Problem 40 - Chemistry: The Central Science

Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) Bromine vapor turns to bromine liquid as it is cooled. (b) Crystals of iodine disappear from an evaporating dish as they stand in a fume hood. (c) Rubbing alcohol in an open container slowly disappears. (d) Molten lava from a volcano turns into solid rock.

Step-by-Step Solution

Answer

(a) Phase transition: Condensation (exothermic) (b) Phase transition: Sublimation (endothermic) (c) Phase transition: Evaporation (endothermic) (d) Phase transition: Freezing (exothermic)

Step 1: (a) Phase transition of bromine vapor to liquid

In this situation, bromine vapor turns into liquid as it is cooled. This is an example of condensation (gas to liquid). Now we need to determine if this is an exothermic or endothermic process. When a substance condenses, the particles are coming closer together, releasing energy. Thus, this process is an exothermic process. (a) Phase transition: Condensation (exothermic)

Step 2: (b) Phase transition of iodine crystals

In this situation, iodine crystals disappear from an evaporating dish as they stand in a fume hood. This is an example of sublimation (solid to gas). Now we need to determine if this is an exothermic or endothermic process. When a substance sublimates, the particles are moving farther apart, and energy is absorbed. Thus, this process is an endothermic process. (b) Phase transition: Sublimation (endothermic)

Step 3: (c) Phase transition of rubbing alcohol

In this situation, rubbing alcohol in an open container slowly disappears. This is an example of evaporation (liquid to gas). Now we need to determine if this is an exothermic or endothermic process. When a substance evaporates, the particles are moving farther apart, and energy is absorbed. Thus, this process is an endothermic process. (c) Phase transition: Evaporation (endothermic)

Step 4: (d) Phase transition of molten lava

In this situation, molten lava from a volcano turns into solid rock. This is an example of freezing (liquid to solid). Now we need to determine if this is an exothermic or endothermic process. When a substance freezes, the particles are coming closer together, releasing energy. Thus, this process is an exothermic process. (d) Phase transition: Freezing (exothermic)

Key Concepts

Condensation

Imagine a warm shower on a cold day, and the steam hits the cool surface of the bathroom mirror. This is an everyday instance of condensation, a phase transition where gas transforms into a liquid. On a microscopic level, gas particles, which are initially far apart and move rapidly, slow down and come closer together as they lose energy. This released energy warms up the surrounding environment, which is why condensation is considered an exothermic process.

Sublimation

Sublimation might sound mysterious, but it's a simple concept you might have observed without realizing. Have you ever noticed how dry ice seems to vanish without a trace? That's sublimation, where a solid turns directly into a gas without passing through the liquid state. It requires energy to break the bonds holding the solid's molecules together, which is absorbed from the surroundings, making sublimation an endothermic process. The iodine crystals in the exercise demonstrate sublimation by transitioning from solid directly to gas.

Evaporation

A puddle drying up after a rainy day is a common sight and a classic example of evaporation. It's a process where the molecules at the surface of a liquid absorb enough energy to break away and become a gas. This phase transition is individual, occurring one molecule at a time and it's an endothermic process since energy is absorbed to allow for this particle freedom. Rubbing alcohol evaporating from a container is another typical example of evaporation in our everyday experiences.

Freezing

During winter, water turns to ice as it freezes. Freezing is the transition from liquid to solid, and it results in the molecules slowing down and arranging themselves into a fixed structure. This process is exothermic, as it releases heat to the surroundings. So when molten lava cools and solidifies into rock, as mentioned in the exercise, it's releasing heat into the cooler air or sea, which is this release of energy that is characteristic of an exothermic process.

Exothermic and Endothermic Processes

In chemistry, changes in phase are often accompanied by heat exchange. Exothermic processes, like condensation and freezing, release heat into the surroundings, which can be felt as warmth. On the other hand, endothermic processes absorb energy, causing the environment to feel cooler, as seen with sublimation and evaporation. Understanding these energy changes is crucial not just in laboratory settings but also for explaining numerous natural phenomena and everyday occurrences.