NTS NAT-IM (Medical / Pre-Medical Track) Chemistry Chemical Equilibrium — Set 2

Chemical Equilibrium MCQs set 2 for NTS NAT-IM (Medical / Pre-Medical Track) Chemistry — 20 solved questions.

NTS NAT-IM (Medical / Pre-Medical Track) Chemistry Chemical Equilibrium — Set 2

  1. Question 1

    Q1. A chemist observes a reaction where products never reach 100% yield despite infinite time. This behavior distinguishes it from an irreversible reaction because:

    • A) Dynamic equilibrium is established
    • B) Static equilibrium is established
    • C) Reactants are completely consumed
    • D) The reaction occurs in an open vessel

    Answer: Dynamic equilibrium is established

    Explanation: Dynamic equilibrium involves ongoing forward and reverse reactions at equal rates, unlike static equilibrium where all motion ceases entirely.

  2. Question 2

    Q2. In the synthesis of ammonia (N2 + 3H2 ⇌ 2NH3), the relationship between Kp and Kc differs from the H2 + I2 ⇌ 2HI reaction because:

    • A) Kp is always numerically equal to Kc
    • B) Kc depends on pressure while Kp does not
    • C) Kp equals Kc only if the change in gaseous moles is zero
    • D) Kp is used for liquids while Kc is for gases

    Answer: Kp equals Kc only if the change in gaseous moles is zero

    Explanation: Kp equals Kc only when the number of moles of gaseous reactants and products are equal; otherwise, units differ significantly.

  3. Question 3

    Q3. During the industrial production of SO3, a technician calculates that the reaction quotient (Qc) is 1.5 while Kc is 2.5. This indicates that:

    • A) The reaction proceeds in the reverse direction
    • B) The reaction proceeds in the forward direction
    • C) The system has already reached equilibrium
    • D) The value of Kc will increase to match Qc

    Answer: The reaction proceeds in the forward direction

    Explanation: When Qc is less than Kc, the system must produce more products to reach equilibrium, unlike Qc > Kc which shifts backward.

  4. Question 4

    Q4. A student compares the effect of heating on the Haber process (exothermic) and the formation of NO (endothermic). The equilibrium constant Kc behaves differently because:

    • A) Kc increases for both types of reactions
    • B) Kc decreases for both types of reactions
    • C) Kc remains constant regardless of temperature
    • D) Kc decreases for exothermic and increases for endothermic reactions

    Answer: Kc decreases for exothermic and increases for endothermic reactions

    Explanation: For exothermic reactions, increasing temperature decreases Kc; for endothermic reactions, Kc increases with temperature, showing opposite thermodynamic dependencies.

  5. Question 5

    Q5. The decomposition of calcium carbonate (CaCO3 ⇌ CaO + CO2) is a heterogeneous equilibrium. This differs from the homogeneous N2O4 ⇌ 2NO2 system because:

    • A) The concentration of CaCO3 is doubled in the expression
    • B) Pure solids like CaCO3 are omitted from the Kc expression
    • C) The pressure of solids affects the equilibrium position
    • D) Only the liquid phase is considered in the calculation

    Answer: Pure solids like CaCO3 are omitted from the Kc expression

    Explanation: In heterogeneous equilibria, the concentration of pure solids is taken as unity, unlike homogeneous systems where all species appear in Kc.

  6. Question 6

    Q6. Increasing the pressure of a reaction vessel shifts the equilibrium of N2 + 3H2 ⇌ 2NH3 but has no effect on H2 + I2 ⇌ 2HI because:

    • A) The HI reaction involves no change in the number of gaseous moles
    • B) The HI reaction is faster than the ammonia synthesis
    • C) Ammonia synthesis is an endothermic process
    • D) Hydrogen acts as a catalyst in the HI reaction

    Answer: The HI reaction involves no change in the number of gaseous moles

    Explanation: Pressure shifts equilibrium toward the side with fewer moles; reactions with equal moles on both sides remain unaffected by pressure changes.

