Worksheet: Le Chatelier's Principle

In this worksheet, we will practice using energy changes and stoichiometry to predict the effects of temperature and pressure on equilibrium yields.

Q1:

A suspension contains solid Mg(OH)2 in equilibrium with dissolved Mg2+ and OH ions. If solid KOH is added to the suspension, which of the following effects is not observed?

  • A [ M g ] 2 + decreases.
  • B [ O H ] decreases.
  • CThe pH increases.
  • D [ K ] + increases.
  • EThe amount of solid Mg(OH)2 increases.

Q2:

A suspension contains solid Mg(OH)2 in equilibrium with dissolved Mg2+ and OH ions. If solid MgCl2 is added to the suspension, which of the following effects is not observed?

  • A [ O H ] increases.
  • BThe pH decreases.
  • C [ C l ] increases.
  • D [ M g ] 2 + increases.
  • EThe amount of solid Mg(OH)2 increases.

Q3:

A suspension contains solid NiCO3 in equilibrium with dissolved [Ni]2+ and CO32 ions. If solid KCO23 is added to the suspension, which of the following effects is not observed?

  • A [ K ] + increases.
  • B [ N i ] 2 + stays constant.
  • CThe pH increases.
  • DThe amount of solid NiCO3 increases.
  • E [ C O ] 3 2 increases.

Q4:

A suspension contains solid NiCO3 in equilibrium with Ni2+ and CO32 ions. If solid Ni(NO)32 is added to the suspension, which of the following effects is not observed?

  • A [ C O ] 3 2 decreases.
  • BThe amount of solid NiCO3 increases.
  • C [ N O ] 3 increases.
  • D [ N i ] 2 + increases.
  • EThe solubility of solid NiCO3 increases.

Q5:

A suspension contains solid Mg(OH)2 in equilibrium with dissolved Mg2+ and OH ions. If more solid Mg(OH)2 is added to the suspension, which of the following effects is not observed?

  • A [ O H ] increases.
  • B [ M g ] + stays constant.
  • CThe addition of solid Mg(OH)2 has no effect on the equilibrium.
  • DThe amount of solid Mg(OH)2 increases.
  • EThe amount of HO2 stays constant.

Q6:

A suspension contains solid Mg(OH)2 in equilibrium with dissolved Mg2+ and OH ions. If an acid is added to the suspension, which of the following effects is not observed?

  • A [ H ] + increases.
  • B [ M g ] 2 + remains constant.
  • CThe amount of solid Mg(OH)2 decreases.
  • D [ O H ] decreases.
  • EThe amount of HO2 increases.

Q7:

A suspension contains solid AgCrO24 in equilibrium with a saturated solution of Ag+ and CrO42 ions. If 99 g of solid AgCrO24 is added to the suspension, how are the concentrations of Ag+ and CrO42 affected?

  • A [ A g ] + increases by a factor of 50 and [CrO]42 increases by a factor of 100.
  • B [ A g ] + increases by a factor of 100 and [CrO]42 increases by a factor of 50.
  • C [ A g ] + and [CrO]42 both remain constant.
  • D [ A g ] + increases by a factor of 200 and [CrO]42 increases by a factor of 100.
  • E [ A g ] + and [CrO]42 both increase by a factor of 100.

Q8:

Solutes A and B react to produce solutes C and D in a reversible reaction, as shown by the equation. A+BC+D In a solution at equilibrium, the concentrations of A, B, C, and D are [A], [B], [C], and [D] respectively. When a small quantity of compound C is added to this solution, the composition of the solution changes according to Le Chatelier’s principle.

According to Le Chatelier’s principle, how and why does the position of equilibrium change?

  • AThe position of equilibrium moves to the right to lower [C].
  • BThe position of equilibrium moves to the left to raise [C].
  • CThe position of equilibrium is unchanged as the reaction has no effect on the number of solute molecules.
  • DThe position of equilibrium moves to the left to lower [C].
  • EThe position of equilibrium moves to the right to raise [C].

