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Worksheet: Le Chatelier's Principle

Q1:

Solutes A and B react to produce solutes C and D in a reversible reaction, as shown by the equation. 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?

  • A The position of equilibrium moves to the left to raise [ C ] .
  • B The position of equilibrium moves to the right to lower [ C ] .
  • C The position of equilibrium moves to the right to raise [ C ] .
  • D The position of equilibrium moves to the left to lower [ C ] .
  • E The position of equilibrium is unchanged as the reaction has no effect on the number of solute molecules.

Which equilibrium concentrations increase when compound C is added?

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

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

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

Q2:

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

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

Q3:

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

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

Q4:

Methane reacts with steam in the presence of a catalyst to produce carbon monoxide and hydrogen gases, as shown in the equation:

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?

  • ADecreasing both temperature and pressure
  • BIncreasing both temperature and pressure
  • CIncreasing temperature at constant pressure
  • DIncreasing temperature and decreasing pressure
  • EDecreasing pressure at constant temperature

Which of the following graphs best represents the change in equilibrium yield as the reaction temperature decreases below 1 0 0 C at 1 atm pressure?

  • A
  • B
  • C
  • D
  • E

Q5:

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

  • AThe amount of solid M g ( O H ) 2 increases.
  • BThe addition of solid M g ( O H ) 2 has no effect on the equilibrium.
  • C [ M g ] + stays constant.
  • D [ O H ] increases.
  • EThe amount of H O 2 stays constant.

Q6:

A suspension contains solid M g ( O H ) 2 in equilibrium with dissolved M g 2 + and O H ions. If solid M g C l 2 is added to the suspension, which of the following effects is not observed?

  • AThe amount of solid M g ( O H ) 2 increases.
  • B [ M g ] 2 + increases.
  • C [ C l ] increases.
  • D [ O H ] increases.
  • EThe pH decreases.

Q7:

Gaseous phosphorus pentachloride ( P C l ) 5 decomposes into phosphorus trichloride ( P C l ) 3 and chlorine gases, as shown in the equation:

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

Q8:

The colorless gas dinitrogen tetroxide ( N O ) 2 4 decomposes reversibly into the brown gas nitrogen dioxide ( N O ) 2 with a reaction enthalpy of +57.2 kJ/mol: 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 syringe contains gas molecules with a larger average size.
  • BThe syringe contains a larger mass of gas molecules.
  • CThe temperature of the gas in the syringe is higher.
  • DThe syringe contains a larger number of gas molecules.
  • EThe pressure of the gas in the syringe is higher.

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