ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 5): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Standard enthalpies of combustion (kJ mol⁻¹): C(graphite) −393.5, H₂(g) −285.8, propane C₃H₈(g) −2220. The standard enthalpy of formation of propane is
Answer: B. −103.7 kJ mol⁻¹
Explanation: 3C + 4H₂ → C₃H₈. ΔHf = [3(−393.5) + 4(−285.8)] − (−2220) = (−1180.5 − 1143.2) + 2220 = −103.7 kJ mol⁻¹.
Q2. Burning 0.92 g of liquid ethanol (46 g mol⁻¹) in a bomb calorimeter of heat capacity 10.5 kJ K⁻¹ raises the temperature by 2.60 K. Given C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), what is ΔH (not ΔU) per mole of ethanol at 298 K? (R = 8.314 J K⁻¹ mol⁻¹)
Answer: D. −1367.5 kJ mol⁻¹
Explanation: q = 10.5 × 2.60 = 27.3 kJ for 0.020 mol, so ΔU = −1365 kJ mol⁻¹. Δn(gas) = 2 − 3 = −1; ΔH = ΔU + ΔnRT = −1365 − 2.48 = −1367.5 kJ mol⁻¹.
Q3. A reaction has ΔG° = −5.7 kJ mol⁻¹ at 298 K. Its equilibrium constant is closest to (R = 8.314 J K⁻¹ mol⁻¹)
Answer: C. 10
Explanation: ΔG° = −RT ln K → ln K = 5700/(8.314 × 298) = 2.30, so K = e^2.30 ≈ 10.
Q4. N₂O₄ ⇌ 2NO₂ has Kc = 0.36 mol dm⁻³ at 373 K. If 0.10 mol dm⁻³ of N₂O₄ is initially present, the degree of dissociation at equilibrium is
Answer: C. 0.60
Explanation: Kc = 4c₀α²/(1 − α) → 0.40α² + 0.36α − 0.36 = 0 → α = [−0.36 + √(0.1296 + 0.576)]/0.80 = (−0.36 + 0.84)/0.80 = 0.60.
Q5. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Kc = 6.0 × 10⁵ (mol dm⁻³)⁻² at 500 K. What is Kp in atm⁻²? (R = 0.0821 dm³ atm K⁻¹ mol⁻¹)
Answer: A. 356
Explanation: Kp = Kc(RT)^Δn with Δn = −2: RT = 0.0821 × 500 = 41.05; Kp = 6.0e5/41.05² = 6.0e5/1685 = 356 atm⁻².
Take the timed test (free, no sign-up) to see your score and the answer and explanation for every question.
Q6. For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −198 kJ. Which change increases both the equilibrium yield of SO₃ and the value of Kc?
Q7. For a reaction with ΔH° = +50 kJ mol⁻¹ (assumed constant), by what factor does the equilibrium constant change when the temperature is raised from 300 K to 310 K? (R = 8.314 J K⁻¹ mol⁻¹)
Q8. What is the pH of a 1.0 × 10⁻⁷ mol dm⁻³ solution of HCl at 25 °C (Kw = 1.0 × 10⁻¹⁴)?
Q9. A buffer contains 0.20 mol dm⁻³ NH₃ and 0.10 mol dm⁻³ NH₄Cl. For NH₃, Kb = 1.8 × 10⁻⁵. Its pH is closest to
Q10. 100 cm³ of a buffer containing 0.20 mol dm⁻³ acetic acid and 0.20 mol dm⁻³ sodium acetate (pKa = 4.76) receives 10.0 cm³ of 0.10 mol dm⁻³ HCl. What is the new pH?
Q11. CaF₂ has Ksp = 4.0 × 10⁻¹¹ (mol dm⁻³)³. Equal volumes of 2.0 × 10⁻³ mol dm⁻³ Ca(NO₃)₂ and 3.0 × 10⁻⁴ mol dm⁻³ NaF are mixed. What happens?
Q12. A first-order reaction has a half-life of 20 min. How long does it take for 90 % of the reactant to be consumed?
Q13. The concentration of A in a zero-order decomposition falls from 0.50 mol dm⁻³ to 0.30 mol dm⁻³ in 20 min. What is the half-life of the reaction starting from 0.50 mol dm⁻³?
Q14. A second-order reaction 2A → products has k = 0.25 dm³ mol⁻¹ s⁻¹, defined by −d[A]/dt = k[A]². How long is needed for [A] to fall from 0.40 to 0.10 mol dm⁻³?
Q15. Initial-rate data: (1) [A] = 0.10, [B] = 0.10, rate = 2.0 × 10⁻³; (2) [A] = 0.20, [B] = 0.10, rate = 8.0 × 10⁻³; (3) [A] = 0.20, [B] = 0.20, rate = 1.6 × 10⁻² (mol dm⁻³ s⁻¹). What rate is expected when [A] = 0.30 and [B] = 0.40 mol dm⁻³?
Q16. The rate constant of a reaction triples when the temperature is raised from 300 K to 320 K. Its activation energy is closest to (R = 8.314 J K⁻¹ mol⁻¹)
Q17. A catalyst changes the activation energy of a reaction from 120 kJ mol⁻¹ to 100 kJ mol⁻¹ with the same pre-exponential factor. At 400 K, by what factor does the rate constant increase? (R = 8.314 J K⁻¹ mol⁻¹)
Q18. The enthalpy of neutralisation of HF with NaOH is −68.6 kJ mol⁻¹, while that of a strong acid with a strong base is −57.3 kJ mol⁻¹. The enthalpy of ionisation of HF is therefore
Q19. 100 cm³ of 0.50 mol dm⁻³ H₂SO₄ is mixed with 100 cm³ of 1.00 mol dm⁻³ NaOH in an insulated cup. Taking ΔH = −57.3 kJ per mole of water formed, 200 g total solution and specific heat 4.18 J g⁻¹ K⁻¹, the temperature rise is
Q20. For ½N₂ + 3/2 H₂ ⇌ NH₃, K = 4.0 at a given temperature. For the reaction 2NH₃ ⇌ N₂ + 3H₂ at the same temperature, K is
Q21. N₂O₄ ⇌ 2NO₂ is 50 % dissociated at a total pressure of 1.0 atm and 333 K. What is the degree of dissociation at the same temperature when the total equilibrium pressure is 4.0 atm?
Q22. Solid NH₄HS dissociates: NH₄HS(s) ⇌ NH₃(g) + H₂S(g). When equilibrium is reached in an evacuated vessel the total pressure is 0.60 atm. What is Kp?
Q23. At a certain temperature Kw = 1.0 × 10⁻¹³. What is the pH of 0.010 mol dm⁻³ NaOH at that temperature?
Q24. What is the pH of 0.10 mol dm⁻³ NaCN? (Ka of HCN = 6.2 × 10⁻¹⁰; Kw = 1.0 × 10⁻¹⁴; assume little hydrolysis)
Q25. Ice melts reversibly at 273 K and 1 atm with ΔH = +6.01 kJ mol⁻¹. The entropy change for melting 1 mol of ice is
Q26. A gas absorbs 1500 J of heat and expands from 5.0 dm³ to 10.0 dm³ against a constant external pressure of 2.0 atm (1 dm³ atm = 101.3 J). What is ΔU?
Q27. For 2H₂(g) + O₂(g) → 2H₂O(l) at 298 K, the difference ΔH − ΔU is (R = 8.314 J K⁻¹ mol⁻¹)
Q28. Using ΔHf° (kJ mol⁻¹): NH₃(g) −46, NO(g) +90, H₂O(g) −242, the ΔH° of 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) is
Q29. 2NO + O₂ → 2NO₂ is proposed to occur by: (1) NO + NO ⇌ N₂O₂ (fast equilibrium); (2) N₂O₂ + O₂ → 2NO₂ (slow). The rate law is
Q30. The hydrolysis CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH is found to be first order in ester when carried out in a large excess of water. This is because