All Free Chemistry MCQs with Answers

31. Adding a catalyst to a system at equilibrium

• A. shifts the equilibrium towards the products
• B. shifts the equilibrium towards the reactants
• C. increases the value of Kc
• D. speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but is unchanged

Explanation: A catalyst lowers the activation energy of both directions by the same amount, so both rates rise in proportion and they still become equal at exactly the same position. What changes is the time taken to get there, which is commercially vital in a process such as the Haber synthesis. Only temperature changes the value of Kc itself.

32. In the Haber process for manufacturing ammonia, the catalyst used is

• A. finely divided iron with promoters
• B. platinum
• C. vanadium pentoxide
• D. nickel

Explanation: Iron with small amounts of potassium and aluminium oxide as promoters is used because it is effective and cheap enough for a process operating on this scale. Vanadium pentoxide is the catalyst of the Contact process for sulphuric acid and nickel is used in the hydrogenation of oils, both of which appear here as familiar but wrong answers. The gases are recycled repeatedly since a single pass converts only about fifteen per cent.

33. The conditions actually used in the Haber process are about

• A. 1000 degrees Celsius and 1 atmosphere
• B. 25 degrees Celsius and 1000 atmospheres
• C. 450 degrees Celsius and 200 atmospheres
• D. 450 degrees Celsius and 1 atmosphere

Explanation: A moderately high temperature is a compromise, since a low temperature would give a better yield but far too slowly for industry, while high pressure genuinely improves the yield and is limited only by the cost and strength of the plant. Ammonia is removed by liquefaction and the unreacted gases are recycled, so the overall conversion is high even though each pass is poor.

34. The solubility product Ksp of a sparingly soluble salt is

• A. the total mass of the salt dissolved
• B. the product of the concentrations of its ions in a saturated solution, each raised to the power of its coefficient
• C. the concentration of the undissolved solid
• D. the ratio of dissolved to undissolved salt

Explanation: For a saturated solution in contact with excess solid, an equilibrium exists between the solid and its ions, and Ksp is the equilibrium constant for that dissolving process. The concentration of the solid itself is constant and so is not included in the expression. If the ionic product exceeds Ksp a precipitate forms, which is the basis of qualitative analysis.

35. 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.