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Consider the equilibrium system shown - HSC - SSCE Chemistry - Question 36 - 2022 - Paper 1
Question 36
Consider the equilibrium system shown. H₂O(l) ⇌ 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... show full transcript
Worked Solution & Example Answer:Consider the equilibrium system shown - HSC - SSCE Chemistry - Question 36 - 2022 - Paper 1
Step 1
Explain the differences in evaporation for these TWO samples
Answer
The evaporation of water from the beaker occurs freely into the atmosphere, while the water in the sealed bottle experiences restricted evaporation. This leads to distinct differences in the evaporation rates between the two systems due to differing physical constraints. In the open system (beaker), water molecules can escape into the air freely, while in the closed system (sealed bottle), the vapor pressure builds up, reaching an equilibrium that limits further evaporation.
Additionally, the forward reaction (evaporation) is endothermic, and in both cases, the temperature of 23°C influences the rate of evaporation. The system in the sealed bottle may reach a state where the rate of evaporation equals the rate of condensation, leading to an equilibrium state.
Step 2
Consider changes in enthalpy and entropy
Answer
For the beaker, evaporation increases the system's entropy as water transitions from a liquid state, which is more ordered, to a gaseous state, which is more disordered. The enthalpy increases as energy is absorbed during the phase change.
In contrast, the sealed bottle reaches a dynamic equilibrium where the entropy changes are less pronounced over time. The enthalpy remains constant as the system stabilizes, maintaining the vapor pressure without allowing additional evaporation unless the temperature is increased. Both systems exemplify the principles of thermodynamics, indicating that spontaneity relates to changes in both enthalpy (H) and entropy (S) such that H > T*S for reactions that are endothermic, with the system's directionality influenced by external conditions.