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 MgCl2 is added to the suspension, which of the following effects is not observed?

  • A[OH] increases.
  • BThe pH decreases.
  • C[Cl] increases.
  • D[Mg]2+ increases.
  • EThe amount of solid Mg(OH)2 increases.

Q2:

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[Ni]2+ stays constant.
  • CThe pH increases.
  • DThe amount of solid NiCO3 increases.
  • E[CO]32 increases.

Q3:

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[CO]32 decreases.
  • BThe amount of solid NiCO3 increases.
  • C[NO]3 increases.
  • D[Ni]2+ increases.
  • EThe solubility of solid NiCO3 increases.

Q4:

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[OH] increases.
  • B[Mg]+ 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.

Q5:

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[Mg]2+ remains constant.
  • CThe amount of solid Mg(OH)2 decreases.
  • D[OH] decreases.
  • EThe amount of HO2 increases.

Q6:

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

Q7:

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]

Q8:

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

  • A[Cr(HO)(OH)]()+3OH()[Cr(OH)]()+3HO()233632saqaql
  • BNHCl()NH()+HCl()43saqaq
  • CAgNO()+Cl()AgCl()+NO()33aqaqsaq
  • D[Cu(HO)]()+4Cl()[CuCl]()+6HO()262+422aqaqaqaq
  • EBiCl()+HO()BiOCl()+2HCl()32aqlsaq

Q9:

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

  • ASCl+Cl2SCl2222
  • BOF+SOSO+F2232
  • CPClPCl+Cl532
  • D2NO+CHOHCHONO+HNO2333
  • ECH+HOCO+3H422

Q10:

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

Q11:

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

Q12:

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
  • C
  • D
  • E

The student compresses the gas by pushing the plunger and allows the reaction to reach equilibrium at room temperature, with the plunger fixed in its new position. Which diagram best illustrates the gas syringe under the new reaction conditions once equilibrium has been reached?

  • A
  • B
  • C
  • D
  • E

Q13:

Gaseous sulfur dioxide reacts with oxygen to produce sulfur trioxide gas, as shown in the equation: 2SO+O2SO223. The enthalpy change for this reaction is 198 kJ/mol. Five sets of reaction conditions are listed in the table.

ConditionsTemperature (C)Pressure (atm)Catalyst
A4002No
B4001Yes
C3002No
D4002Yes
E3001Yes

Which set of reaction conditions produces the highest yield at equilibrium?

  • AA
  • BD
  • CC
  • DB
  • EE

Which two sets of reaction conditions produce similar yields at equilibrium?

  • AB and D
  • BC and E
  • CA and C
  • DA and D
  • EB and E

Which set of reaction conditions produces the highest reaction rate?

  • AE
  • BB
  • CA
  • DC
  • ED

Q14:

In which of the following gas-phase reactions could the equilibrium yield be increased by increasing temperature or decreasing pressure?

  • ANO2NO,+57.2kJ/mol242Δ𝐻=
  • BCl+OClO+O,-29.9kJ/mol32Δ𝐻=
  • CCO+HOCO+H,-41.0kJ/mol222Δ𝐻=
  • D2SClSCl+Cl,+40.6kJ/mol2222Δ𝐻=
  • E2SO+O2SO,-196kJ/mol223Δ𝐻=

Q15:

Gases A and B react exothermically to produce gas C and solid D, as shown by the equation: A()+B()C()+2D()gggs. Which of the following changes to the reaction conditions would most increase the equilibrium yield of C?

  • AAdding a catalyst and increasing the temperature
  • BAdding a catalyst and increasing the pressure
  • CDecreasing both the temperature and the pressure
  • DIncreasing the pressure and decreasing the temperature
  • EDecreasing the pressure and increasing the temperature

Q16:

Gaseous hydrogen and iodine react reversibly to produce hydrogen iodide gas, as shown in the equation: H+I2HI22.

The enthalpy change in this reaction is 9.48 kJ/mol. Which of the following statements correctly describes the effect of temperature or pressure on the position of equilibrium?

  • AHeating shifts the position of equilibrium to the right.
  • BDecreasing pressure has no effect on the equilibrium yield of hydrogen iodide.
  • CCooling and removing iodine both increase the equilibrium yield of hydrogen iodide.
  • DIncreasing pressure strongly favors the backward reaction.
  • ERemoving hydrogen iodide strongly favors the backward reaction.

Q17:

In which of the following gas-phase reactions could the equilibrium yield be increased by increasing either temperature or pressure?

  • ANO+ONO+O,-200kJ/mol322Δ𝐻=
  • BN+2O2NO,+66.4kJ/mol222Δ𝐻=
  • CN+O2NO,+181kJ/mol22Δ𝐻=
  • DN+3H2NH,-92.2kJ/mol223Δ𝐻=
  • ENO4NO+O,+110kJ/mol2522Δ𝐻=

Q18:

Molecules A and B react exothermically and reversibly to produce a third molecule C, as shown by the equation.

A box contains five molecules each of A, B, and C at equilibrium, as shown. The temperature inside the box is 25C, and the pressure is 1 bar.

