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The Hidden Power of Halogens: Properties of Group 17 Elements

  The Hidden Power of Halogens – Understanding Group 17 Elements When you hear the word halogen , you might not feel any excitement at first. But these elements, hidden in Group 17 of the periodic table, are some of the most fascinating, dangerous, and useful substances on Earth. From the fluoride in your toothpaste to the chlorine that keeps swimming pools clean, halogens are all around us — changing lives, saving lives, and sometimes even threatening them. Let’s take a deeper, human look into these elements and uncover what makes them so unique. What Makes Group 17 So Special? Group 17 contains a lineup of nonmetals known as halogens. The group includes fluorine, chlorine, bromine, iodine, and astatine — and a very rare artificial member, tennessine. What binds them together is not just their position on the periodic table, but their desperate need for one more electron. That’s right. All halogens have seven electrons in their outermost shell, and they want eight. This mis...
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Chapter 5 energetics class 9 exercise solutions with mcqs and short question in pdf

 


Exercise: Tick (✓) the Correct Answer

(i) The following reaction is an exothermic reaction:


H2 + Cl2 = 2HCl

(a) By collisions between the molecules
(b) From sunlight
(c) From the surrounding
(d) By collisions of the molecules with the walls of the container

Answer: (b) From sunlight
Explanation: This is a photochemical reaction, meaning light energy (from sunlight) initiates bond breaking in H₂ and Cl₂, leading to HCl formation.

(ii) Which of the following reactions has the least value of activation energy?

(a) H₂(g) + ½ O₂(g) → H₂O(g)
(b) C₆₀(s) + O₂(g) → CO₂(g)
(c) NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)
(d) H₂(g) + I₂(g) → 2HI(g)

Answer: (a) H₂(g) + ½ O₂(g) → H₂O(g)
Explanation: Combustion reactions typically have low activation energy and are highly exothermic. The reaction between hydrogen and oxygen to form water releases significant energy and has low activation energy.

(iii) Formation of which hydrogen halide from the elements is an endothermic reaction?

(a) HCl
(b) HF
(c) HBr
(d) HI

Answer: (b) HF
Explanation: The formation of HF requires significant energy input due to the high bond energy of fluorine, making it an endothermic reaction.

(iv) What are the products of anaerobic respiration?

(a) ATP + CO₂ + H₂O
(b) CO₂ + H₂O
(c) ATP + Ethanol + H₂O
(d) Ethanol + H₂O

Answer: (d) Ethanol + H₂O
Explanation: Anaerobic respiration in microorganisms like yeast produces ethanol and water in the absence of oxygen.

(v) Which reaction do you expect to be a reversible reaction?

(a) Graph with a steep energy difference
(b) Graph with moderate energy difference
(c) Graph with smaller activation energy both ways
(d) Graph with very high activation energy

Answer: (c) Graph with smaller activation energy both ways
Explanation: Reversible reactions have similar energy levels for reactants and products and a low activation energy barrier.

(vi) What does it show when a chemical reaction is exothermic?

(a) It shows the bonds that break are weaker than those that are formed.
(b) It shows the bonds that break are stronger than those that are formed.
(c) Exothermic nature of the reaction is not concerned with bond formation or bond breakage.
(d) It shows that the reactants are more stable than the products.

Answer: (a) It shows the bonds that break are weaker than those that are formed.
Explanation: Exothermic reactions release heat because the bonds formed in products are stronger than those broken in reactants.

(vii) When NaOH and HCl are mixed, the temperature increases. The reaction is:

(a) Endothermic with a positive enthalpy change.
(b) Endothermic with a negative enthalpy change.
(c) Exothermic with a positive enthalpy change.
(d) Exothermic with a negative enthalpy change.

Answer: (d) Exothermic with a negative enthalpy change.
Explanation: Neutralization reactions (like NaOH + HCl) release heat, making them exothermic with ΔH < 0 (negative enthalpy change).

(viii) The average bond dissociation energy for the C-H bond is 412 kJ/mol. Which of the following processes will have enthalpy change close to 412 kJ/mol?

(a) CH₄(g) → C(g) + 2H₂(g)
(b) CH₄(g) → CH₃(g) + H(g)
(c) CH₄(g) → C(g) + 4H(g)
(d) CH₄(g) → CH₂(g) + H₂(g)

Answer: (b) CH₄(g) → CH₃(g) + H(g)
Explanation: Breaking one C-H bond requires 412 kJ/mol. Option (b) involves the removal of one hydrogen atom, making its enthalpy change closest to 412 kJ/mol.

(ix) The average bond energies for O-H and O=O are 146 kJ/mol and 496 kJ/mol, respectively. Find the enthalpy in kJ for the following reaction:

(a) -102 kJ
(b) +102 kJ
(c) +350 kJ
(d) +394 kJ

Answer: (a) -102 kJ
Explanation:

  • Breaking bonds absorbs energy, and forming bonds releases energy.


