Chemical Energetics

Chemistry, HSSC-Punjab First Year. This chapter is split into 41 topics and has 961 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Enthalpy of neutralisation from calorimetry 21 questions
2. Enthalpy change of reaction from bond energies 26 questions
3. Enthalpy change and calorie content of food 26 questions
4. Standard enthalpy of reaction, combustion and formation 22 questions
5. Enthalpy of formation from a measured heat of combustion 20 questions
6. Standard conditions and standard states 20 questions
7. Heat, temperature and conservation of energy 22 questions
8. Heat change from temperature change (q = mc Delta T) 26 questions
9. Exothermic and endothermic processes 22 questions
10. Energy absorbed in bond breaking and released in bond making 26 questions
11. Thermodynamics, thermochemistry and enthalpy 21 questions
12. Energetics of solution and hydration 23 questions
13. Comparing the entropy of substances 21 questions
14. Spontaneity and temperature change 28 questions
15. Enthalpy of atomisation, neutralisation and electron affinity 23 questions
16. Factors affecting the electron affinity of elements 20 questions
17. Solubility trends of group 2 hydroxides and sulfates 21 questions
18. Energy profile diagrams and activation energy 24 questions
19. Bond energy problems from the book data 20 questions
20. Calculating Delta G from Delta H and Delta S 26 questions
21. Sign of the entropy change of a reaction 27 questions
22. Hess's law of heat summation 21 questions
23. Factors affecting the enthalpy of hydration 21 questions
24. Free energy change from formation data and standard entropies 22 questions
25. Lattice energy and the factors affecting it 21 questions
26. Exercise items on thermochemistry 23 questions
27. Calculating the entropy change of a reaction 22 questions
28. Entropy change of the surroundings and total entropy change 30 questions
29. Gibbs free energy and spontaneity 25 questions
30. Entropy and the number of ways of arranging particles 27 questions
31. Comparing bond energies and bond strength 24 questions
32. Hess's law with given thermochemical equations 24 questions
33. Calorimeter and the measurement of enthalpy change 20 questions
34. Born-Haber cycle: steps and meaning 23 questions
35. Enthalpy of reaction from enthalpies of formation 28 questions
36. Bond energy and bond dissociation energy 30 questions
37. Born-Haber cycle calculations 26 questions
38. Other calorimetry calculations 21 questions
39. Enthalpy of solution from lattice energy and hydration enthalpies 26 questions
40. Exercise numerical problems 20 questions
41. Enthalpy of formation from enthalpies of combustion 22 questions

Sample questions: Chemical Energetics

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. The first electron affinity of chlorine is −348.8 kJ mol⁻¹. This is:

• A. the same value as for fluorine and for bromine
• B. a positive value, as for the noble gases
• C. among the most negative values for any element(correct answer)
• D. equal to zero, as for the alkaline earths

Answer: C.Chlorine's first electron affinity is greater (more negative) than fluorine's, because fluorine is a very small atom with strong repulsion in its compact shell (standard knowledge).

Q2. Use the bond energies (kJ mol⁻¹): E(H–H) = 436, E(O=O) = 498, E(O–H) = 463 to estimate ΔH for 2H₂(g) + O₂(g) → 2H₂O(g).

• A. +482 kJ mol⁻¹
• B. +16.0 kJ mol⁻¹
• C. −964 kJ mol⁻¹
• D. −482 kJ mol⁻¹(correct answer)

Answer: D.Bonds broken (reactants): 1370 kJ; bonds formed (products): 1852 kJ; ΔH = ΣE(reactants) − ΣE(products) = 1370 − 1852 = −482 kJ mol⁻¹ (per equation as written).

Q3. Use the bond energies (kJ mol⁻¹): E(C–H) = 413, E(O=O) = 498, E(C=O) = 728, E(O–H) = 463 to estimate ΔH for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).

• A. −660 kJ mol⁻¹(correct answer)
• B. −1160 kJ mol⁻¹
• C. −1320 kJ mol⁻¹
• D. +660 kJ mol⁻¹

Answer: A.Bonds broken (reactants): 2648 kJ; bonds formed (products): 3308 kJ; ΔH = ΣE(reactants) − ΣE(products) = 2648 − 3308 = −660 kJ mol⁻¹ (per equation as written).

Q4. Use the bond energies (kJ mol⁻¹): E(C=C) = 615, E(C–H) = 413, E(H–H) = 436, E(C–C) = 348 to estimate ΔH for C₂H₄(g) + H₂(g) → C₂H₆(g).

