Important Thermodynamic Terminology, Processes and Functions – MCAT Content

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Thermodynamics is the study of heat, energy, and work. In chemistry, it helps us understand how energy changes during chemical reactions. This guide covers key terms, processes, and functions in thermodynamics.

I. Introduction to Thermodynamics

Thermodynamics explains how heat, energy, and work move and change in chemical reactions. Here are some basic terms:

A. System and Surroundings

A system is the part of the universe we are studying. Everything outside the system is the surroundings. For example, in a reaction inside a beaker, the system is the chemicals in the beaker. The surroundings are the beaker and the air around it.

B. Types of Systems

There are three types of systems:

  • Open System: Can exchange both energy and matter with its surroundings. Example: A boiling pot of water.
  • Closed System: It can exchange energy but does not matter with its surroundings. Example: A sealed flask.
  • Isolated System: Cannot exchange energy or matter with its surroundings. Example: A thermos flask.

C. State Functions

State functions depend only on the state of the system, not how it got there. Examples include temperature, pressure, and volume. These are important because they help us understand the energy changes in reactions.

II. Key Thermodynamic Terms and Equations

Understanding these terms and equations helps in studying thermodynamics:

A. Internal Energy (U)

Internal energy is the total energy within a system. It includes all kinetic (motion) and potential (stored) energy of the particles. For example, heating water in a pot increases its internal energy.

ฮ”U=q+w

where q is heat added to the system, and w is work done on the system.

B. Enthalpy (H)

Enthalpy is the heat content of a system at constant pressure. When water boils, it absorbs heat, changing its enthalpy.

ฮ”H=ฮ”U+Pฮ”V

where P is pressure, and ฮ”V is the change in volume.

C. Entropy (S)

Entropy measures the disorder or randomness in a system. Higher entropy means more disorder. For example, when ice melts into water, the entropy increases because the water molecules move more freely.

Where qแตฃโ‚‘แตฅ is the reversible heat, and T is temperature in Kelvin.
Entropy

D. Gibbs Free Energy (G)

Gibbs Free Energy determines if a reaction is spontaneous (happens on its own). A negative ฮ”G means the reaction is spontaneous. For example, the rusting of iron is spontaneous because ฮ”G is negative.

ฮ”G=ฮ”Hโˆ’Tฮ”S

III. Thermodynamic Processes

Different processes affect the system in various ways:

A. Isothermal Process

An isothermal process occurs at a constant temperature. In this process, the internal energy change (ฮ”U) is zero.

B. Adiabatic Process

An adiabatic process occurs without heat exchange. All energy changes come from work done on or by the system.

C. Isochoric Process

An isochoric process occurs at constant volume. No work is done because the volume does not change. Any heat added changes the internal energy.

D. Isobaric Process

An isobaric process occurs at constant pressure. The heat added or removed changes the enthalpy.

IV. Important Thermodynamic Functions

These functions are key to understanding how energy changes:

A. Heat Capacity (C)

Heat capacity is the amount of heat needed to change the temperature of a substance by 1 degree Celsius. It is defined as:

Heat Capacity

Where q is heat added, and ฮ”T is the change in temperature.

B. Standard Enthalpy of Formation (ฮ”Hfยฐ)

The standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their standard states.

C. Standard Entropy (Sยฐ)

Standard entropy is the absolute entropy of a substance at 1 atm pressure and 298 K.

D. Standard Gibbs Free Energy Change (ฮ”Gยฐ)

The standard Gibbs Free Energy change is the change in free energy for a reaction under standard conditions (1 atm, 298 K).

V. Chemical Equations and Examples

Including equations helps illustrate thermodynamic concepts:

A. Combustion of Methane

The combustion of methane (CHโ‚„) is an exothermic reaction:

CHโ‚„+2Oโ‚‚โ†’COโ‚‚+2Hโ‚‚O

ฮ”H for this reaction is negative, indicating it releases heat. This is why methane is used as fuel.

B. Formation of Water

The formation of water from hydrogen and oxygen is another example:

2Hโ‚‚+Oโ‚‚โ†’2Hโ‚‚Oโ‚‚

ฮ”H is also negative. This reaction is important in respiration and combustion.

C. Dissolution of Sodium Chloride

The dissolution of sodium chloride (NaCl) in water is an endothermic process:

NaCl(s)โ†’Na+(aq)+Clโˆ’(aq)

ฮ”H is positive, meaning it absorbs heat from the surroundings.

VI. Bridge/Overlap

Thermodynamics connects with many other areas in chemistry:

A. Chemical Kinetics

Understanding energy changes helps explain reaction rates. Higher temperatures often increase reaction rates by providing more energy. This explains why food cooks faster at higher temperatures.

B. Physical Chemistry

Thermodynamics is crucial in physical chemistry, explaining how energy changes in different states of matter. For example, it helps understand phase transitions like melting and boiling.

C. Environmental Chemistry

Thermodynamic principles help explain how pollutants behave in the environment. For example, they predict how gases will dissolve in oceans, affecting marine life and the atmosphere.

D. Biochemistry

In biochemistry, thermodynamics helps us understand metabolic pathways. For example, the breakdown of glucose in cellular respiration releases energy that cells use to perform work.

VII. Wrap-Up and Key Terms

Understanding thermodynamics involves mastering several key terms and concepts. Let's review:

Key Terms

  • System: Part of the universe being studied.
  • Surroundings: Everything outside the system.
  • Internal Energy (U): Total energy within a system.
  • Enthalpy (H): Heat content at constant pressure.
  • Entropy (S): Measure of disorder.
  • Gibbs Free Energy (G): Determines if a reaction is spontaneous.

VIII. Practice Questions

Sample Practice Question 1

What is the change in internal energy (ฮ”U) if a system absorbs 50 J of heat and does 20 J of work on the surroundings?

A. 30 J
B. 70 J
C. -30 J
D. -70 J

Click to reveal answer

Ans. A

ฮ”U = q + w. Here, q = 50 J (heat absorbed) and w = -20 J (work done by the system). So, ฮ”U = 50 J + (-20 J) = 30 J.

Sample Practice Question 2

In which type of process does the volume of the system remain constant?

A. Isothermal
B. Adiabatic
C. Isochoric
D. Isobaric

Click to reveal answer

Ans. C

An isochoric process occurs at constant volume, meaning no work is done because the volume does not change.

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