  • 2. Questions for Short Answers

    i. What is the difference between enthalpy and enthalpy change?

    • Enthalpy (H) is the total heat content of a system at constant pressure.
    • Enthalpy change (ΔH) is the heat absorbed or released during a reaction at constant pressure.

    ii. Why is breaking of a bond an endothermic process?

    • Breaking a bond requires energy to overcome the attractive forces between atoms. Since energy is absorbed, it is an endothermic process.

    iii. Depict the transition state for the following reaction:
    H₂ + Cl₂ → 2HCl

    • In the transition state, H-H and Cl-Cl bonds start breaking, and H-Cl bonds start forming. This state is highly unstable and has the highest energy in the reaction.

    iv. Draw the reaction profiles for two exothermic reactions, one of which moves faster than the other.

    • Exothermic reaction profiles show that the products have lower energy than the reactants. The faster reaction will have a lower activation energy compared to the slower one.

    v. What is the role of glycogen in our body?

    • Glycogen is the storage form of glucose in animals and humans. It is stored in the liver and muscles and provides energy when needed.

    3. Constructed Response Questions

    i. Identify whether the following physical changes are exothermic or endothermic:

    3. Constructed Response Questions

    i. Identify whether the following physical changes are exothermic or endothermic:

    • Conversion of hydrated salt into anhydrous salt → Endothermic
    • Burning paper → Exothermic
    • Vaporizing liquid nitrogen → Endothermic
    • Evaporation of dry ice → Endothermic
    • Conduction of electricity by metals → Neither (Not a heat-related process)
    • Dissolving ammonium chloride in water → Endothermic
    • Formation of rain from clouds → Exothermic
    • Dissolving sodium carbonate in water → Endothermic

    ii. Why does the reaction between atmospheric oxygen and nitrogen not take place under normal conditions?

    • The reaction requires high activation energy to break the strong triple bond in nitrogen (N≡N). Under normal conditions, this energy is not available, so no reaction occurs. However, lightning provides enough energy for nitrogen and oxygen to react, forming NO. Once lightning stops, the reaction ceases because there is no further energy input.

    iii. Why doesn’t natural gas (CH₄) react with atmospheric oxygen unless ignited?

    • CH₄ and O₂ do not react at room temperature because the activation energy is too high. When a burning matchstick is introduced, it provides heat energy to initiate the reaction. The reaction then continues on its own as it releases further heat energy, sustaining itself until the reactants are consumed.


    ii. Why does the reaction between atmospheric oxygen and nitrogen not take place under normal conditions?

    • The reaction requires high activation energy to break the strong triple bond of nitrogen (N≡N). Lightning provides this energy, allowing nitrogen and oxygen to form NO. The reaction stops once lightning stops because no additional energy is available.

    iii. Why doesn’t natural gas (CH₄) react with atmospheric oxygen unless ignited?

    • CH₄ and O₂ do not react at room temperature because activation energy is too high. A burning matchstick provides the required heat energy to initiate the reaction, which then continues due to the heat released.

    4. Descriptive Questions

    i. Find the enthalpy change for:
    N₂ + O₂ → 2NO
    Given bond energies:

    • N≡N = 958.38 kJ/mol
    • O=O = 498 kJ/mol
    • NO formation = -626 kJ/mol (per NO molecule)

    Total energy required to break bonds:
    958.38 + 498 = 1456.38 kJ

    Total energy released in bond formation:
    2 × (-626) = -1252 kJ

    Enthalpy change:
    ΔH = Energy required - Energy released
    = 1456.38 - 1252
    = +204.38 kJ
    (Since ΔH is positive, the reaction is endothermic.)

    ii. Difference between heat and enthalpy:

    • Heat (q): Energy transfer due to temperature difference.
    • Enthalpy (H): Total energy in a system, including internal energy and pressure-volume work.

    iii. Why is bond formation always an exothermic process?

    • When a bond forms, atoms move to a lower energy state and release excess energy in the form of heat, making it exothermic.

    iv. Role of lipids in our body:

    • Lipids store energy, form cell membranes, and insulate the body.

    v. Explanation of key terms:

    • Activation Energy: Minimum energy required for a reaction to occur.
    • Transition State: High-energy unstable state between reactants and products.
    • Aerobic Respiration: The breakdown of glucose using oxygen to release energy.

    5. Investigative Questions

    i. Why is it essential to cook some foods while others can be eaten raw?

    • Cooking softens food, kills bacteria, and breaks down complex molecules into simpler, digestible forms. Some foods, like fruits, are naturally soft and digestible, so they don’t need cooking.

    ii. Why do fireworks look spectacular? What type of chemical compounds are used?

    • Fireworks contain oxidizers (e.g., potassium nitrate, chlorates, or perchlorates) and metal salts (e.g., strontium, copper, barium), which produce different colors when burned. The reaction releases light, heat, and sound energy, creating the visual and auditory effects.

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