Phase changes also prefer dynamic equilibrium conditions in closed systems such as a sealed water bottle.

Question

H2O (l) + heat ⇌ H2O (g)


A bottle of water at room temperature will have a certain amount of water vapor present. If you leave the bottle in the sun, you will notice water droplets begin to form in the top on the plastic where it appears that only air should be. Use your knowledge of Le Chatelier's Principle to determine if the statements below are true or false.

(3 points)
True False
The system is in dynamic equilibrium as the added heat from the sun caused more water to form vapor then it had to condense back into water due to increased pressure in the bottle.

The system will favor a shift to the left or reverse if you toss the sealed bottle of water back into a cooler.

The system is in dynamic equilibrium with equal numbers of moles of liquid water and water vapor inside the bottle.

Answer 1

Let's analyze each statement using knowledge of Le Chatelier's Principle and phase changes.

1. Statement: The system is in dynamic equilibrium as the added heat from the sun caused more water to form vapor then it had to condense back into water due to increased pressure in the bottle.

   Assessment: False. While the system initially reaches a dynamic equilibrium, adding heat increases the vaporization rate (more liquid turns into gas). If the vaporization rate exceeds the condensation rate, the system is no longer in equilibrium. Higher temperatures typically lead to a higher vapor pressure (more water vapor), which can result in a shift toward the right, not dynamic equilibrium. Therefore, this statement is false.

2. Statement: The system will favor a shift to the left or reverse if you toss the sealed bottle of water back into a cooler.

   Assessment: True. According to Le Chatelier's Principle, if the temperature of the system decreases (as it would in a cooler environment), the system will favor the exothermic reaction to produce heat to counteract the change. In this case, the reverse reaction, which is condensation (liquid water from water vapor), will be favored, resulting in a shift to the left.

3. Statement: The system is in dynamic equilibrium with equal numbers of moles of liquid water and water vapor inside the bottle.

   Assessment: False. While a system at equilibrium does have constant rates of evaporation and condensation, it does not necessarily mean that the moles of liquid and vapor are equal. In many cases, especially at room temperature, there will generally be more water in the liquid phase than in the vapor phase due to the relatively low volume of vapor in comparison to the liquid. Therefore, this statement is false.

In summary:

1. False
2. True
3. False