General Science MCQ – 24 (Chemistry)

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1. Which of the following substance has the lowest standard molar entropy (So) at 25o C?
(A) CH3OH(l)
(B) CO(g)
(C) MgO(s)
(D) H₂O(l)

2. When crystalline solid barium hydroxide octahydrate and crystalline solid ammonium nitrate are mixed in a beaker at room temperature, a spontaneous occurs. The temperature of the beaker contents rapidly falls to below 0o use this information to decide whether the reaction is exothermic or endothermic and what the sign of ΔH and ΔS are?
(A) Endothermic; ΔH > 0; ΔS > 0
(B) Endothermic; ΔH < 0; ΔS > 0
(C) Endothermic; ΔH < 0; ΔS < 0
(D) Endothermic; ΔH > 0; ΔS < 0

3. Sodium carbonate can be made by heating sodium hydrogen carbonate:
2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g)
For this reaction, ΔH^° = 128.9 kJ and ΔS^° = 321 J/K. At approximately what temperature will K = 1?
(A) 401.6 K
(B) 401.6^o K
(C) 402.6 C
(D) 33.1 K

4. For the overall hypothetical reaction A + 5B → 4C, the rate of appearance of C given by Δ[C]/Δt is the same as?
(A) -(5/4)(Δ[B]/Δt)
(B) Δ[A]/Δt
(C) -(4/5)(Δ[B]/Δt)
(D) None of these

5. The initial rate of the reaction
PCl₅→ PCl₃ + Cl₂
is increased a factor of four when the concentration of PCl5 is doubled. Therefore, the rate?
(A) Is second order with respect to PCI₃
(B) Is first order with respect to PCI₃
(C) Is second order with respect to PCI₅
(D) Is first order with respect to PCI₅

6. Consider the reaction A -> products. Which of the following plots is consistent with a zero-order reaction?
(A) In [A] plotted against time gives a straight line of negative slope
(B) [A] plotted against time gives a horizontal, straight line.
(C) [A] plotted against time gives a curved line of negative slope, decreasing in magnitude as time increase
(D) [A] plotted against time gives a straight line of negative slope

7. The rate constant of a first-order reaction is 3.68 x 10^-2 s^-1 at 150^o C, and the activation energy is 71 kj/mol. What is the value of the rate constant at 170^o C?
(A) 9.2 x 10^-2 s^-1
(B) 9.5 x 11^-2 s^-1
(C) 3.7 x 10^-2 s^-1
(D) 2.49 s^-1

8. The reaction
3ClO^–(aq) → ClO₃^–(aq)
+ 2Cl^–(aq) has been proposed to occur by the following mechanism.
ClO^–(aq) + ClO^–( aq)→ ClO₂^–(aq) + Cl^–( aq) (slow)
ClO₂^–(aq) + ClO^–(aq) → ClO₃^–(aq) + Cl (aq) (fast)
Which rate law is consistent with this mechanism?
(A) Rate = k[CI^–][CIO^–]²
(B) Rate = k[CIO^–]³
(C) Rate = k[CIO^–]
(D) Rate = k[ClO^–]²

9. A catalyst speeds up a reaction by?
(A) Providing a new reaction pathway for molecules
(B) Increasing the number of high-energy molecules
(C) Increasing the temperature of the molecules in the reaction
(D) Increasing the activation energy for the reaction

10. Consider the following gas-phase equilibrium:
H₂(g) + I₂(g) ↔ 2HI(g) At a certain temperature, the equilibrium constant K_c is 4.0. Starting with equimolar quantities of H₂ and I₂ and no HI, when equilibrium was established, 0.20 moles of HI was present. How much H₂ was used to start the reaction?
(A) 4.0 mil
(B) 0.23 mol
(C) 0.20 mol
(D) 0.10 mol