ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 6): 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. CaCO₃(s) ⇌ CaO(s) + CO₂(g) is at equilibrium in a closed cylinder at fixed temperature. The piston is pushed in until the volume is halved, and equilibrium is re-established at the same temperature. Compared with the original, the CO₂ pressure and the mass of CaCO₃ are now:
Answer: A. pressure unchanged; mass of CaCO₃ greater
Explanation: K_p = p(CO₂) depends only on temperature, so the pressure returns to its original value. Fewer moles of CO₂ are needed at the same pressure in the smaller volume, so CO₂ recombines with CaO, increasing CaCO₃.
Q2. N₂O₄(g) ⇌ 2NO₂(g) is 20% dissociated (α = 0.20) at a total pressure of 1.0 atm. The value of K_p is:
Answer: A. 0.167 atm
Explanation: With initial 1 mol: N₂O₄ = 0.8, NO₂ = 0.4, total 1.2 mol. p(NO₂) = 0.333, p(N₂O₄) = 0.667 atm. K_p = (0.333)²/0.667 = 0.167 atm (= 4α²P/(1 − α²)).
Q3. 2.0 mol each of A and B are mixed in a 1.0 dm³ vessel: A + B ⇌ C + D, K_c = 4.0. The equilibrium concentration of C is:
Answer: C. 1.33 mol dm⁻³
Explanation: x²/(2 − x)² = 4, so x/(2 − x) = 2 and x = 4/3 = 1.33 mol dm⁻³.
Q4. The pH of 0.10 M sodium acetate solution (K_a of acetic acid = 1.8 × 10⁻⁵; pK_a = 4.745, pK_w = 14) is:
Answer: B. 8.87
Explanation: CH₃COO⁻ hydrolyses: pH = 7 + ½(pK_a + log C) = 7 + ½(4.745 − 1) = 8.87.
Q5. 100 cm³ of 0.20 M acetic acid (pK_a = 4.74) is mixed with 50 cm³ of 0.20 M NaOH. The pH of the resulting solution is:
Answer: B. 4.74
Explanation: NaOH (10 mmol) converts half of the acid (20 mmol) to acetate, leaving 10 mmol acid and 10 mmol acetate. pH = pK_a + log(1) = 4.74.
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Q6. K_sp of Mg(OH)₂ is 1.2 × 10⁻¹¹ at 25 °C. The pH of its saturated solution is:
Q7. Equal volumes of 1.0 × 10⁻⁵ M BaCl₂ and 1.0 × 10⁻⁵ M Na₂SO₄ are mixed (K_sp of BaSO₄ = 1.1 × 10⁻¹⁰). Which statement is correct?
Q8. The solubility product of PbI₂ is 7.1 × 10⁻⁹. Its solubility in 0.10 M KI (taking [I⁻] = 0.10 M) is:
Q9. Initial rates for A + B → products: (i) [A] = 0.10, [B] = 0.10 M, rate = 2.0 × 10⁻³ M s⁻¹; (ii) [A] = 0.20, [B] = 0.10, rate = 8.0 × 10⁻³; (iii) [A] = 0.10, [B] = 0.30, rate = 6.0 × 10⁻³. The rate constant k is:
Q10. A first-order reaction has k = 5.0 × 10⁻³ s⁻¹. The time taken for the concentration to fall from 0.80 M to 0.10 M is about (ln 2 = 0.693):
Q11. For a second-order reaction 2A → products with rate constant k = 0.050 dm³ mol⁻¹ s⁻¹, the half-life when the initial concentration is 0.80 M (t½ = 1/(k[A]₀)) is:
Q12. For a reaction with activation energy 50 kJ mol⁻¹, by what factor does the rate constant increase when the temperature is raised from 300 K to 310 K (R = 8.314 J K⁻¹ mol⁻¹; ln k₂/k₁ = (Eₐ/R)(1/T₁ − 1/T₂))?
Q13. An exothermic reaction has ΔH = −40 kJ mol⁻¹ and a forward activation energy of 80 kJ mol⁻¹. A catalyst lowers the forward activation energy to 60 kJ mol⁻¹. The activation energy of the reverse reaction with the catalyst is:
Q14. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) has ΔH = −890 kJ mol⁻¹ at 298 K. Taking R = 8.314 J K⁻¹ mol⁻¹, ΔU is (ΔH = ΔU + ΔnRT):
Q15. ΔH_comb (kJ mol⁻¹): C(graphite) −393.5, H₂(g) −285.8, C₃H₈(g) −2220.0. The standard enthalpy of formation of propane (3C + 4H₂ → C₃H₈) is:
Q16. 4.0 g of NaOH (0.10 mol) dissolves in 200 g of water and the temperature rises by 5.0 °C. Taking the specific heat of the solution as 4.18 J g⁻¹ K⁻¹ and using a mass of 200 g, the enthalpy of solution of NaOH is:
Q17. Standard enthalpies of combustion are: C(graphite) −394, H₂(g) −286 and CH₄(g) −890 kJ mol⁻¹. The enthalpy of formation of CH₄(g) from its elements is:
Q18. 100 cm³ of 1.00 M HCl is mixed with 100 cm³ of 1.00 M NaOH in an insulated cup and the temperature rises by 6.5 °C. Taking the mixture as 200 g of solution with specific heat 4.2 J g⁻¹ K⁻¹, the molar enthalpy of neutralisation is:
Q19. For N₂ + 3H₂ ⇌ 2NH₃ at a certain temperature K_c = 40 (dm⁶ mol⁻²). A mixture has [N₂] = 0.10, [H₂] = 0.20 and [NH₃] = 0.20 mol dm⁻³. The reaction will proceed:
Q20. N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is at equilibrium in a flexible container kept at constant total pressure and temperature. Helium is added, so the volume increases. The equilibrium:
Q21. In which of these changes does the entropy of the system decrease?
Q22. 100 cm³ of 0.10 M HCl is mixed with 100 cm³ of 0.050 M NaOH. The pH of the mixture is:
Q23. The pH of 1.0 × 10⁻⁷ M HCl at 25 °C, allowing for the ionisation of water (K_w = 1.0 × 10⁻¹⁴), is:
Q24. The pH of 0.10 M NH₄Cl solution (K_b of NH₃ = 1.8 × 10⁻⁵, pK_b = 4.745, pK_w = 14) is:
Q25. The reaction NO₂ + CO → NO + CO₂ has the experimental rate law rate = k[NO₂]². Which mechanism is consistent with it?
Q26. The rate of a reaction is proportional to [A]^(3/2). The units of its rate constant (concentration in mol dm⁻³, time in s) are:
Q27. For 2N₂O₅ → 4NO₂ + O₂, the rate of formation of NO₂ is 4.0 × 10⁻³ mol dm⁻³ s⁻¹. Taking the rate of reaction as defined by rate = −½d[N₂O₅]/dt, the rate of formation of O₂ is:
Q28. In a first-order decomposition of H₂O₂, 20% of the initial amount decomposes in the first 10 minutes. The percentage of the initial amount remaining after 30 minutes is:
Q29. For the equilibrium 2A ⇌ B, K_c = 4.0 (concentrations in mol dm⁻³). For A ⇌ ½B at the same temperature, K_c is:
Q30. How much heat is needed to raise the temperature of 2.0 mol of a monatomic ideal gas by 10 K at constant pressure (C_p = 5R/2, R = 8.314 J K⁻¹ mol⁻¹)?