Chemistry, University Course Notes

Introduction to Thermodynamics

[latexpage]

Summary of various disciplines

  • Thermodynamics = study of transformation of energy on a macroscopic level (i.e., “large” scale)
  • Chemical dynamics (kinetics) = study of the rates and mechanisms of chenical reactions
    • Describes how fast things are changing and why
  • Quantum mechanics = mathematical description of matter, light, and energy at the microscopic (i.e., atomic/molecular) scale
  • Stastistical mechanics = framework for relating propeties of atoms and molecules to thier microscopic behavior

Applications/Examples of Thermodynamics

Note: These are from the first lecture of Chem 341

  • Powering a car
    • Which biofuel works best?
  • Optimization of chemical reactions
  • Calories in food
  • Protein folding

Basic Definitions

Note: These are from the second lecture of Chem 341

System = all materials involved in a process

  • Types of systems:
    • Isolated systems = cannot exchange energy or matter
    • Closed systems = cannot exchange matter (but can exchange energy)
    • Open systems = can exchange matter (and energy as well)

Surroundings = the rest of the universe

Examples:

  • The Earth: closed system
    • Surroundings with respect to the earth:
      • Energy input from the Sun
      • Energy output into space
  • Coffee
    • Consider two cases
      1. Coffee is contained in a styrofoam cup with a lid
        • Aside: the the styrofoam walls are adiabiotic → blocks heat transfer
      2. Coffee is contained in a copper cup with a lid
        • Aside: the copper walls are diathermal → allows heat transfer
    • In both cases, the coffee corresponds to a closed system whose surroundings is everything else in the universe
    • However, since the walls in the cooper cups are diathermal, the coffee will cool down more quickly than the coffee in the styrofoam cup

Intensive variables = variables that do not depend on the size of a system (e.g., temperature and pressure)

Extensive variables = variables that depend on the size of a system (e.g., heat and volume)

  • Dividing an extensive variable by mass or the number of moles in a system converts it to an intensive variable

Ideal gases = gases comprised of molecules that do not interact or take up space

  • Importantly, ideal gases don’t actually exist. They are just useful hypothetical concepts for understanding thermodynamics

Work = any quantity of energy that flows across the boundary between a system and its surroundings

  • Work is transitory ⇒ it only exists during processes of change
  • SI unit: Joules

Heat = any quantity of energy that flows between a system and its surroundings due to a temperature difference

  • Like work, heat is transitory
  • Exothermic = heat is transferred into the system
  • Endothermic = heat is transferred out of the system

Entropy = not quite a measure of randomness/disorder, but more of a meausre of the number of available states (called configurations) a system has

Equations

Ideal gas law: PV = nRT

  • P = pressure (SI units: 1 \mathrm{N} / \mathrm{m^2} = 1 \mathrm{Pa} )
  • V = volume (SI units: \mathrm{m^3} = 1 \mathrm{L}
  • n = number of moles
  • R = 1.834 \mathrm{J} / \mathrm{mol}    \mathrm{K} = ideal gas constant
  • T = temperature (SI unit: \mathrm{K})

Van der Waals equation (“real gas law”): 

(P + a \frac{n^2}{V^2})(V - nb) = nRT

  • Variables
    • a = strength of interactions between molecules/atoms on a given parameter
    • b = size of the molecule/atom
  • This is a much more realistic equation for describing basic interactions between molecules
    • Accounts for both molecule interactions and the size molecules

Laws of Thermodynamics

  • 0th Law of Thermodynamics = two systems that are separately in thermal equilibrium with a third system are in thermal equilibrium with eachother
    • Let T_i be the temperature of system i
      • If T_1 = T_3 and T_2 = T_3, then T_1 = T_2
  • 1st Law of Thermodyamics (Law of Conservation of Energy) = in an isolated system, energy cannot be created or destroyed
  • 2nd Law of Thermodynamics = for any spontaneous process to occur, the change in entropy \nabla S must be greater than or equal to zero
    • Note: spontaneous chemical reactions can occur that results in a negative change in entropy (as long as there is a sufficient enough change in enthalpy)
  • 3rd Law of Thermodynamics = the entropy of a system approaches a constant value as T \rightarrow 0K
    • Thus, perfectly crystalline solid only has one state at T = 0K ⇒ S = k \ln{1} = 0
      • This holds if the perfect crystal only has one state with minimum energy
    • Allows for calculation of absolute entropies for elements and compounds at any value of T

Sources:

Fall 2014 Chem 341 Lectures (taught by Dr. Joshua Patterson – Chemistry Department at Christopher Newport University)

Physical Chemistry, 3rd Edition, Thomas Engel and Philip Reid Pearson Education Inc. (2013)

https://courses.lumenlearning.com/boundless-chemistry/chapter/the-laws-of-thermodynamics/

 

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.