ECAT — Engineering College Admission Test (UET)

ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 4)

Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 4): 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.

Q1. Standard enthalpies of combustion (kJ mol⁻¹) are: C(graphite) −393.5, H₂(g) −285.8 and CH₃OH(l) −726.0. What is the standard enthalpy of formation of CH₃OH(l)?
- A. −239.1 kJ mol⁻¹ ✓
- B. −66.3 kJ mol⁻¹
- C. +239.1 kJ mol⁻¹
- D. −1405.4 kJ mol⁻¹
Answer: A. −239.1 kJ mol⁻¹
Explanation: C + 2H₂ + ½O₂ → CH₃OH. ΔHf = [ΔHc(C) + 2ΔHc(H₂)] − ΔHc(CH₃OH) = (−393.5 − 571.6) − (−726.0) = −239.1 kJ mol⁻¹.
Q2. A reaction has ΔH = −30 kJ mol⁻¹ and ΔS = −100 J mol⁻¹ K⁻¹. Below what temperature is it spontaneous?
- A. 30 K
- B. It is spontaneous at all temperatures
- C. 300 K ✓
- D. 3000 K
Answer: C. 300 K
Explanation: ΔG = ΔH − TΔS = −30 000 + 100T < 0 requires T < 300 K. At higher temperatures the unfavourable entropy term wins.
Q3. The vapour density of an equilibrium mixture from the dissociation N₂O₄ ⇌ 2NO₂ is 36.8 (relative to H₂ = 1). Taking the vapour density of undissociated N₂O₄ as 46, the degree of dissociation is
- A. 0.50
- B. 0.25 ✓
- C. 0.125
- D. 0.20
Answer: B. 0.25
Explanation: α = (D − d)/[d(n − 1)] with n = 2: (46 − 36.8)/36.8 = 0.25.
Q4. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at equilibrium in a cylinder with a movable piston at constant temperature and pressure, argon is added. What happens?
- A. Kp increases
- B. The equilibrium shifts toward NH₃
- C. There is no change in the position of equilibrium
- D. The equilibrium shifts toward the reactants ✓
Answer: D. The equilibrium shifts toward the reactants
Explanation: At constant total pressure the volume increases when argon is added, so partial pressures fall. The reaction shifts toward the side with more gas moles (reactants, 4 mol). Kp is unchanged. At constant volume argon would have no effect.
Q5. Taking water's own ionisation into account, what is the approximate pH of a 1.0×10⁻⁸ M HCl solution at 25 °C?
- A. 7.00
- B. 8.00
- C. 6.00
- D. 6.98 ✓
Answer: D. 6.98
Explanation: Water's own ionization cannot be ignored: [H⁺] = 10⁻⁸ + [OH⁻] gives x² − 10⁻⁸x − 10⁻¹⁴ = 0, so x = 1.05×10⁻⁷ M and pH = 6.98 (slightly acidic, never above 7).

The other 25 questions

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Q6. What is the pH of 0.10 M NH₃(aq)? (Kb = 1.8×10⁻⁵, log 1.34 = 0.127)
- A. 11.13
- B. 2.87
- C. 12.00
- D. 9.25
Q7. A buffer is prepared by mixing 0.30 mol NH₃ and 0.10 mol NH₄Cl in 1 L of solution. pKa of NH₄⁺ = 9.25 and log 3 = 0.48. What is the pH?
- A. 4.77
- B. 8.77
- C. 9.25
- D. 9.73
Q8. A saturated solution of Mg(OH)₂ has pH 10. What is the Ksp of Mg(OH)₂?
- A. 1×10⁻¹²
- B. 5×10⁻¹³
- C. 5×10⁻⁹
- D. 1×10⁻⁸
Q9. 50 mL of 2.0×10⁻⁵ M AgNO₃ is mixed with 50 mL of 2.0×10⁻⁵ M NaCl. Ksp of AgCl = 1.8×10⁻¹⁰. What is the result?
- A. No precipitate forms because Q < Ksp
- B. No precipitate forms because Q > Ksp
- C. A precipitate forms because Q = Ksp
- D. A precipitate forms because Q > Ksp
Q10. For A + B → products the initial rate data are: [A] = 0.1, [B] = 0.1 M, rate 2.0×10⁻³; [A] = 0.2, [B] = 0.1 M, rate 8.0×10⁻³; [A] = 0.1, [B] = 0.2 M, rate 4.0×10⁻³ (rates in M s⁻¹). What is the rate when [A] = 0.3 M and [B] = 0.2 M?
- A. 1.08×10⁻¹ M s⁻¹
- B. 3.6×10⁻² M s⁻¹
- C. 1.2×10⁻² M s⁻¹
- D. 2.4×10⁻² M s⁻¹
Q11. A second-order reaction (rate = k[A]²) has a half-life of 40 min when [A]₀ = 0.10 M. What is the half-life when [A]₀ = 0.40 M?
- A. 40 min
- B. 160 min
- C. 20 min
- D. 10 min
Q12. A zero-order reaction has k = 0.010 M s⁻¹ and an initial concentration of 0.50 M. What is its half-life?
- A. 69.3 s
- B. 25 s
- C. 100 s
- D. 50 s
Q13. A reaction has activation energy 50 kJ mol⁻¹. By what factor does its rate constant increase when the temperature is raised from 300 K to 320 K? (R = 8.314 J mol⁻¹ K⁻¹)
- A. About 1.07
- B. About 12
- C. About 2.0
- D. About 3.5
Q14. The forward reaction of an exothermic reversible reaction has Ea = 80 kJ mol⁻¹ and ΔH = −30 kJ mol⁻¹. What is the activation energy of the reverse reaction?
- A. 110 kJ mol⁻¹
- B. 50 kJ mol⁻¹
- C. −110 kJ mol⁻¹
- D. 80 kJ mol⁻¹
Q15. The reaction NO₂ + CO → NO + CO₂ proceeds by: step 1 (slow) 2NO₂ → NO₃ + NO; step 2 (fast) NO₃ + CO → NO₂ + CO₂. What is the experimental rate law?
- A. rate = k[NO₂]²
- B. rate = k[NO₂][CO]
- C. rate = k[NO₂]
- D. rate = k[NO₃][CO]
Q16. The equilibrium constant of a reaction at 300 K is 10. What is ΔG° for the reaction? (R = 8.314 J mol⁻¹ K⁻¹, ln 10 = 2.303)
- A. −24.9 kJ mol⁻¹
- B. −5.74 kJ mol⁻¹
- C. +5.74 kJ mol⁻¹
- D. −2.49 kJ mol⁻¹
Q17. Graphite → diamond has ΔH = +1.9 kJ mol⁻¹, and the enthalpy of combustion of graphite is −393.5 kJ mol⁻¹. What is the enthalpy of combustion of diamond?
- A. −391.6 kJ mol⁻¹
- B. +395.4 kJ mol⁻¹
- C. −395.4 kJ mol⁻¹
- D. −393.5 kJ mol⁻¹
Q18. For which order of reaction is the half-life independent of the initial concentration?
- A. First order
- B. Zero order
- C. Second order
- D. Third order
Q19. The equilibrium constant of an exothermic reaction, when the temperature is raised, will
- A. become zero
- B. decrease
- C. increase
- D. remain unchanged
Q20. 50 mL of 1.0 M HCl is mixed with 50 mL of 1.0 M NaOH in a calorimeter and the temperature rises by 6.8 °C. Taking the solution mass as 100 g and specific heat 4.18 J g⁻¹ °C⁻¹, the enthalpy of neutralization is about
- A. −56.8 kJ mol⁻¹
- B. −2.84 kJ mol⁻¹
- C. −113.6 kJ mol⁻¹
- D. −28.4 kJ mol⁻¹
Q21. For A(g) + B(g) ⇌ C(g) + D(g), Kc = 4. If 1.0 mol each of A and B is placed in a 1 L vessel, how many moles of C are present at equilibrium?
- A. 0.50
- B. 1.33
- C. 0.67
- D. 0.80
Q22. At 60 °C the ionic product of water is Kw = 1.0×10⁻¹³. What is the pH of pure water at this temperature, and is it acidic, basic or neutral?
- A. 6.5, neutral
- B. 7.0, neutral
- C. 7.0, basic
- D. 6.5, acidic
Q23. What is the pH of a 0.0010 M solution of Ba(OH)₂ at 25 °C? (log 2 = 0.30)
- A. 2.7
- B. 11.0
- C. 10.7
- D. 11.3
Q24. 25 mL of 0.10 M HCl is mixed with 15 mL of 0.10 M NaOH. What is the pH of the resulting solution? (log 2.5 = 0.40)
- A. 1.00
- B. 1.60
- C. 1.40
- D. 2.40
Q25. A first-order reaction is 25% complete in 10 minutes. What is its half-life? (ln 2 = 0.693, ln 4/3 = 0.288)
- A. 40 min
- B. 20 min
- C. 24 min
- D. 30 min
Q26. For CaCO₃(s) ⇌ CaO(s) + CO₂(g) in a closed vessel at fixed temperature, the equilibrium pressure of CO₂
- A. is independent of the amounts of CaCO₃ and CaO present
- B. depends on the volume of the vessel
- C. falls if more CaO is added
- D. doubles if the mass of CaCO₃ is doubled
Q27. What are the units of Kp for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) when pressures are in atm?
- A. no units
- B. atm⁻²
- C. atm²
- D. atm
Q28. Using S° (J mol⁻¹ K⁻¹): H₂(g) 131, O₂(g) 205, H₂O(l) 70, what is ΔS° for 2H₂(g) + O₂(g) → 2H₂O(l)?
- A. −327 J K⁻¹
- B. +327 J K⁻¹
- C. −187 J K⁻¹
- D. −47 J K⁻¹
Q29. Which of the following has a standard enthalpy of formation equal to zero at 25 °C and 1 atm?
- A. O₃(g)
- B. Br₂(l)
- C. Br₂(g)
- D. C(diamond)
Q30. For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) at constant temperature, the volume of the container is doubled. Which statement is correct?
- A. Equilibrium shifts to the right and Kp increases
- B. Equilibrium shifts to the right, but Kp is unchanged
- C. No shift occurs and Kp is unchanged
- D. Equilibrium shifts to the left and Kp decreases