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CHEMISTRY, M5 EQ-Bank 5 MC

The conversion of calcium carbonate to calcium oxide and carbon dioxide is a reversible reaction and will reach equilibrium under certain conditions.

In which diagram is the system most likely to have reached equilibrium?
 

 

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`D`

Show Worked Solution
  • In order to reach equilibrium, the reaction must occur in a closed system. In this system, a lid must be on the container.
  • When the reaction  \(\ce{CaCO3(s) \rightleftharpoons CaO(s) + CO2(g)} \)  has reached equilibrium, both the forwards and reverse reactions will be occurring.
  • All three species will be present.

`=>D`

CHEMISTRY, M5 2022 HSC 36

Consider the equilibrium system shown.

 \( \ce{H2O(l) \rightleftharpoons H2O(g)} \)

In a laboratory at 23°C, a 100 mL sample of water is held in a beaker and another 100 mL sample is held in a sealed bottle.

Explain the differences in evaporation for these TWO samples. In your answer, consider changes in enthalpy and entropy for this process.   (4 marks)

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  • The evaporation of water absorbs energy, hence is an endothermic reaction and results in a positive change in enthalpy (`ΔH > 0`).
  • Additionally, the process converts a liquid into a gaseous state, and thus increases the disorder of the system, as a result, entropy increases (`ΔS > 0`).
  • Since enthalpy and entropy are both positive, according to `ΔG = ΔH − T ΔS`, the evaporation of water is spontaneous at high temperatures, ie when `ΔG < 0`. 

Beaker sample:

  • The evaporation of water in a beaker represents an open system, where vapour molecules are able to escape the system.
  • As a result, there would be a continuous disturbance to the equilibrium, and according to Le Chatelier’s Principle, the equilibrium will shift to counteract the change, and thus produce more gaseous water until there is no liquid water left.
  • Thus, dynamic equilibrium will not be established in a beaker. 

Sealed bottle sample:

  • On the other hand, the evaporation of water in a sealed bottle represents a closed system where the water vapour cannot escape from the system.
  • In this reaction liquid water would evaporate, shifting the equilibrium to the right until the rate of the forward reaction and the rate of the reverse reaction is equal.
  • At this point, there would be virtually no change in the concentration of liquid water and gaseous water, and thus dynamic equilibrium will be established.
Show Worked Solution
  • The evaporation of water absorbs energy, hence is an endothermic reaction and results in a positive change in enthalpy (`ΔH > 0`).
  • Additionally, the process converts a liquid into a gaseous state, and thus increases the disorder of the system, as a result, entropy increases (`ΔS > 0`).
  • Since enthalpy and entropy are both positive, according to `ΔG = ΔH − T ΔS`, the evaporation of water is spontaneous at high temperatures, ie when `ΔG < 0`. 

Beaker sample:

  • The evaporation of water in a beaker represents an open system, where vapour molecules are able to escape the system.
  • As a result, there would be a continuous disturbance to the equilibrium, and according to Le Chatelier’s Principle, the equilibrium will shift to counteract the change, and thus produce more gaseous water until there is no liquid water left.
  • Thus, dynamic equilibrium will not be established in a beaker. 

Sealed bottle sample:

  • On the other hand, the evaporation of water in a sealed bottle represents a closed system where the water vapour cannot escape from the system.
  • In this reaction liquid water would evaporate, shifting the equilibrium to the right until the rate of the forward reaction and the rate of the reverse reaction is equal.
  • At this point, there would be virtually no change in the concentration of liquid water and gaseous water, and thus dynamic equilibrium will be established.

♦ Mean mark 49%.