Free Chemical Equilibrium MCQs with Answers

11. For the sparingly soluble salt AgCl, the solubility product expression is

• A. Ksp = [Ag+] + [Cl-]
• B. Ksp = [AgCl] divided by [Ag+][Cl-]
• C. Ksp = [Ag+][Cl-] divided by [AgCl]
• D. Ksp = [Ag+][Cl-]

Explanation: Silver chloride dissociates into one silver ion and one chloride ion, so Ksp is simply the product of the two concentrations, and the solid does not appear because its concentration does not vary. If the solubility is s moles per cubic decimetre then both ion concentrations equal s and Ksp equals s squared. Concentrations are always multiplied in such expressions, never added.

12. The solubility of silver chloride in water decreases when sodium chloride is added. This is an example of

• A. the common ion effect
• B. the buffer action
• C. hydrolysis
• D. the catalytic effect

Explanation: Sodium chloride supplies extra chloride ions, and since the ionic product must not exceed Ksp the equilibrium shifts towards the undissolved solid, so less silver chloride can dissolve. This is Le Chatelier's principle applied to a solubility equilibrium and is used deliberately to wash a precipitate without dissolving it. Adding an ion not present in the salt would not have this effect.

13. The ionisation of a weak acid such as acetic acid is suppressed by the addition of sodium acetate because

• A. sodium acetate is a strong acid
• B. the added acetate ions shift the ionisation equilibrium back towards the un-ionised acid
• C. sodium ions react with the acid
• D. the temperature of the solution falls

Explanation: Sodium acetate is fully ionised and floods the solution with acetate, the common ion, so by Le Chatelier's principle the acid dissociates less and the hydrogen ion concentration falls. This mixture of a weak acid with its salt is exactly what makes an acidic buffer work. Sodium ions are spectators and take no part in the equilibrium.

14. A buffer solution is one that

• A. has a pH of exactly 7
• B. neutralises all acids completely
• C. resists a change in pH when a small amount of acid or base is added
• D. conducts electricity better than water

Explanation: A buffer contains a reservoir of both a weak acid and its conjugate base, so added hydrogen ions are mopped up by the base and added hydroxide ions by the acid, and the pH barely moves. A buffer can be acidic or basic; a pH of 7 is not part of the definition. Blood is buffered near pH 7.4 mainly by the carbonic acid and hydrogen carbonate system.

15. Which mixture would act as an acidic buffer?

• A. Hydrochloric acid and sodium chloride
• B. Sodium hydroxide and sodium chloride
• C. Ammonia and ammonium chloride
• D. Acetic acid and sodium acetate

Explanation: An acidic buffer needs a weak acid together with the salt of that acid, and acetic acid with sodium acetate is the standard example. Ammonia with ammonium chloride is also a buffer but a basic one, since it is built from a weak base and its salt, which is why it is the closest distractor. Hydrochloric acid is strong and fully ionised, so it has no reservoir of un-ionised acid and cannot buffer.

16. In a reversible reaction, the equilibrium constant Kc changes only when there is a change in

• A. concentration of the reactants
• B. pressure of the system
• C. temperature
• D. the catalyst used

Explanation: Concentration and pressure changes shift the position of equilibrium so that the same value of Kc is restored, and a catalyst affects only the rate, so none of them alters the constant. Temperature is different because it changes the relative rates of the forward and reverse reactions unequally, giving a genuinely new value of Kc. For an exothermic reaction Kc falls as the temperature rises.

17. For an endothermic reaction at equilibrium, raising the temperature will

• A. shift the equilibrium towards the products and increase Kc
• B. shift the equilibrium towards the reactants
• C. have no effect
• D. decrease the rate of both reactions

Explanation: Heat can be treated as a reactant in an endothermic reaction, so supplying more of it drives the system forward to absorb the extra energy, which is Le Chatelier's principle applied to temperature. Because the position genuinely moves rather than being restored, the value of Kc increases as well. For an exothermic reaction the same reasoning gives the opposite result.

18. A saturated solution of a sparingly soluble salt is one in which

• A. no more solid can be added to the container
• B. the dissolved ions are in equilibrium with the undissolved solid
• C. the salt has completely dissolved
• D. the solubility product has been exceeded

Explanation: In a saturated solution ions leave the solid surface and return to it at the same rate, so the concentration of dissolved ions stays constant and equals the value fixed by Ksp. If the ionic product were pushed beyond Ksp the excess would precipitate until equilibrium was restored. Undissolved solid must be present for the equilibrium to exist at all.

19. In the reaction A + B gives C + D at equilibrium, adding more of substance A will

• A. decrease the amount of C formed
• B. have no effect on the position of equilibrium
• C. shift the equilibrium to the right, forming more C and D
• D. change the value of Kc

Explanation: The system responds to the extra A by consuming some of it, which means running the forward reaction further and producing more products until the ratio required by Kc is restored. This is why one reactant is often used in excess industrially when it is the cheaper one. The value of Kc is unchanged, since only temperature can alter it.

20. The pH of a buffer solution made from a weak acid and its salt depends on

• A. the total volume of the solution
• B. the temperature only
• C. the presence of a catalyst
• D. the dissociation constant of the acid and the ratio of salt to acid concentration

Explanation: The Henderson Hasselbalch relationship gives pH as pKa plus the logarithm of the ratio of salt to acid, so it is the ratio that matters rather than the absolute amounts. Diluting the buffer changes both concentrations equally and so leaves the pH almost unaltered, which is one of its most useful features. Buffer capacity, however, does depend on how concentrated the components are.