Passage 9: Thermodynamics

In a series of experiments that investigated the thermodynamics of phase changes,

the journey of a water molecule as it transitions from ice to steam and back to

liquid water was documented. It began with a block of ice at -10°C, which was

gradually heated with a controlled heat source. The ice melted, boiled into steam,

and finally condensed back into water. Temperature changes were meticulously

recorded, and heat input at each stage was determined via sensors on the heat

source.

The previous results applied to an open system (the water was open to the

atmosphere). The team also monitored how energy input values changed when a

closed system was used instead. They found that the amount of energy required

to boil the water was dramatically affected by this adjustment, but not the energy

required to melt the ice.

As observed in the passage, it takes a significant amount of energy to convert

water from the solid phase to the gas phase. How is it possible, then, that water

left at room temperature readily evaporates over time?

A) Temperature fluctuations at room temperature provide sufficient energy for

evaporation

B) Water molecules have a lower energy barrier to overcome for evaporation at

room temperature

C) The large increase in entropy during evaporation makes the process

thermodynamically favorable even without large inputs of energy

D) Air pressure at room temperature is lower than the vapor pressure of water,

facilitating evaporation

Click to reveal answer

Correct answer: C. Thermodynamic favorability is a function

of enthalpy change and entropy change. A process is always spontaneous if

enthalpy change is negative and entropy change is positive via the relationship

ΔG = ΔH - TΔS (with a negative ΔG corresponding with a spontaneous process).

As we can see in the experiment, the process of turning ice into steam is an

endothermic process with a positive ΔH, so the process is not readily spontaneous

at room temperature. This implicit input of energy is necessary because of the

intermolecular forces of attraction between water molecules. At room temperature,

water does not boil for this reason, but keep in mind that temperature is simply an

average of the kinetic energy of individual particles. At any given moment in time,

a water molecule at the surface of a body of water may possess enough energy

to overcome IMFs and enter into the gas phase. This, combined with the fact that

entropy change between the liquid and gas phase is very positive, enables the

phenomenon of evaporation at room temperature.

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Day 55 MCAT Practice Question