All 24 MCQs of 9th Class Chemistry Chapter 5, Energetics, with answers and the reason each option is right or wrong: the 10 textbook exercise MCQs plus 14 more board-style MCQs. Punjab Board new syllabus.
Try each one first: tap an option to check it, then read the reasoning and why the others are wrong.
A note about the questions
The wording follows published solved exercises and every answer was re-checked by hand, so the wording or the order of options may differ slightly from your print.
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Set A: Textbook exercise 10 MCQs
The multiple-choice questions at the end of Chapter 5 in your book, in the book's option order.
Q5.1Bond Energies
The following reaction is an exothermic reaction: H₂ + Cl₂ → 2HCl (in sunlight). From where does the energy come to break the bonds of H₂ and Cl₂?
Answer(b) From sunlight
Step by step
Bond breaking needs energy (endothermic).
Here that energy is supplied as light: the condition written over the arrow.
Why the others are wrong
(a) at room temperature, in the dark, collisions do not give enough energy; H₂ and Cl₂ can be kept mixed in the dark.
(c) heat from the surroundings is not what starts this reaction.
(d) collisions with the walls do not break bonds.
Remember
Read what is written over the arrow.
Q5.2Activation Energy and Catalysts
Which of the following reactions has the least value of activation energy?
(a), (b), (d) must break H–H, O=O or I–I bonds first: they need a spark, flame or heat.
(c) is an ionic reaction: it happens the moment the solutions mix.
Why the others are wrong
(a) a hydrogen–oxygen mixture does nothing until a spark is applied.
(b) carbon must be heated before it burns.
(d) hydrogen reacts with solid iodine only at high temperature.
Remember
Instant reaction = lowest activation energy.
Q5.3Exothermic and Endothermic Reactions
Formation of which hydrogen halide from the elements is an endothermic reaction?
Answer(d) HI
Step by step
HF, HCl, HBr: ΔH negative (exothermic).
HI: ΔH = +26.5 kJ per mole (+53 kJ for 2HI).
Why the others are wrong
(a) H₂ + Cl₂ → 2HCl is exothermic (it can explode in sunlight).
(b) HF forms very exothermically.
(c) HBr formation is exothermic.
Remember
Bond strength falls down the group: HF strongest, HI weakest.
Q5.4Respiration and Lipids
What are the products of anaerobic respiration?
Answer(c) ATP + Ethanol + H₂O
Step by step
Anaerobic (yeast): glucose → ethanol + CO₂ + energy (ATP).
(a) and (b) are the products of aerobic respiration.
Why the others are wrong
(a) CO₂ + H₂O + energy is aerobic respiration.
(b) that is aerobic respiration without the energy.
(d) no energy (ATP) is listed, and that is the whole point of respiration.
Remember
Anaerobic in yeast = ethanol (alcohol) is made.
Q5.5Activation Energy and Catalysts
Which reaction do you expect to be a reversible reaction? (Energy profiles from A + B to C + D.) (a) C + D far below A + B; (b) C + D at the same level as A + B; (c) C + D somewhat above A + B; (d) C + D far above A + B, almost as high as the peak.
Answer(b) Products at the same energy level as the reactants
Step by step
Backward Eₐ = forward Eₐ − ΔH.
In (b) ΔH = 0, so both are equal.
In (a) the backward barrier is huge: practically one-way.
Why the others are wrong
(a) the products would need a very large energy to go back: practically irreversible.
(c) the forward barrier is larger than the backward one.
(d) the products sit just below the peak; they go back far more easily than they are formed, so few products remain.
Remember
Same height both sides of the hill = traffic both ways.
Q5.6Bond Energies
What does it show when a chemical reaction is exothermic?
Answer(a) It shows the bonds which break are weaker than those formed.
Step by step
Breaking: energy in.
Making: energy out.
Out > in: exothermic.
Why the others are wrong
(b) that gives an endothermic reaction.
(c) the heat of reaction comes from bond breaking and making.
(d) in an exothermic reaction the products are lower in energy, so they are the more stable.
Remember
Strong bonds made = heat given out.
Q5.7Exothermic and Endothermic Reactions
When NaOH and HCl are mixed the temperature increases. The reaction is:
Answer(d) exothermic with a negative enthalpy change
Step by step
Surroundings warmer → exothermic.
Exothermic → ΔH negative.
Why the others are wrong
(a) an endothermic reaction would cool the solution.
(b) endothermic changes have positive ΔH.
(c) exothermic changes have negative ΔH.
Remember
Hot = exo = minus.
Q5.8Bond Energies
The average bond dissociation energy for the C–H bond is 412 kJ mol⁻¹. Which of the following processes will have enthalpy change close to 412 kJ mol⁻¹?
Answer(d) CH₄(g) → CH₃(g) + H(g)
Step by step
One C–H bond broken: ≈ 412 kJ (measured 435 for the first one; 412 is the average).
Options (a) and (b) also form H–H bonds, so their values are different again.
Why the others are wrong
(a) four C–H bonds break and two H–H bonds form.
(b) this is the reverse of methane's formation (about +75 kJ).
(c) four C–H bonds: about 1648 kJ.
Remember
One bond broken, nothing else formed.
Q5.9Bond Energies
The average bond energies for O–O and O=O are 146 and 496 kJ mol⁻¹ respectively. Find the enthalpy in kJ for the reaction H–O–O–H(g) → H–O–H(g) + ½O=O(g).
Answer(a) −102 kJ
Step by step
Energy in: 146 kJ.
Energy out: 248 kJ.
ΔH = +146 − 248 = −102 kJ (exothermic).
Why the others are wrong
(b) Wrong sign — more energy is released than absorbed.
