ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 1): 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. Given C + O₂ → CO₂, ΔH = −393.5 kJ, and CO + ½O₂ → CO₂, ΔH = −283.0 kJ, the enthalpy change for C + ½O₂ → CO is:
Answer: C. −110.5 kJ
Explanation: Hess's law: subtract the second equation from the first: −393.5 − (−283.0) = −110.5 kJ.
Q2. The enthalpy of neutralization of a strong acid by a strong base is about:
Answer: C. −57.3 kJ mol⁻¹
Explanation: In every case the reaction is H⁺ + OH⁻ → H₂O, which releases about 57.3 kJ per mole of water formed.
Q3. Which change releases heat to the surroundings?
Answer: C. Freezing of water
Explanation: Freezing forms hydrogen bonds and releases the enthalpy of fusion; the other three processes absorb heat.
Q4. In a calorimeter, a reaction raises the temperature of 100 g of water by 5°C. The heat released is (c = 4.18 J g⁻¹ K⁻¹):
Answer: D. 2.09 kJ
Explanation: q = mcΔT = 100 × 4.18 × 5 = 2090 J = 2.09 kJ.
Q5. The standard enthalpy of formation of an element in its standard state is:
Answer: A. zero
Explanation: By convention, ΔHf° of elements in their most stable form at 298 K and 1 atm is taken as zero.
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Q6. For N₂(g) + 3H₂(g) → 2NH₃(g) at 300 K, the value of ΔH − ΔE is about (R = 8.314 J K⁻¹ mol⁻¹):
Q7. Which of the following is a state function?
Q8. The units of Kc for N₂ + 3H₂ ⇌ 2NH₃ are:
Q9. For H₂ + I₂ ⇌ 2HI, the equilibrium concentrations are [H₂] = 0.1, [I₂] = 0.2 and [HI] = 1.0 mol dm⁻³. The value of Kc is:
Q10. For which reaction does Kp equal Kc?
Q11. Increasing the pressure on the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g):
Q12. Adding a catalyst to a reaction at equilibrium:
Q13. For the endothermic equilibrium N₂O₄(g, colourless) ⇌ 2NO₂(g, brown), cooling the mixture:
Q14. The pH of a 0.001 M HCl solution is:
Q15. The pH of a 0.005 M Ba(OH)₂ solution is:
Q16. The solubility of AgCl in water decreases when NaCl is added. This is due to:
Q17. The solubility of CaF₂ in water is 2 × 10⁻⁴ mol dm⁻³. Its solubility product Ksp is:
Q18. A buffer contains 0.1 M CH₃COOH and 1.0 M CH₃COONa. If the pKa of acetic acid is 4.74, the pH of the buffer is:
Q19. The pH of a 0.1 M solution of a weak monobasic acid with Ka = 1 × 10⁻⁵ is:
Q20. 1 mol of PCl₅ is heated in a 1 dm³ vessel and is 40% dissociated at equilibrium (PCl₅ ⇌ PCl₃ + Cl₂). The value of Kc is about:
Q21. The rate constant of a reaction has units of mol dm⁻³ s⁻¹. The order of the reaction is:
Q22. The rate constant of a first-order reaction is 0.0693 min⁻¹. Its half-life is:
Q23. What percentage of a first-order reactant remains after three half-lives?
Q24. For the rate law rate = k[A]²[B], if the concentrations of both A and B are doubled, the rate becomes:
Q25. A first-order reaction has a half-life of 20 min. How long does it take for 75% of the reactant to decompose?
Q26. A 10 K rise in temperature often roughly doubles the rate of a reaction mainly because:
Q27. When the concentration of the only reactant is doubled, the rate becomes four times larger. The order of the reaction is:
Q28. For a zero-order reaction, the half-life is:
Q29. The rate constant of a reaction doubles when the temperature is raised from 300 K to 310 K. The activation energy is about (R = 8.314 J K⁻¹ mol⁻¹, ln 2 = 0.693):
Q30. In the titration of oxalic acid with acidified KMnO₄, the reaction is slow at first and then speeds up. This is because: