AS Level Chemistry chapter 5: Chemical energetics MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
46 exam-style questions on Chemical energetics, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A reaction is exothermic when
A
more energy is absorbed in breaking bonds than is released in making bonds
B
the energy released in forming the new bonds exceeds the energy needed to break the old bonds
C
the activation energy is zero
D
the products have higher enthalpy than the reactants
Show answer
Answer: B. Breaking bonds always absorbs energy and forming bonds releases it. If more is released than absorbed, the surroundings get warmer and ΔH is negative.
2. Use these bond energies (kJ mol−1): C–H 413, O=O 498, C=O 805, O–H 464. What is the enthalpy change for CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)?
A
−818 kJ mol−1
B
−417 kJ mol−1
C
+417 kJ mol−1
D
+818 kJ mol−1
Show answer
Answer: A. Broken: 4(413) + 2(498) = 2648. Made: 2(805) + 4(464) = 3466. ΔH = 2648 − 3466 = −818 kJ mol−1. +417 comes from ignoring the 2s in the equation.
3. Which standard enthalpy change can only be found indirectly, from a Hess's law cycle, rather than by direct experiment?
A
the enthalpy change of formation of methane
B
the enthalpy change of solution of ammonium nitrate
C
the enthalpy change of combustion of ethanol
D
the enthalpy change of neutralisation of hydrochloric acid by sodium hydroxide
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Answer: A. Carbon and hydrogen do not react directly to give only methane, so its ΔHf is calculated from combustion data. The other three can be measured in a simple calorimeter.
4. Which statement is Hess's law?
A
the enthalpy change of a reaction depends only on the initial and final states, not on the route taken
B
the enthalpy change of a reaction is always negative
C
the total bond energy of the reactants equals the total bond energy of the products
D
the enthalpy change of a reaction equals its activation energy
Show answer
Answer: A. Hess's law follows from conservation of energy: whichever route is taken from reactants to products, the overall ΔH is the same. It lets us find enthalpy changes that cannot be measured directly.
5. 2.00 g of ammonium nitrate, NH4NO3 (Mr 80.0), is dissolved in 50.0 g of water and the temperature falls by 3.1 °C. Taking c = 4.18 J g−1 K−1 and using the mass of water only, what is the enthalpy change of solution?
A
−25.9 kJ mol−1
B
+0.648 kJ mol−1
C
+25.9 kJ mol−1
D
+27.0 kJ mol−1
Show answer
Answer: C. q = 50.0 × 4.18 × 3.1 = 648 J for 0.0250 mol, so ΔH = +0.648/0.0250 = +25.9 kJ mol−1. The temperature fell, so the process is endothermic and ΔH is positive.
6. Bond energies in kJ mol−1: N–N 163, N–H 391, O=O 498, N≡N 945, O–H 464. Hydrazine vapour burns: N2H4(g) + O2(g) → N2(g) + 2H2O(g). What is ΔH for this reaction?
A
−576 kJ mol−1
B
+352 kJ mol−1
C
+576 kJ mol−1
D
+739 kJ mol−1
Show answer
Answer: A. Broken: 163 + 4(391) + 498 = 2225; made: 945 + 4(464) = 2801; ΔH = 2225 − 2801 = −576 kJ mol−1. +352 counts only two O–H bonds; +739 leaves out the N–N bond and reverses the sign.
7. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) ΔH = −467 kJ mol−1 MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ΔH = −151 kJ mol−1 The standard enthalpy change of formation of H2O(l) is −286 kJ mol−1. What is the standard enthalpy change of formation of MgO(s)?
A
−904 kJ mol−1
B
−602 kJ mol−1
C
−316 kJ mol−1
D
−30 kJ mol−1
Show answer
Answer: B. Mg + ½O2 → MgO = (reaction 1) − (reaction 2) + (formation of water): −467 + 151 − 286 = −602 kJ mol−1. −316 leaves out the water step; −904 adds reaction 2 instead of reversing it.
8. The enthalpy change of solution of anhydrous MgSO4 is −91 kJ mol−1 and that of MgSO4·7H2O is +16 kJ mol−1. What is ΔH for MgSO4(s) + 7H2O(l) → MgSO4·7H2O(s)?
A
+75 kJ mol−1
B
−75 kJ mol−1
C
+107 kJ mol−1
D
−107 kJ mol−1
Show answer
Answer: D. Both salts dissolve to give the same solution, so the direct route (anhydrous salt to solution, −91) equals hydration (ΔH) followed by dissolving the hydrate (+16). ΔH + 16 = −91, so ΔH = −107 kJ mol−1. Simply adding the two values gives the wrong −75.
9. Which bond energy is an exact value rather than an average value?
A
C–H
B
O–H
C
H–H
D
C–C
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Answer: C. H–H occurs only in the H2 molecule, so its bond energy is measured for that one bond. C–H, O–H and C–C occur in many different compounds and their values are averaged.
10. Which equation represents both a standard enthalpy change of formation and a standard enthalpy change of combustion?
A
CO(g) + ½O2(g) → CO2(g)
B
C(s) + ½O2(g) → CO(g)
C
C(s) + O2(g) → CO2(g)
D
2H2(g) + O2(g) → 2H2O(l)
Show answer
Answer: C. Burning one mole of carbon completely forms one mole of CO2 from its elements. CO is incomplete combustion, CO + ½O2 starts from a compound, and the hydrogen equation makes two moles.
46, each with the answer and a short explanation, across 2 topics: Enthalpy change, ΔH, Hess’s law. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level Chemistry chapter 5 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