Which equilibrium concentrations increase when compound C is added?

  • A [ A ] , [ B ] , [ C ] , and [D]
  • B [ A ] , [ B ] , and [C] only
  • C [ A ] and [B] only
  • D [ C ] only
  • E [ C ] and [D] only

Which of the following ratios of equilibrium concentrations does not change when compound C is added?

  • A [ A ] [ B ]
  • B [ B ] [ D ]
  • C [ A ] [ C ]
  • D [ C ] [ D ]

Q9:

In which of the following reactions would the addition of hydrochloric acid decrease the quantity of solid material?

  • A [ C r ( H O ) ( O H ) ] ( ) + 3 O H ( ) [ C r ( O H ) ] ( ) + 3 H O ( ) 2 3 3 6 3 2 s a q a q l
  • B N H C l ( ) N H ( ) + H C l ( ) 4 3 s a q a q
  • C A g N O ( ) + C l ( ) A g C l ( ) + N O ( ) 3 3 a q a q s a q
  • D [ C u ( H O ) ] ( ) + 4 C l ( ) [ C u C l ] ( ) + 6 H O ( ) 2 6 2 + 4 2 2 a q a q a q a q
  • E B i C l ( ) + H O ( ) B i O C l ( ) + 2 H C l ( ) 3 2 a q l s a q

Q10:

In which of the following gas-phase reactions would the equilibrium yield increase with increasing pressure?

  • A S C l + C l 2 S C l 2 2 2 2
  • B O F + S O S O + F 2 2 3 2
  • C P C l P C l + C l 5 3 2
  • D 2 N O + C H O H C H O N O + H N O 2 3 3 3
  • E C H + H O C O + 3 H 4 2 2

Q11:

Methane reacts with steam in the presence of a catalyst to produce carbon monoxide and hydrogen gases, as shown in the equation: CH+HOCO+3H422.

The enthalpy change for this reaction is +206 kJ/mol.

Which of the following changes would produce the greatest increase in the yield of hydrogen?

  • AIncreasing temperature at constant pressure
  • BDecreasing both temperature and pressure
  • CIncreasing temperature and decreasing pressure
  • DIncreasing both temperature and pressure
  • EDecreasing pressure at constant temperature

Which of the following graphs best represents the change in equilibrium yield as the reaction temperature decreases below 100C at 1 atm pressure?

  • A
  • B
  • C
  • D
  • E

Q12:

Gaseous phosphorus pentachloride (PCl)5 decomposes into phosphorus trichloride (PCl)3 and chlorine gases, as shown in the equation: PClPCl+Cl532.

The enthalpy change for this reaction is +87.9 kJ/mol. The rates of the forward and backward reactions are shown schematically in the diagram:

Which diagram represents the rates of the forward and backward reactions when the temperature is increased at constant pressure?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the rates of the forward and backward reactions when the pressure is decreased at constant temperature?

  • A
  • B
  • C
  • D
  • E

Q13:

The colorless gas dinitrogen tetroxide (NO)24 decomposes reversibly into the brown gas nitrogen dioxide (NO)2 with a reaction enthalpy of +57.2 kJ/mol: NO2NO242. A student stores a sample of dinitrogen tetroxide in a gas syringe at room temperature and 1 atm pressure and allows the plunger of the syringe to move freely. The gas is left to decompose until equilibrium is reached, as shown.

Why is the position of the plunger at equilibrium different from its position at the start of the reaction?

  • AThe pressure of the gas in the syringe is higher.
  • BThe syringe contains gas molecules with a larger average size.
  • CThe temperature of the gas in the syringe is higher.
  • DThe syringe contains a larger number of gas molecules.
  • EThe syringe contains a larger mass of gas molecules.

The student heats the gas syringe and allows the plunger of the syringe to move freely. Which diagram best illustrates the gas syringe under the new reaction conditions once equilibrium has been reached?

  • A
  • B