Question

Which of the following phase changes is not spontaneous at room temperature? Assume only the first phase is initially present. A) H2O(g) → H2O(1) B) H2O(s) → H2O(g) C) H2O(s) → H2O(1) D) H2O(g) → H2O(s) E) They are all spontaneous.

Answer 1

At room temperature, sublimation of ice (H2O(s) → H2O(g)) is not a spontaneous phase change because it requires additional energy to occur. Other phase changes like condensation and melting are spontaneous under these conditions.

Step-by-step explanation:

The student's question pertains to the spontaneity of phase changes at room temperature. A spontaneous phase change occurs without needing to continually add energy once initiated and is driven by a decrease in Gibbs free energy. At room temperature, the following phase changes can happen spontaneously:

• H2O(g) → H2O(l): Condensation, where water vapor becomes liquid.
• H2O(s) → H2O(l): Melting, where ice turns into liquid water.

However, the phase change:

• H2O(s) → H2O(g): Sublimation, where ice turns directly into water vapour.

is not spontaneous at room temperature because ice needs additional energy to change directly into water vapour without first becoming liquid.

Answer 2

The phase change H2O(g) → H2O(s) is not spontaneous at room temperature. While melting and condensation of water are spontaneous, deposition typically requires specific conditions and is not usually spontaneous at room temperatures.

Step-by-step explanation:

The phase change that is not spontaneous at room temperature, assuming only the first phase is initially present, is H2O(g) → H2O(s). While conversion of water from gas to liquid (H2O(g) → H2O(l)) and from solid to liquid (H2O(s) → H2O(l)) are spontaneous at room temperature, the direct deposition of water vapor into a solid state is typically not spontaneous without the presence of specific conditions, such as high humidity and subfreezing temperatures.

Phase changes like the melting of ice (H2O(s) → H2O(l)) or the condensation of water vapor (H2O(g) → H2O(l)) are driven by Gibbs energy changes and involve entropy considerations. At room temperature, the spontaneous phase changes are those that occur without energy input: gas condenses to liquid and solid melts to liquid. However, the reverse process of gas turning into solid, known as deposition, does not happen spontaneously at room temperature conditions because it typically requires energy removal and specific conditions that promote this change.