ECAT — Engineering College Admission Test (UET)

ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 3)

Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 3): 30 multiple-choice questions (40 minutes) with instant scoring and a full…

What this test covers

• 30 ECAT Chemistry

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

1. Q1. Given N₂ + O₂ → 2NO, ΔH = +180 kJ and 2NO + O₂ → 2NO₂, ΔH = −114 kJ, the ΔH for N₂ + 2O₂ → 2NO₂ is:

   • A. +66 kJ ✓
   • B. −66 kJ
   • C. −294 kJ
   • D. +294 kJ

   Answer: A. +66 kJ

   Explanation: Add the two equations: 180 + (−114) = +66 kJ (NO cancels).

2. Q2. From ΔHf°: CH₄ = −74.8, CO₂ = −393.5, H₂O(l) = −285.8 kJ/mol, the standard enthalpy of combustion of methane is:

   • A. −1040 kJ/mol
   • B. −604.5 kJ/mol
   • C. −890.3 kJ/mol ✓
   • D. −178.8 kJ/mol

   Answer: C. −890.3 kJ/mol

   Explanation: ΔH = ΣΔHf(products) − ΣΔHf(reactants) = −393.5 + 2(−285.8) + 74.8 = -890.3 kJ/mol.

3. Q3. How much heat is released when 3.2 g of methane is burned if its enthalpy of combustion is −890 kJ/mol? (C = 12, H = 1)

   • A. 178 kJ ✓
   • B. 890 kJ
   • C. 17.8 kJ
   • D. 284.8 kJ

   Answer: A. 178 kJ

   Explanation: 3.2/16 = 0.2 mol; 0.2 × 890 = 178 kJ.

4. Q4. Using bond energies (C–H 414, Cl–Cl 243, C–Cl 339, H–Cl 431 kJ/mol), the ΔH for CH₄ + Cl₂ → CH₃Cl + HCl is:

   • A. −113 kJ ✓
   • B. +113 kJ
   • C. −339 kJ
   • D. +27 kJ

   Answer: A. −113 kJ

   Explanation: ΔH = bonds broken − bonds formed = (414 + 243) − (339 + 431) = -113 kJ.

5. Q5. For N₂(g) + 3H₂(g) → 2NH₃(g) at 298 K, ΔH differs from ΔU by about (R = 8.314 J K⁻¹ mol⁻¹):

   • A. +4.96 kJ, with ΔH less negative
   • B. −4.96 kJ, with ΔH more negative ✓
   • C. −9.9 kJ, with ΔH less negative
   • D. zero

   Answer: B. −4.96 kJ, with ΔH more negative

   Explanation: ΔH = ΔU + ΔnRT, Δn = −2: ΔnRT = -4.96 kJ, so ΔH = ΔU − 4.96 kJ.

The other 25 questions

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6. Q6. A gas absorbs 100 J of heat and does 40 J of work on its surroundings. The change in its internal energy is:

   • A. −60 J
   • B. −140 J
   • C. +60 J
   • D. +140 J

7. Q7. A gas expands from 1 L to 6 L against a constant external pressure of 2 atm. The work done by the gas is about (1 L atm = 101.3 J):

   • A. 2026 J
   • B. 1013 J
   • C. 506 J
   • D. 202.6 J

8. Q8. A reaction has ΔH = +30 kJ and ΔS = +100 J K⁻¹. It becomes spontaneous at temperatures above:

   • A. 3000 K
   • B. 130 K
   • C. 30 K
   • D. 300 K

9. Q9. For which change is the entropy change ΔS clearly positive?

   • A. H₂O(g) → H₂O(l)
   • B. CaCO₃(s) → CaO(s) + CO₂(g)
   • C. N₂(g) + 3H₂(g) → 2NH₃(g)
   • D. 2H₂(g) + O₂(g) → 2H₂O(l)

10. Q10. 1 mol of ethanol and 1 mol of acetic acid are mixed in 1 L and reach equilibrium with Kc = 4 for CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The moles of ester at equilibrium are:

    • A. 0.33
    • B. 0.50
    • C. 0.80
    • D. 0.67

11. Q11. 1 mol of PCl₅ in a 1 L vessel dissociates to the extent of 40% (PCl₅ ⇌ PCl₃ + Cl₂). The value of Kc is:

    • A. 0.267
    • B. 0.16
    • C. 0.67
    • D. 0.40

12. Q12. For the reaction N₂ + 3H₂ ⇌ 2NH₃ the relation between Kp and Kc is:

    • A. Kp = Kc(RT)²
    • B. Kp = Kc(RT)⁻¹
    • C. Kp = Kc(RT)⁻²
    • D. Kp = Kc

13. Q13. If Kc for N₂ + 3H₂ ⇌ 2NH₃ is K, the Kc for ½N₂ + 3/2 H₂ ⇌ NH₃ is:

    • A. √K
    • B. K²
    • C. K/2
    • D. 1/K

14. Q14. For an equilibrium A + B ⇌ C with Kc = 50, the reaction mixture has [A] = [B] = 0.1 M and [C] = 1.0 M. What happens?

