Chapter Three · Law of mass action & Le Chatelier's principle: practice questions

A reversible reaction has an equilibrium constant Kc = 1 × 10⁻⁵ at a certain temperature. This value indicates that:

• Athe forward reaction is almost complete.
• Bthe reaction is not a reversible reaction.
• Cproducts and reactants are present in equal concentrations.
• Dthe backward reaction is predominant and reactants prevail at equilibrium.

A suitable catalyst is added to a gaseous reaction that has already reached equilibrium. Which of the following statements is correct?

• AThe position of equilibrium is unchanged, but equilibrium is reached in a shorter time.
• BThe equilibrium shifts in the backward direction, increasing the amount of reactant.
• CThe value of the equilibrium constant increases.
• DThe equilibrium shifts in the forward direction, increasing the amount of product.

What happens when a strip of magnesium metal is placed in hydrochloric acid solution?

• AA white precipitate of silver chloride is formed
• BHydrogen gas evolved
• CEthyl acetate ester and water are formed
• DVapour pressure equals saturated water vapour pressure

For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which of the following correctly represents Kp in terms of partial pressures?

• AKp = p(SO₃)² / (p(SO₂)² × p(O₂))
• BKp = p(SO₃) / (p(SO₂) × p(O₂))
• CKp = p(SO₃)² / (p(SO₂) × p(O₂)²)
• DKp = (p(SO₂)² × p(O₂)) / p(SO₃)²

For the esterification equilibrium CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l), which of the following correctly represents the equilibrium constant Kc?

• AKc = [CH₃COOC₂H₅] / ([CH₃COOH][C₂H₅OH])
• BKc = ([CH₃COOC₂H₅][H₂O]) / [CH₃COOH]
• CKc = ([CH₃COOC₂H₅][H₂O]) / ([CH₃COOH][C₂H₅OH])
• DKc = [CH₃COOH][C₂H₅OH] / ([CH₃COOC₂H₅][H₂O])

In the equilibrium reaction between iron(III) chloride and ammonium thiocyanate, what is the colour observed upon the formation of iron(III) thiocyanate?

• ABlood red
• BPale yellow
• CColourless
• DReddish brown

Which of the following is cited as an example of a complete (irreversible) reaction?

• AReaction between acetic acid and ethyl alcohol
• BEvaporation and condensation of water in a closed vessel
• CAddition of sodium chloride solution to a silver nitrate solution
• DDissolution of ester and water in a closed container

In the esterification reaction between acetic acid and ethyl alcohol, what is observed when testing the reaction solution using blue litmus paper?

• AIt remains blue because the products have a neutral effect
• BIt turns red in spite of the produced materials having neutral effect on litmus
• CIt turns colourless because the reaction goes to completion
• DIt turns green due to the formation of ethyl acetate

For the reaction H2(g) + I2(g) ⇌ 2HI(g), 2 mol of H2 and 2 mol of I2 are placed in a 1 L vessel. At equilibrium, 3 mol of HI are present. What is the value of Kc?

• A36
• B12
• C18
• D9

According to the Law of Mass Action, if an equilibrium is established for the reaction FeCl3(aq) + 3NH₄SCN(aq) ⇌ Fe(SCN)₃(aq) + 3NH₄Cl(aq), how is the equilibrium constant Kc mathematically expressed in terms of rate constants and concentrations?

• AKc = (k₂)/(k₁) = ([FeCl₃][NH₄SCN]³)/([Fe(SCN)₃][NH₄Cl]³)
• BKc = (k₁)/(k₂) = ([Fe(SCN)₃] [NH₄Cl]³)/([FeCl₃] [NH₄SCN]³)
• CKc = (k₁)/(k₂) = ([FeCl₃] [NH₄SCN])/([Fe(SCN)₃] [NH₄Cl])
• DKc = k₁ × k₂ = [Fe(SCN)₃][NH₄Cl]³ [FeCl₃][NH₄SCN]³