Third Law of Thermodynamics

Thermodynamics

Presenter: Dr. Luis Bello

​•16.1 Spontaneity

•16.2 Entropy

•16.3 The Second and Third Laws of Thermodynamics

•16.4 Free Energy

​Thermodynamics is the branch of science that deals with the study of energy and its transformations.

It is concerned with the relationships between heat, work, energy, and temperature, and how they affect the behavior of matter.

The 3 main laws are:

​The first law, which states that energy cannot be created or destroyed, only transferred or converted from one form to another.

​The second law, which states that the total entropy (or disorder) of a system and its surroundings always increases over time, meaning that energy tends to flow from hotter to cooler objects and systems naturally move towards a state of maximum entropy.

​The third law, which states that as the temperature of a system approaches absolute zero, the entropy of the system approaches a constant minimum.

​•Will a reaction occur “naturally” at a given temperature and pressure, without the exertion of any outside force?

•In other words, is a reaction spontaneous?

•A process that does occur naturally under a specific set of conditions is called a spontaneous process.

•A process that does not occur naturally under a specific set of conditions is called nonspontaneous.

​•A spontaneous process will “just happen.”

•A spontaneous process occurs naturally at a given set of conditions, without outside forces.

•Example: A mixture of hydrogen and oxygen will react in the presence of a spark.

2H₂(g) + O₂(g) + spark → 2H₂O(g)
•It is important to not confuse spontaneous with fast.

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•If a reaction is spontaneous in one direction, it will be nonspontaneous in the reverse direction under the same conditions.

  2H2(g) + O2(g) → 2H2O(l)       Spontaneous

  2H2O(l) → 2H2(g) + O2(g)      Nonspontaneous

•A nonspontaneous reaction is still possible with the continual input of energy.

•Many spontaneous processes proceed with a decrease in energy.

•Recall that exothermic reactions also proceed with a decrease in energy.

•Spontaneous reactions are often exothermic, but not always.

2H₂(g) + O₂(g) + spark → 2H₂O(g)

•Which is spontaneous at room temperature and a pressure of 1 atm?

  H2O(s)  →  H2O(l)   DH = +6.01 kJ/mol

  H2O(l)  →  H2O(s)   DH = –6.01 kJ/mol

​•Two objects at different temperature are placed in contact.

•Heat spontaneously flows from the hotter object to the colder object.

•But overall, there is no change in energy.

​•Some factors other than energy are important to the spontaneity of a process.

•It appears that greater, more uniform dispersal of matter and energy can also be the driving force of a spontaneous process.

•This other factor is entropy.

entropy (S)

state function that is a measure of the matter and/or energy dispersal within a system, determined by the number of system microstates; often described as a measure of the disorder of the system

Entropy plays a crucial role in chemical reactions and phase transitions. For example, in chemical reactions, entropy changes can indicate whether a reaction is spontaneous (if the total entropy increases) or non-spontaneous (if the total entropy decreases).

Multiple Choice

Do molecules generally move faster at 45 C or 106 C?

• 45 C because molecules have more energy than in 106C

• 106 C because molecules have more energy than in 45C

• 106 C because molecules have less energy than in 45C

• 45C because molecules have less energy than in 106 C

Multiple Choice

DON'T RUSH, YOU HAVE TIME.

Water droplets condensing on the inside of a window on a cold day. Is this process exothermic or endothermic?

• Gases condense to form liquids. Gases are moving faster than liquids, the air around the window has a higher temperature/kinetic energy. Releasing energy to get water droplets, which is is Exothermic

• Gases condense to form liquids. Gases are moving faster than liquids, the air around the window has a higher temperature/kinetic energy. Releasing energy to get water droplets, which is is Endothermic

• Gases condense to form liquids. Gases particles are moving slower than liquids. Releasing energy to get water droplets, which is is Endothermic

• Gases condense to form liquids. Gases particles are moving slower than liquids. Releasing energy to get water droplets, which is is Exothermic

Multiple Choice

The diagram represents which type of reaction

• endothermic

• exothermic

Multiple Choice

Which best describes ENTROPY?

• Useful heat energy needed to drive an engine

• Heat that can be stored to be used at a later date

•   Random energy that escapes from the system

• A state of order in a physical system

Multiple Choice

Which state of H₂O  has the greatest entropy?

• Ice

• Water

• Vapor