  7. Question 7

    Q7. A researcher compares two acetate buffers: one with [Salt]/[Acid] = 1 and another with [Salt]/[Acid] = 10. The first solution is superior because:

    • A) Maximum buffer capacity is achieved
    • B) The solution becomes a neutral mixture
    • C) The buffer range is at its minimum
    • D) The pH becomes independent of the pKa value

    Answer: Maximum buffer capacity is achieved

    Explanation: Buffer capacity is highest when [Salt] equals [Acid], providing maximum resistance to pH change, whereas buffer range is the pH interval.

  8. Question 8

    Q8. When mixing AgNO3 and NaCl, a white precipitate forms only if the ionic product (Qsp) exceeds the Ksp. This contrast is used to determine:

    • A) The solution is saturated and no precipitate forms
    • B) The solution is unsaturated and precipitate forms
    • C) The solution is supersaturated and precipitate forms
    • D) The ionic product is exactly equal to the solubility product

    Answer: The solution is supersaturated and precipitate forms

    Explanation: Precipitation only occurs when the ionic product exceeds Ksp; if it is lower, the solution remains unsaturated and clear.

  9. Question 9

    Q9. Comparing 0.1 M HCl and 0.1 M CH3COOH, the HCl solution has a much lower pH. This difference in acidity is because:

    • A) HCl has a lower Ka value than acetic acid
    • B) HCl ionizes completely while acetic acid ionizes partially
    • C) Acetic acid produces more H+ ions than HCl
    • D) HCl forms a buffer while acetic acid does not

    Answer: HCl ionizes completely while acetic acid ionizes partially

    Explanation: Strong acids like HCl ionize completely (100%), whereas weak acids like CH3COOH only partially ionize in aqueous solutions.

  10. Question 10

    Q10. In a comparison of a weak acid and its conjugate base, the relationship between Ka and Kb is defined by which constant fact?

    • A) The product of Ka and Kb equals the ionic product of water
    • B) The sum of Ka and Kb equals 14
    • C) Ka is always greater than Kb for any salt
    • D) They are independent of the temperature of the solution

    Answer: The product of Ka and Kb equals the ionic product of water

    Explanation: The product of Ka and its conjugate base's Kb always equals Kw (10^-14), showing an inverse relationship between their strengths.

  11. Question 11

    Q11. An acidic buffer consists of CH3COOH and CH3COONa. In contrast, a basic buffer like NH4OH/NH4Cl must contain:

    • A) A strong acid and its salt with a weak base
    • B) A weak acid and a strong base
    • C) A weak base and its salt with a weak acid
    • D) A weak base and its salt with a strong acid

    Answer: A weak base and its salt with a strong acid

    Explanation: Acidic buffers use weak acids and their salts, while basic buffers require a weak base and its salt for pH stabilization.

  12. Question 12

    Q12. In the industrial Contact Process, V2O5 is used. Unlike a change in temperature, the addition of this catalyst ensures that:

    • A) The catalyst increases the yield of products
    • B) The catalyst increases the value of Kc
    • C) The catalyst only decreases the time to reach equilibrium
    • D) The catalyst shifts the equilibrium to the right

    Answer: The catalyst only decreases the time to reach equilibrium

    Explanation: A catalyst increases the rate of reaching equilibrium but does not change the Kc value, which is solely temperature-dependent.

  13. Question 13

    Q13. A student compares the addition of NaCl to CH3COOH (common ion effect) and the dissolution of Na2CO3 in water (hydrolysis). These differ because:

    • A) Both processes increase the solubility of the salt
    • B) Common ion effect suppresses ionization while hydrolysis involves water reaction
    • C) Both processes always result in a neutral solution
    • D) Hydrolysis occurs only in saturated solutions of AgCl

    Answer: Common ion effect suppresses ionization while hydrolysis involves water reaction

    Explanation: Common ion effect suppresses ionization of weak electrolytes, while salt hydrolysis involves ions reacting with water to change the pH.