(c) this is 496 − 146, the wrong way round and without the ½.
(d) this does not follow from the data.
Remember
ΔH = bonds broken − bonds formed.
Q5.10Activation Energy and Catalysts
Why does the following exothermic reaction not occur? C (diamond) → C (graphite), ΔH = −3 kJ mol⁻¹
Answer(c) The change from diamond to graphite has high activation energy.
Step by step
Exothermic does not mean fast.
Rate depends on the activation energy, not on ΔH.
Why the others are wrong
(a) ΔH is negative, so graphite is lower in energy and more stable.
(b) bond strength alone does not explain why a favourable change does not happen.
(d) density has nothing to do with whether a reaction takes place.
Remember
Diamonds are "forever" because of the energy hill, not because they are more stable.
Set B: More MCQs from this chapter 14 MCQs
Extra questions on the same ideas, of the kind set in board papers and class tests.
QB1System and Surroundings
When energy is transferred from the surroundings to the system, the change is:
Answer(b) endothermic, positive sign
Step by step
Into the system: endothermic, +.
Out of the system: exothermic, −.
Why the others are wrong
(a) that is energy leaving.
(c) exothermic is negative.
(d) endothermic is positive.
Remember
Endo = enter.
QB2System and Surroundings
When water is boiled in a beaker, the beaker and burner are called the:
Answer(a) surrounding
Step by step
System: what is studied.
Surrounding: everything else.
Why the others are wrong
(b) the water is the system.
(c) nothing is catalysed here.
(d) a transition state is part of a reaction path.
Remember
Surrounding = everything outside the system.
QB3Enthalpy
The unit of enthalpy is:
Answer(b) kJ mol⁻¹
Step by step
Energy per mole.
Why the others are wrong
(a) that is power (watt).
(c) that is concentration.
(d) that is temperature.
Remember
Energy per mole.
QB4Enthalpy
Which can be measured directly?
Answer(a) The enthalpy change ΔH
Step by step
H includes all the energy of all the particles: no zero to measure from.
ΔH = heat given out or taken in at constant pressure.
Why the others are wrong
(b) H cannot be measured.
(c) only ΔH.
(d) ΔH can be measured.
Remember
We measure changes, not totals.
QB5Exothermic and Endothermic Reactions
H₂ and O₂ react to give liquid water: 2H₂(g) + O₂(g) → 2H₂O(l). The heat evolved is 571.6 kJ. ΔH is:
Answer(c) −571.6 kJ
Step by step
Evolved → minus sign.
Why the others are wrong
(a) Wrong sign.
(b) that is per mole and the wrong sign.
(d) heat is evolved.
Remember
Given out = negative.
QB6Exothermic and Endothermic Reactions
Formation of NO from N₂ and O₂ during lightning is:
Answer(d) endothermic
Step by step
ΔH positive: endothermic.
Why the others are wrong
(a) ΔH is positive.
(b) a new substance, NO, is formed.
(c) Wrong.
Remember
Needs lightning = needs energy.
QB7Exothermic and Endothermic Reactions
Which of these is endothermic?
Answer(b) Melting of ice
Step by step
Solid → liquid needs energy.
Why the others are wrong
(a) combustion is exothermic.
(c) respiration is exothermic.
(d) condensation gives out heat.
Remember
Melting, boiling, evaporating, subliming: all endothermic.
QB8Bond Energies
Bond breaking is:
Answer(a) always endothermic
Step by step
Breaking: energy in.
Making: energy out.
Why the others are wrong
(b) that is bond making.
(c) never.
(d) Wrong.
Remember
Break = take; make = give.
QB9Bond Energies
Using bond energies H–H 436, I–I 151 and H–I 299 kJ mol⁻¹, ΔH for H₂(g) + I₂(g) → 2HI(g) is:
Answer(c) −11 kJ
Step by step
Energy in 587 kJ.
Energy out 598 kJ.
ΔH = −11 kJ.
Why the others are wrong
(a) Wrong sign.
(b) counts only one H–I bond.
(d) that is one H–I bond alone.
Remember
Two H–I bonds are formed, not one.
QB10Activation Energy and Catalysts
The minimum energy that colliding particles must have for a successful collision is called:
Answer(d) activation energy
Step by step
Height of the hump from the reactants.
Why the others are wrong
(a) enthalpy is heat content.
(b) molecules have kinetic energy, but Eₐ is the minimum needed.
(c) bond energy is for one bond.
Remember
Eₐ = energy to climb the hill.
QB11Activation Energy and Catalysts
A catalyst increases the rate of a reaction because it:
Answer(b) provides a path of lower activation energy
Step by step
Lower hump, same start and finish.
Why the others are wrong
(a) ΔH is unchanged.
(c) reactants and products keep their energies.
(d) it undergoes no permanent chemical change.
Remember
A catalyst lowers the hill, not the valley.
QB12Activation Energy and Catalysts
Which catalyst is used in the hydrogenation of oil to make banaspati ghee?
Answer(c) Nickel
Step by step
Oil + H₂ (Ni) → ghee.
Why the others are wrong
(a) the book links platinum to H₂SO₄.
(b) iron is the Haber-process catalyst.
(d) chlorine catalyses ozone breakdown.
Remember
Ni for ghee.
QB13Respiration and Lipids
During glycolysis, one molecule of glucose splits into:
Answer(a) two molecules of pyruvate
Step by step
C₆H₁₂O₆ → 2C₃H₄O₃ + 2 ATP.
Why the others are wrong
(b) CO₂ is made later, in the mitochondria.
(c) ethanol comes only from anaerobic respiration.
(d) glycogen is a store, not a product of glycolysis.