The temperature of the box changes to 50C, and the box expands to maintain a pressure of 1 bar. Which of the following diagrams represents the box at equilibrium under these conditions?

  • A
  • B
  • C
  • D
  • E

The volume of the box increases at a fixed temperature of 25C. Which of the following diagrams represents the box at equilibrium under these conditions?

  • A
  • B
  • C
  • D
  • E

At a fixed temperature of 25C, the concentration of B is increased and the box changes in volume to maintain a pressure of 1 bar. Which of the following diagrams represents the box at equilibrium under these conditions?

  • A
  • B
  • C
  • D
  • E

At a fixed temperature of 25C, the concentration of C is decreased and the box changes in volume to maintain a pressure of 1 bar. Which of the following diagrams represents the box at equilibrium under these conditions?

  • A
  • B
  • C
  • D
  • E

Q19:

Chloromethane reacts with sodium iodide to produce iodomethane and sodium chloride, as shown by the equation.

The molar solubilities of sodium chloride and sodium iodide in five organic solvents at 20C are shown in the table.

SolventSolubility of NaCl (M)Solubility of NaI (M)
Dimethylformamide6.8×100.96
Formamide1.63.8
Acetone5.5×102.0
Methanol0.244.9
Ethanol1.1×103.1

Assuming that chloromethane and iodomethane are highly soluble in all the solvents listed, which solvent would produce the greatest percentage yield of iodomethane at equilibrium?

  • AEthanol
  • BMethanol
  • CAcetone
  • DFormamide
  • EDimethylformamide

Q20:

Under certain conditions, nitric oxide (NO) and nitrogen dioxide (NO)2 gases react reversibly to produce liquid dinitrogen trioxide (NO)23, as shown in the equation. NO()+NO()NO()ggl223 The enthalpy change for this reaction is 41 kJ/mol. Below the boiling point of dinitrogen trioxide, which of the following graphs correctly illustrates the effects of temperature (𝑇) and pressure on the equilibrium yield of this reaction?

  • A
  • B
  • C
  • D

Q21:

In the Haber process, nitrogen and hydrogen gases react exothermically to produce gaseous ammonia, as shown by the given equation: N+3H2NH223.

The reaction mixture is transferred between two vessels, A and B, at different temperatures. The arrows in the diagram indicate the addition of gases to vessel A, the transfer of gases between the two vessels, and the removal of a liquid compound X from reactor B.

Identify compound X.

  • AH2
  • BCO2
  • CN2
  • DNH3
  • EHO2

Why is the pressure in vessel A set to a value higher than 1 atm?

  • ABecause the ammonia yield is increased, although the reaction rate is reduced
  • BBecause the reaction rate is increased, although the ammonia yield is reduced
  • CBecause the reaction rate is increased and the ammonia yield is approximately the same
  • DBecause both the reaction rate and ammonia yield are increased
  • EBecause a higher pressure allows the product to be isolated more easily

For which of the following reasons does the choice of conditions in vessel B drive the position of equilibrium to the right?

  • AIncreasing the pressure favors the forward reaction.
  • BCondensation of compound X favors the forward reaction.
  • CIncreasing the temperature favors the endothermic process.
  • DLowering the pressure favors the forward reaction.
  • ELowering the temperature favors the endothermic process.

Q22:

At equilibrium, a box contains five molecules each of gaseous compounds A, B, C and D. Changing the temperature, 𝑇, or pressure, 𝑃, of the system alters the composition of the gas mixture at equilibrium, as shown in the diagram.

Write a balanced chemical equation for the reaction taking place in the gas mixture.

  • A2AB+C
  • BA+2CD
  • CA+CD
  • DA+2CB+D
  • E2A+2CB

The temperature of the box is decreased and the system is allowed to reach equilibrium. Assuming the pressure remains at the original value, which of the following situations could be observed?

  • A
  • B
  • C
  • D
  • E

The pressure is increased and the system is allowed to reach equilibrium. Assuming the temperature remains at the original value, which of the following situations could be observed?

  • A
  • B
  • C
  • D
  • E

Q23:

Solid blue [Cr(HO)(OH)]233 reacts with excess hydroxide ions in water to produce aqueous [Cr(OH)]63 ions, as shown by the following equation [Cr(HO)(OH)]()+3OH()[Cr(OH)]()+3HO()233632saqaql. Solutions of [Cr(OH)]63 are blue green in color and darken with increasing concentration. All other aqueous ions in this reaction produce colorless solutions. A suspension X is produced by adding [Cr(HO)(OH)]233 to a test tube of aqueous sodium hydroxide. At equilibrium, suspension X contains both solid [Cr(HO)(OH)]233 and aqueous [Cr(OH)]63 ions, as illustrated in the diagram.

Which diagram represents the situation at equilibrium when [Cr(OH)]63 ions are added to suspension X?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the situation at equilibrium when the pH of suspension X is decreased?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the situation at equilibrium when [Cr(HO)(OH)]233 is added to suspension X?

  • A
  • B
  • C
  • D
  • E

The diagram illustrates the situation at equilibrium following one or more changes to suspension X. Which of the following changes could have resulted in the situation shown?