  14. Question 14

    Q14. When comparing the physiological pH of human blood and gastric juice, the primary difference is that:

    • A) Blood is slightly basic while gastric juice is highly acidic
    • B) Blood is highly acidic while gastric juice is neutral
    • C) Both fluids have the same pH of 7.4
    • D) Blood pH is maintained by HCl while gastric juice uses buffers

    Answer: Blood is slightly basic while gastric juice is highly acidic

    Explanation: Human blood is slightly basic (pH 7.35-7.4) and highly buffered, whereas gastric juice is highly acidic (pH 1-2) for digestion.

  15. Question 15

    Q15. The Henderson-Hasselbalch equation for an acidic buffer differs from that of a basic buffer because the basic version:

    • A) Both use pKa to calculate the final pH
    • B) Acidic buffers use the ratio of [Base]/[Salt]
    • C) Basic buffers use pKb to calculate pOH first
    • D) The log term is subtracted in the acidic buffer equation

    Answer: Basic buffers use pKb to calculate pOH first

    Explanation: The Henderson equation for acidic buffers uses pKa and [Salt]/[Acid], whereas the basic version uses pOH, pKb, and [Salt]/[Base].

  16. Question 16

    Q16. If a saturated solution of AgCl is prepared in 0.1 M NaCl instead of pure water, the solubility of AgCl changes because:

    • A) Solubility increases due to the formation of a complex
    • B) Solubility remains the same as Ksp is constant
    • C) Solubility increases because NaCl is a strong electrolyte
    • D) Solubility decreases due to the common ion effect

    Answer: Solubility decreases due to the common ion effect

    Explanation: The presence of Cl- ions from NaCl shifts the AgCl equilibrium to the left, reducing its solubility compared to pure water.

  17. Question 17

    Q17. In a gaseous equilibrium like PCl5 ⇌ PCl3 + Cl2, reducing the volume of the container at constant temperature differs from adding an inert gas because:

    • A) The system shifts to the side with more moles
    • B) The concentration of all gaseous species initially increases
    • C) The value of the equilibrium constant Kc increases
    • D) The reaction rate of the forward reaction decreases

    Answer: The concentration of all gaseous species initially increases

    Explanation: Reducing volume increases the concentration of all species; the system shifts to the side with fewer moles to relieve the pressure.

  18. Question 18

    Q18. The equilibrium constant Kc for the reaction PCl5 ⇌ PCl3 + Cl2 has specific units. These units differ from the H2 + I2 ⇌ 2HI reaction because:

    • A) Kc has no units for this reaction
    • B) The units are mol^2/dm^6
    • C) The units are mol/dm^3
    • D) The units are dm^3/mol

    Answer: The units are mol/dm^3

    Explanation: Units of Kc depend on the difference in moles (Δn); for PCl5 dissociation, Δn is 1, resulting in mol/dm3, unlike Δn=0 reactions.

  19. Question 19

    Q19. In the titration of CH3COOH with NaOH, phenolphthalein is a better indicator than methyl orange. This is because:

    • A) The equivalence point pH is greater than 7
    • B) The equivalence point pH is exactly 7
    • C) Methyl orange is only used for organic acids
    • D) Phenolphthalein reacts chemically with the weak acid

    Answer: The equivalence point pH is greater than 7

    Explanation: Phenolphthalein changes color in the basic range (8.3-10), making it suitable for weak acid-strong base titrations where the equivalence point is basic.

  20. Question 20

    Q20. A student measures the ionic product of water (Kw) at 25°C and 100°C. The value at 100°C is different because:

    • A) Kw remains 10^-14 at all temperatures
    • B) Kw increases as temperature increases
    • C) Kw decreases as temperature increases
    • D) The pH of neutral water remains 7 at 100°C

    Answer: Kw increases as temperature increases

    Explanation: The auto-ionization of water is endothermic, so Kw increases from 10^-14 at 25°C to approximately 10^-12 at 100°C.

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Level 1

A chemist observes a reaction where products never reach 100% yield despite infinite time. This behavior distinguishes it from an irreversible reaction because: