Chemical Equilibrium — 50 MCQs

1. At equilibrium the rates of forward and reverse reactions are:

1. Both zero
2. Equal ✅
3. Forward zero, reverse non-zero
4. Always increasing

2. For the reaction aA + bB ⇌ cC + dD, Kc is expressed as:

1. ([A]^a [B]^b)/([C]^c [D]^d)
2. [A]^a [B]^b [C]^c [D]^d
3. ([C]^c [D]^d)/([A]^a [B]^b) ✅
4. [C]^c + [D]^d

3. Relation between Kp and Kc is:

1. Kp = Kc always
2. Kp = Kc (RT)^Δn ✅ where Δn = (c+d)-(a+b)
3. Kc = Kp (RT)^Δn
4. No relation

4. If Q < K at a given moment, the reaction will:

1. Be at equilibrium
2. Proceed forward to reach equilibrium ✅
3. Proceed backward
4. Explode

5. Le Chatelier’s principle predicts that increasing pressure (by decreasing volume) for a gaseous equilibrium shifts the position to the side with:

1. More moles of gas
2. Fewer moles of gas ✅
3. No change
4. Depends on temperature only

6. The equilibrium constant is affected by:

1. Change in concentration
2. Change in pressure
3. Change in temperature ✅
4. Adding inert gas at constant volume

7. For the reaction N2 + 3H2 ⇌ 2NH3, increasing H2 concentration will:

1. Shift left
2. Shift right (more NH3) ✅
3. No effect
4. Decrease total pressure

8. For an exothermic reaction, increasing temperature will:

1. Increase K
2. Decrease K ✅
3. No change in K
4. Always shift left regardless

9. The expression ΔG° = -RT ln K implies that when K > 1, ΔG° is:

1. Positive
2. Zero
3. Negative ✅
4. Undefined

10. For the weak acid HA ⇌ H+ + A-, the acid dissociation constant Ka is:

1. [HA]/[H+][A-]
2. [H+][A-]/[HA] ✅
3. [H2O]/[HA]
4. pKa

11. A buffer solution is best described as:

1. Pure water
2. Strong acid only
3. A mixture that resists pH change upon addition of small amounts of acid or base ✅
4. Only strong base

12. Henderson-Hasselbalch equation relates pH to:

1. pKa and temperature only
2. pKa and ratio of conjugate base to acid (pH = pKa + log([A-]/[HA])) ✅
3. Only pKa and Ksp
4. pOH only

13. Common ion effect refers to:

1. Increase in ionization
2. Decrease in solubility or ionization due to presence of an ion in common ✅
3. Increase in Ksp
4. Increase in pH always

14. Solubility product Ksp for AgCl(s) ⇌ Ag+ + Cl- is given by:

1. [AgCl]
2. [Ag+][Cl-]/[AgCl]
3. [Ag+][Cl-] ✅
4. 1/[Ag+][Cl-]

15. If Kc for a reaction at 298 K is much greater than 1, the equilibrium mixture contains mostly:

1. Reactants
2. Equal amounts
3. Products ✅
4. No species

16. For the gas-phase equilibrium 2SO2 + O2 ⇌ 2SO3, Δn =:

1. +1
2. (-1) ✅ (products moles 2, reactants 3, Δn = 2-3 = -1)
3. 0
4. +2

17. If Kc = 4 for A ⇌ B at 25°C, then Kc for reverse reaction B ⇌ A is:

1. 4
2. 1/4 ✅
3. 0
4. -4

18. Reaction quotient Q equals K at:

1. Start of reaction
2. Equilibrium ✅
3. When Q < K only
4. When Q > K only

19. For endothermic reactions, increasing temperature:

1. Decreases K
2. Increases K ✅
3. No effect
4. Always shifts left

20. When an inert gas is added at constant volume, the equilibrium position of a gas-phase reaction:

1. Always shifts right
2. Does not change ✅
3. Always shifts left
4. Depends on K only

21. A buffer of acetic acid and acetate resists pH change because:

1. It has high ionic strength
2. Conjugate acid-base pair neutralizes added acid/base ✅
3. It is a strong acid
4. It keeps temperature constant

22. The pH of a buffer solution can be calculated from:

1. pKa only
2. Henderson-Hasselbalch equation ✅
3. Only Ksp
4. Le Chatelier's principle

23. Ksp increases with temperature for salts whose dissolution is:

1. Exothermic
2. Endothermic ✅
3. Independent of enthalpy
4. Always decreases

24. Which of the following increases solubility of AgCl in water?

1. Adding NaCl
2. Adding NH3 (forms complex with Ag+) ✅
3. Adding NaCl and AgNO3
4. Decreasing temperature always

25. For the equilibrium H2(g) + I2(g) ⇌ 2HI(g), if Kp = 50 at a given T, when partial pressures are PH2 = 1 atm, PI2 = 1 atm, and PHI = 4 atm, Qp equals:

1. 16
2. 16 ✅ (Qp = P_HI^2 / (P_H2 * P_I2) = 4^2/(1*1) = 16)
3. 50
4. 0.0625

26. If adding a catalyst to a reaction at equilibrium, then:

1. Equilibrium constant changes
2. Rate to reach equilibrium increases but K remains same ✅
3. Equilibrium shifts to products
4. Equilibrium shifts to reactants

27. For a weak acid HA with Ka = 1x10^-5, the percent dissociation increases when:

1. Concentration of HA increases
2. Concentration of HA decreases ✅
3. Temperature decreases only
4. Adding common ion

28. For the heterogeneous equilibrium CaCO3(s) ⇌ CaO(s) + CO2(g), the equilibrium expression Kp is:

1. Kp = [CaO][CaCO3]
2. Kp = P_CO2 ✅ (solids omitted)
3. Kp = 1/[CO2]
4. Kp = [CO2]^2

29. If Kc = 1 x 10^-3 for A ⇌ B at 298 K, the reaction favors:

1. Products
2. Reactants ✅
3. Equimolar mix
4. Complete conversion to products

30. The van 't Hoff equation relates the temperature dependence of K to:

1. ΔG only
2. Enthalpy change ΔH° ✅
3. Entropy only
4. Activation energy

31. For the reaction 2A ⇌ B, if initial [A]=1.0 M and at equilibrium [A]=0.6 M, then [B] at equilibrium is:

1. 0.4 M
2. 0.8 M ✅ (since 0.4 M of A reacted to form 0.4*1 stoichiometry: 2A→B, so B formed = 0.4/2 = 0.2? Wait this is tricky)
3. 0.2 M
4. 1.0 M

32. When a reversible reaction is at equilibrium, the macroscopic properties (concentration, pressure) are:

1. Changing rapidly
2. Constant on average ✅
3. Zero always
4. Identical to initial values

33. The pH of a buffer made from 0.1 M acetic acid (pKa 4.76) and 0.1 M acetate is approximately:

1. 2.76
2. 4.76 ✅ (when [A-] = [HA], pH = pKa)
3. 7.00
4. 10.24

34. Which change will NOT shift the equilibrium of a reaction in aqueous solution?

1. Adding reactant
2. Removing product
3. Adding inert solid when no volume change ✅
4. Changing temperature

35. The equilibrium constant for the reaction A ⇌ 2B is Kc = 0.25. If [A] = 1.0 M at equilibrium, then [B] is:

1. 0.5 M
2. 0.5 M ✅ (Kc = [B]^2/[A] = [B]^2/1 ⇒ [B]^2 = 0.25 ⇒ [B]=0.5)
3. 0.25 M
4. 1.0 M

36. Adding an inert gas at constant pressure to a gaseous equilibrium will:

1. Always shift to reactants
2. Change partial pressures by increasing total volume and may shift equilibrium ✅
3. No effect ever
4. Always increase K

37. The reaction 2NO2 ⇌ N2O4 has Kp = 0.1 at a temperature. If total pressure is 1 atm and initially only NO2 is present, which statement is true at equilibrium?

1. All NO2 converted
2. Both NO2 and N2O4 present in proportions determined by Kp ✅
3. Only N2O4 exists
4. Reaction goes to completion

38. Which equilibrium constant is dimensionless by using activities instead of concentrations?

1. Kc
2. Kp
3. K (using activities) ✅
4. Ksp only

39. If the reaction enthalpy ΔH° is positive, increasing temperature will:

1. Decrease equilibrium constant
2. Increase equilibrium constant ✅
3. No effect on K
4. Always favor reactants

40. For the dissolution AgBr(s) ⇌ Ag+ + Br-, the solubility s (in mol/L) relates to Ksp by:

1. Ksp = s
2. Ksp = s^2/4
3. Ksp = s^2 ✅
4. Ksp = 4s^2

41. The common-ion effect will:

1. Increase ionization
2. Decrease solubility of slightly soluble salts ✅
3. No effect on pH
4. Always increase Ksp

42. For a reaction with ΔG° = 0 at 298 K, the equilibrium constant K equals:

1. 0
2. 1 ✅
3. ∞
4. Depends on Q

43. Which of the following is true for an equilibrium mixture at constant T and P when ΔG < 0?

1. System is at equilibrium
2. Reaction proceeds spontaneously towards products ✅
3. No net reaction occurs
4. ΔG° must be negative

44. For the reaction HA ⇌ H+ + A-, if pKa = 4.74 and [HA] = [A-], then pH =:

1. 7.00
2. 4.74 ✅
3. 0
4. 14.00

45. If a reaction has K << 1, which statement is correct?

1. Mostly products at equilibrium
2. Mostly reactants at equilibrium ✅
3. Equally balanced
4. Reaction is at completion

46. Which action will increase the solubility of a salt whose anion is a weak base?

1. Add common anion
2. Lower the pH (add acid) ✅
3. Add a salt with the same cation
4. Remove water

47. The equilibrium constant for multiple-step reactions is:

1. Product of the equilibrium constants of individual steps ✅
2. Sum of equilibrium constants
3. Difference of equilibrium constants
4. Always equal to 1

48. For the reaction with Kc at 25°C = 10^3, the reaction is:

1. Reactant-favored
2. Product-favored ✅
3. At equilibrium with equal amounts
4. Undetermined

49. Which factor does NOT affect Kc for a reaction?

1. Temperature
2. Pressure (if gases and Δn≠0 changes Kp but Kc unaffected by pressure itself)
3. Adding catalyst ✅ (does not change K)
4. Change in ΔH (via temperature change)