu), its decomposition can be controlled by adding small amounts of acid, alcohol, or acetanilide.
In simple words: Hydrogen peroxide can oxidize or reduce other chemicals, add to certain compounds, form peroxide salts with bases, and it naturally breaks down into water and oxygen.

🎯 Exam Tip: For hydrogen peroxide, remember its dual nature (oxidizing/reducing), its role in addition reactions, the formation of peroxide salts, and its decomposition into water and oxygen.

Question 40. Explain the following reactions using hydrogen peroxide:
(a) How it acts as an oxidising and reducing agent
(b) What happens in addition reactions
(c) How it forms peroxide
(d) How it dissociates
Answer:
(a) Hydrogen peroxide is a strong chemical that can either make other things lose electrons (oxidise) or gain electrons (reduce). It can do this in both acidic and basic conditions.
For example, in an acidic liquid: \( \mathrm{K_2Cr_2O_7 + 4H_2SO_4 + 3H_2O_2 \rightarrow K_2SO_4 + Cr_2(SO_4)_3 + 7H_2O + 3O_2} \)
And in a basic liquid:
\( \mathrm{I_2 + H_2O_2 + 2OH^- \rightarrow 2I^- + 2H_2O + O_2} \)
(b) Hydrogen peroxide can join with other molecules. For instance, when it reacts with ethene, they combine to create a new substance called ethylene glycol.
Here is the reaction:
\( \mathrm{CH_2=CH_2 + H_2O_2 \rightarrow CH_2OH-CH_2OH} \)
(c) When hydrogen peroxide (which is acidic) reacts with a base, it can form new salts called peroxides.
For example, with sodium carbonate:
\( \mathrm{Na_2CO_3 + H_2O_2 \rightarrow Na_2O_2 + H_2O + CO_2} \)
(d) Hydrogen peroxide is not very stable. If you leave it in the open air or heat it up, it will break apart into water and oxygen gas. This process releases heat.
The breakdown reaction is:
\( \mathrm{2H_2O_2 \rightarrow 2H_2O + O_2} \)
Some substances like platinum or gold can make this breakdown happen faster. You can slow it down by adding a tiny bit of acid, alcohol, or acetanilide.
In simple words: Hydrogen peroxide is a versatile chemical that can oxidise or reduce other substances, combine with molecules in addition reactions, create peroxide salts with bases, and it naturally breaks down into water and oxygen.

🎯 Exam Tip: When explaining chemical properties of a compound, always provide balanced chemical equations as examples to support your points. Also mention the conditions (acidic/basic) if they are relevant.