Looking for complete solved exercises of Chapter 5 new syllabus 2025 Punjab boards: Energetics from the 9th class Chemistry book? Find detailed, step-by-step solutions designed to help students understand core concepts. Download the free PDF and ace your exams with ease.
Question 1:
Tick (✓) the correct answer.
(i) The following reaction is an exothermic reaction:
H2+Cl2→(Sunlight) 2HCl
From where does the energy come to break the bond of H2and Cl2?
Options:
(a) By collisions between the molecules
(b) From sunlight
(c) From the surroundings
(d) By collisions of the molecules with the walls of the container
Correct Answer:
(b) From sunlight
Explanation:
The reaction requires sunlight to break the bonds of H2 and Cl2, as the photons provide the energy needed to overcome the bond energy.
(ii) Which of the following reactions has the least value of activation energy?
Options:
(a) H2(g)+12O2(g)→H2O(g)
(b) C(s)+O2(g)→CO2(g)
(c) NaCl(s)+AgNO3(aq)→AgCl(s)+NaNO3(aq)
(d) H2(g)+I2(g)→2HI(g)
Correct Answer:
(c) NaCl(s)+AgNO3(aq)→AgCl(s)+NaNO3(aq)
Explanation:
The reaction between NaCl and AgNO3 involves ionic compounds, which react readily in aqueous solutions with negligible activation energy, unlike other reactions that require bond-breaking or bond formation.
(iii) Formation of which hydrogen halide from the elements is an endothermic reaction?
Options:
(a) HCl
(b) HF
(c) HBr
(d) HI
Correct Answer:
(d) HI
Explanation:
The formation of HI is endothermic because the bond dissociation energy of iodine is high, requiring more energy to form the product compared to the energy released.
(iv) What are the products of anaerobic respiration?
Options:
(a) ATP + CO2 + H2O
(b) CO2 + H2O
(c) ATP + Ethanol + H2O
(d) Ethanol + H2O
Correct Answer:
(c) ATP + Ethanol + H2O
Explanation:
Anaerobic respiration in organisms such as yeast produces ethanol, water, and ATP, as it occurs in the absence of oxygen.
(v) Which reaction do you expect to be a reversible reaction?
Options:
(a) (Graph showing energy decrease after reaction)
(b) (Graph showing energy increase after reaction)
Correct Answer:
(b)
Explanation:
The graph labeled (b) shows a smaller energy difference between the reactants and products, indicating that the reaction is more likely to reach equilibrium and be reversible.
(vi) What does it show when a chemical reaction is exothermic?
Options:
(a) It shows the bonds which break are weaker than those formed.
(b) It shows the bonds which break are stronger than those 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.
Correct Answer:
(a) It shows the bonds which break are weaker than those formed.
Explanation:
In an exothermic reaction, the energy released during bond formation is greater than the energy required to break the bonds. Hence, the bonds formed are stronger than the bonds broken.
(vii) When NaOH and HCl are mixed, the temperature increases. The reaction is:
Options:
(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.
Correct Answer:
(d) Exothermic with a negative enthalpy change.
Explanation:
The reaction between NaOH and HCl releases heat, causing an increase in temperature. This indicates an exothermic reaction with a negative enthalpy change.
(viii) The average bond dissociation energy for the C-H bond is 412 kJ mol−1. Which of the following processes will have an enthalpy change close to 412 kJ mol−1?
Options:
(a) CH4(g)→CH3(g)+H2(g)
(b) CH4(g)→C2H2(g)+H2(g)
(c) CH4(g)→C5(g)+4H2(g)
(d) CH4(g)→CH3(g)+H(g)
Correct Answer:
(d) CH4(g)→CH3(g)+H(g)
The process involves breaking one C-H bond, with the enthalpy change equal to the bond dissociation energy (412 kJ mol−1.
(ix) The average bond energies for O-O and O=O are 146 kJ mol−1and 496 kJ mol−1, respectively. Find the enthalpy in kJ for the following reaction:
H−O−O-H →H−O-H +1/2O2(g)
Options:
(a) -102 kJ
(b) +102 kJ
(c) +350 kJ
(d) +394 kJ
Correct Answer:
(a) -102 kJ
Explanation:
The reaction involves breaking one O-O bond (146 kJ) and forming a half O=O bond (496/2 = 248 kJ).
Enthalpy = Energy of bonds broken – Energy of bonds formed
= 146 – 248
= -102 kJ.
Question 1 (x):
Why does the following exothermic reaction not occur?
C (Diamond)→C (Graphite)ΔH=−3 kJ mol−1
Options:
(a) Structure of diamond is more stable than that of graphite.
(b) Diamond has strong covalent bonds than does the graphite.
(c) The change from diamond to graphite has high activation energy.
(d) Density of graphite is less than that of diamond.
Correct Answer:
(c) The change from diamond to graphite has high activation energy.
Explanation:
Even though the reaction is exothermic, it does not occur easily because breaking the strong bonds in diamond requires a lot of energy, which makes the activation energy very high.
2. Questions for Short Answers:
i. What is the difference between enthalpy and enthalpy change?
- Enthalpy (H): It is the total heat content of a system.
- Enthalpy Change (ΔH): It is the difference in enthalpy between the products and reactants during a reaction.
ii. Why is breaking of a bond an endothermic process?
- Breaking a bond requires energy to overcome the attractive forces holding the atoms together. This energy input makes it an endothermic process.
iii. Depict the transition state for the following reaction: H2+Cl2→2HCl
- The transition state is the highest energy point during the reaction. For this reaction, it is when the bonds in H2 and Cl2 are partially broken, and new H-Cl bonds are partially formed.
iv. Draw the reaction profiles for two exothermic reactions, one of which moves faster than the other.