• A. +123 kJ mol⁻¹
• B. +313 kJ mol⁻¹
• C. −123 kJ mol⁻¹(correct answer)
• D. −246 kJ mol⁻¹

Answer: C.Bonds broken (reactants): 2703 kJ; bonds formed (products): 2826 kJ; ΔH = ΣE(reactants) − ΣE(products) = 2703 − 2826 = −123 kJ mol⁻¹ (per equation as written).

Q5. Use the bond energies (kJ mol⁻¹): E(C–H) = 413, E(Cl–Cl) = 243, E(C–Cl) = 328, E(H–Cl) = 432 to estimate ΔH for CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g).

• A. −104 kJ mol⁻¹(correct answer)
• B. −208 kJ mol⁻¹
• C. +104 kJ mol⁻¹
• D. +139 kJ mol⁻¹

Answer: A.Bonds broken (reactants): 1895 kJ; bonds formed (products): 1999 kJ; ΔH = ΣE(reactants) − ΣE(products) = 1895 − 1999 = −104 kJ mol⁻¹ (per equation as written).

Q6. Use the bond energies (kJ mol⁻¹): E(C≡C) = 812, E(C–H) = 413, E(H–H) = 436, E(C–C) = 348 to estimate ΔH for C₂H₂(g) + 2H₂(g) → C₂H₆(g).

• A. −316 kJ mol⁻¹(correct answer)
• B. −752 kJ mol⁻¹
• C. +316 kJ mol⁻¹
• D. −632 kJ mol⁻¹

Answer: A.Bonds broken (reactants): 2510 kJ; bonds formed (products): 2826 kJ; ΔH = ΣE(reactants) − ΣE(products) = 2510 − 2826 = −316 kJ mol⁻¹ (per equation as written).

Q7. Use the bond energies (kJ mol⁻¹): E(C–H) = 413, E(Cl–Cl) = 243, E(C–Cl) = 328, E(H–Cl) = 432 to estimate ΔH for CH₄(g) + 2Cl₂(g) → CH₂Cl₂(g) + 2HCl(g).

• A. −451 kJ mol⁻¹
• B. −416 kJ mol⁻¹
• C. +208 kJ mol⁻¹
• D. −208 kJ mol⁻¹(correct answer)

Answer: D.Bonds broken (reactants): 2138 kJ; bonds formed (products): 2346 kJ; ΔH = ΣE(reactants) − ΣE(products) = 2138 − 2346 = −208 kJ mol⁻¹ (per equation as written).

Q8. Use the bond energies (kJ mol⁻¹): E(C–C) = 348, E(C–H) = 413, E(Cl–Cl) = 243, E(C–Cl) = 328, E(H–Cl) = 432 to estimate ΔH for C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g).

• A. −208 kJ mol⁻¹
• B. +139 kJ mol⁻¹
• C. −104 kJ mol⁻¹(correct answer)
• D. +104 kJ mol⁻¹

Answer: C.Bonds broken (reactants): 3069 kJ; bonds formed (products): 3173 kJ; ΔH = ΣE(reactants) − ΣE(products) = 3069 − 3173 = −104 kJ mol⁻¹ (per equation as written).

Q9. Use the bond energies (kJ mol⁻¹): E(C–C) = 348, E(C–H) = 413, E(O=O) = 498, E(C=O) = 728, E(O–H) = 463 to estimate ΔH for 2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(g).

• A. −2740 kJ mol⁻¹
• B. −4480 kJ mol⁻¹
• C. +2240 kJ mol⁻¹
• D. −2240 kJ mol⁻¹(correct answer)

Answer: D.Bonds broken (reactants): 9138 kJ; bonds formed (products): 11380 kJ; ΔH = ΣE(reactants) − ΣE(products) = 9138 − 11380 = −2242 kJ mol⁻¹ (per equation as written).

Q10. Use the bond energies (kJ mol⁻¹): E(C–C) = 348, E(C–H) = 413, E(O=O) = 498, E(C=O) = 728, E(O–H) = 463 to estimate ΔH for C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g).

• A. −3160 kJ mol⁻¹
• B. +1580 kJ mol⁻¹
• C. −2080 kJ mol⁻¹
• D. −1580 kJ mol⁻¹(correct answer)

Answer: D.Bonds broken (reactants): 6490 kJ; bonds formed (products): 8072 kJ; ΔH = ΣE(reactants) − ΣE(products) = 6490 − 8072 = −1582 kJ mol⁻¹ (per equation as written).