Thermodynamics (Chemical)

• System: Any specified portion of the universe under thermodynamic study
• Surroundings: Anything outside the system
• Boundary: Imaginary line separating system from surroundings
• Types of Systems:

  • Open System: Exchanges heat energy and matter with surroundings
  • Closed System: Exchanges only energy, not matter with surroundings
  • Isolated System: Neither exchanges energy nor matter with surroundings
• State of a System: Described by observable properties like temperature, pressure, volume
• Standard State of the system: Most stable state at 298K and 1 bar
• State Functions: Value depends only on initial and final state, not path taken
• Isothermal Process: Carried out at constant temperature
• Isobaric Process: Carried out at constant pressure
• Isochoric Process: Carried out at constant volume
• Adiabatic Process: Carried out at constant heat
• Reversible Process:

  • Ideal process
  • Takes infinite time
  • Work obtained is maximum
• Irreversible Process:

  • Real process
  • Takes finite time
  • Work is not maximum
• Extensive Property: Depends on the amount of the system

  • Examples: Internal energy, Enthalpy, entropy, free energy, Mass, Volume
• Intensive Property: Does not depend on the amount of the system

  • Examples: Temperature, Density, refractive index
• Sign Convention for Work and Heat
• Mechanical work done by a gas: w = -P∆V
• Internal Energy: Sum of different kinds of energy possessed by the system
• Zeroth Law of Thermodynamics: Objects in thermal equilibrium with a third object are in thermal equilibrium with each other
• First Law of Thermodynamics: Energy can be converted from one form to another
• Expression for work done during isothermal expansion of an ideal gas: W = -2.303nRT log(V2/V1)
• Free Expansion: Expansion of a gas in vacuum
• Isothermal and free expansion of an ideal gas
• Expression for work done during adiabatic expansion of an ideal gas: W = nCv(T2 - T1)
• Enthalpy or Heat Content (H): Total heat content of a system at constant pressure
• Relation between Cp and Cv for an ideal gas: Cp = Cv + R
• Determination of ∆U using Bomb calorimeter
• Heat evolved per mole of the substance = molar heat capacity x (final temperature - initial temperature)
• Thermochemical equations are balanced chemical equations that indicate the physical states of reactants and products, as well as the amount of heat absorbed or liberated during a reaction
• Exothermic reactions release heat, while endothermic reactions absorb heat
• Differences between exothermic reactions and endothermic reactions:

  • Exothermic reactions liberate heat during the reaction, with a negative change in enthalpy (∆H)
  • Endothermic reactions absorb heat during the reaction, with a positive change in enthalpy (∆H)
• Enthalpy of reaction changes when the number of moles of reactants indicated in the balanced chemical equation have completely reacted under given conditions
• Factors affecting the heat or enthalpy of a reaction:

  1. Amount of reactants

  2. Physical state of reactants and products

  3. Temperature at which the reaction occurs

  4. Allotropic forms of reactants
• Enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its constituent elements
• Standard enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its constituent elements in their standard state, at one bar pressure, at a specified temperature
• Enthalpy of combustion is the change in enthalpy when one mole of a substance is completely burnt in excess air or oxygen
• Enthalpy of solution is the change in enthalpy when one mole of a substance is dissolved in excess water, such that further dilution does not produce any more enthalpy change
• Lattice enthalpy is the change in enthalpy when one mole of an ionic compound dissociates into its constituent ions in the gaseous state
• Enthalpy of transition is the change in enthalpy when one mole of an element undergoes a change from one allotropic form to another, or when one mole of a compound undergoes a polymorphic change at the transition temperature
• Enthalpy of fusion is the change in enthalpy when one mole of a solid substance is converted into its liquid state at its melting point