- A faster reaction will have a lower activation energy, and both will release energy (negative ΔH).
v. What is the role of glycogen in our body?
- Glycogen is the storage form of glucose in the body. It provides energy by breaking down into glucose when required.
3. Constructed Response Questions:
i. Physical changes which usually occur around us are given in the table. Write down whether they are exothermic or endothermic.
| Physical Change | 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 (No heat change) |
| Dissolving ammonium chloride in water | Endothermic |
| Formation of rain from clouds | Exothermic |
| Dissolving sodium carbonate in water | Exothermic |
Q1 (i): Explain why the reaction between atmospheric gases oxygen and nitrogen does not take place under normal conditions. But in the presence of lightning, these gases react to give NO. The reaction stops as soon as the lightning stops.
Answer: Under normal conditions, oxygen (O₂) and nitrogen (N₂) in the atmosphere do not react because of the high activation energy required to break their strong bonds (triple bond in N₂ and double bond in O₂). However, during lightning, the intense heat and energy provide sufficient activation energy to break these bonds, enabling the formation of nitrogen monoxide (NO) through the reaction: N2+O2→2NO
As soon as the lightning stops, the energy source is no longer available, and the reaction ceases because the required activation energy cannot be sustained under normal atmospheric conditions.
Q1 (ii): Explain why a reaction between natural gas (CH₄) and atmospheric oxygen does not take place when you mix them. As soon as you show a burning matchstick, the reaction starts immediately and continues until one or both of the reactants are used up.
Answer: Natural gas (CH₄) and oxygen (O₂) require activation energy to overcome the energy barrier for the combustion reaction. Mixing these gases does not provide sufficient energy for the reaction to start. A burning matchstick acts as an external source of activation energy, initiating the reaction. Once started, the reaction becomes self-sustaining due to the release of heat energy during combustion, which provides the necessary energy to continue until one or both reactants are consumed.
The reaction is: CH4+2O2→CO2+2H2O+energy (heat)
Q4 (i): Find out the enthalpy change of the reaction: N2+O2→2NO
Given data:
- Bond dissociation energy of N2: 958.38 kJ/mol
- Bond dissociation energy of O2: 498 kJ/mol
- Bond formation energy of NO: -626 kJ/mol
Solution: ΔH=Total energy required to break bonds−Total energy released in forming bonds
- Energy to break bonds:
Breaking 1 mol of N2:958.38 kJ/mol
Breaking 1 mol of O2:498 kJ/mol
Total energy required = 958.38+498=1456.38 kJ
- Energy released in forming bonds:
Forming 2 mol of NO:2×(−626)=−1252 kJ
- Enthalpy change:
ΔH=1456.38−1252=204.38 kJ
Answer: The enthalpy change of the reaction is +204.38 kJ (endothermic process).
Q4 (ii): Explain the difference between the terms heat and enthalpy.
Answer:
- Heat: Heat is the transfer of thermal energy between a system and its surroundings due to a temperature difference. It is a path-dependent quantity and depends on the conditions of the transfer.
- Enthalpy: Enthalpy (HH) is a thermodynamic property of a system that represents the total heat content. It is defined as:
H=U+PV
where UU is the internal energy, PP is pressure, and VV is volume. Enthalpy changes (ΔH) occur during chemical reactions and are measured under constant pressure.
Q4 (iii): Explain why the formation of a bond is always an exothermic process.
Answer: The formation of a chemical bond releases energy because atoms move to a lower-energy, more stable state when bonded. The energy released during bond formation is the same as the energy required to break the bond. This release of energy is what makes bond formation an exothermic process.
Q4 (iv): Explain the role of lipids in our body.
Answer: Lipids play several crucial roles in the body:
- Energy Storage: Lipids are a dense source of energy, providing more than twice the energy per gram compared to carbohydrates or proteins.
- Insulation and Protection: They act as thermal insulators and cushion organs against physical shocks.
- Cell Membranes: Lipids, particularly phospholipids, form the structural components of cell membranes.
- Hormone Production: Lipids are precursors for hormones like steroids.
- Absorption of Vitamins: They help in the absorption of fat-soluble vitamins (A, D, E, K).
Q4 (v): Explain the following terms.
- Activation Energy: The minimum energy required to start a chemical reaction by breaking bonds in the reactants.
- Transition State: A high-energy, unstable state during a reaction in which reactants are partially converted into products.
- Aerobic Respiration: A metabolic process in which cells use oxygen to convert glucose into energy (ATP), producing carbon dioxide and water as byproducts.
Q5 (i): Why is it essential to cook some food items while others can be eaten without cooking?
Answer: Cooking is essential for certain food items because it:
- Breaks Down Tough Structures: Cooking softens cellulose in plant cells and denatures proteins, making food easier to digest.
- Destroys Harmful Microorganisms: Cooking kills bacteria and pathogens, ensuring food safety.
- Improves Taste and Texture: It enhances the flavor and makes food more palatable. Some foods (like fruits and vegetables) can be eaten raw because they are naturally safe and digestible without cooking.
Q5 (ii): Why do fireworks look spectacular? What type of chemical compounds undergo reactions during this activity?
Answer: Fireworks look spectacular due to the bright colors and light effects caused by chemical reactions. Metal salts, like:
- Strontium compounds (red)
- Barium compounds (green)
- Sodium compounds (yellow)
These compounds release energy as light of specific wavelengths when they are heated. The combustion of black powder (a mixture of potassium nitrate, sulfur, and charcoal) produces the heat required to excite these metal ions, creating the vibrant displays.