    • A. Q = 100 > Kc, so the reaction shifts to the left
    • B. Q = 10 < Kc, so the reaction shifts to the right
    • C. Q = 100 > Kc, so the reaction shifts to the right
    • D. the system is at equilibrium

15. Q15. For the exothermic reaction N₂ + 3H₂ ⇌ 2NH₃, adding helium at constant volume to the equilibrium mixture:

    • A. has no effect on the position of equilibrium
    • B. shifts it to the left
    • C. shifts it to the right
    • D. increases the value of Kc

16. Q16. A catalyst added to a reversible reaction at equilibrium will:

    • A. change ΔH of the reaction
    • B. increase only the forward rate
    • C. increase Kc
    • D. increase the rates of forward and reverse reactions equally and leave Kc unchanged

17. Q17. The pH of a 0.01 M solution of Ba(OH)₂ (strong base, complete dissociation) is about:

    • A. 2.0
    • B. 11.7
    • C. 12.0
    • D. 12.3

18. Q18. 50 mL of 0.1 M HCl is mixed with 50 mL of 0.04 M NaOH. The pH of the solution is about:

    • A. 7.0
    • B. 2.0
    • C. 1.5
    • D. 1.0

19. Q19. A 10⁻³ M solution of HCl is diluted 100-fold with pure water. The pH of the resulting solution is approximately:

    • A. 5.00
    • B. 7.00
    • C. 3.00
    • D. 5.30

20. Q20. Pure water at 100 °C has Kw = 1 × 10⁻¹². Its pH and nature are:

    • A. pH = 6 and neutral
    • B. pH = 6 and acidic
    • C. pH = 7 and acidic
    • D. pH = 7 and neutral

21. Q21. A weak monobasic acid is 1% ionised in a 0.1 M solution. Its Ka is:

    • A. 1 × 10⁻⁵
    • B. 1 × 10⁻⁷
    • C. 1 × 10⁻⁴
    • D. 1 × 10⁻³

22. Q22. A buffer contains 0.1 mol CH₃COOH and 0.1 mol CH₃COONa per litre (pKa = 4.76). After 0.01 mol of HCl is added to 1 L of it, the pH is about:

    • A. 4.85
    • B. 4.67
    • C. 4.76
    • D. 2.0

23. Q23. Ag₂CrO₄ has Ksp = 3.2 × 10⁻¹¹. Its molar solubility is:

    • A. 5.7 × 10⁻⁶ M
    • B. 8.0 × 10⁻¹² M
    • C. 1.6 × 10⁻⁴ M
    • D. 2.0 × 10⁻⁴ M

24. Q24. Equal volumes of 1 × 10⁻⁴ M AgNO₃ and 1 × 10⁻⁴ M NaCl are mixed (Ksp of AgCl = 1 × 10⁻¹⁰). Which statement is correct?

    • A. A precipitate forms since Q = 1 × 10⁻⁸ > Ksp
    • B. No precipitate forms since Q = 1 × 10⁻⁸ < Ksp
    • C. A precipitate forms since Q = 2.5 × 10⁻⁹ > Ksp
    • D. No precipitate forms since Q = 2.5 × 10⁻⁹ < Ksp

25. Q25. The solubility of AgCl (Ksp = 1.8 × 10⁻¹⁰) in 0.1 M NaCl is:

    • A. 1.8 × 10⁻⁹ M
    • B. 1.34 × 10⁻⁵ M
    • C. 0.1 M
    • D. 1.8 × 10⁻¹¹ M

26. Q26. A reaction has a rate constant with units of mol L⁻¹ s⁻¹. Its order is:

    • A. third
    • B. zero
    • C. second
    • D. first

27. Q27. A first-order reaction is 90% complete in 23 minutes. The rate constant is about:

    • A. 0.10 min⁻¹
    • B. 0.043 min⁻¹
    • C. 10 min⁻¹
    • D. 0.23 min⁻¹

28. Q28. The rate of a reaction doubles when the temperature rises from 300 K to 310 K. The activation energy is approximately (R = 8.314 J K⁻¹ mol⁻¹, ln 2 = 0.693):

    • A. 536 kJ/mol
    • B. 53.6 kJ/mol
    • C. 5.36 kJ/mol
    • D. 26.8 kJ/mol

29. Q29. For a zero-order reaction the rate:

    • A. is directly proportional to concentration
    • B. depends on the square of concentration
    • C. falls to zero immediately
    • D. is independent of the reactant concentration

30. Q30. In the reaction 2NO + O₂ → 2NO₂ doubling [O₂] doubles the rate, and doubling [NO] quadruples it. The rate law and overall order are:

    • A. rate = k[NO][O₂]²; third order
    • B. rate = k[NO]²[O₂]; third order
    • C. rate = k[NO][O₂]; second order
    • D. rate = k[NO]²[O₂]²; fourth order