thermodynamics

• Thermodynamics deals with the interaction of heat and work associated with various physical and chemical transformations
• Forms of energy dealt in chemical thermodynamics are electrical energy and heat energy
• Work involved in chemical reactions is either expansion or compression of gases
• All chemical reactions take place with either absorption or liberation of heat
• Understanding thermodynamics helps chemists study the conditions under which reactions can take place spontaneously
• System is a specified portion of the universe under thermodynamic study, separated from the rest of the universe with a definite boundary surface
• Example: Water in a beaker
• Types of systems:
• Open system:

  • Can exchange both matter and energy with the surroundings
  • Example: Hot water in an open beaker
• Closed system:

  • Cannot exchange matter but can exchange energy in the form of heat, work, etc.
  • Example: Hot water in equilibrium with its vapor in a closed beaker
• Isolated system:

  • Cannot exchange either matter or energy with the surroundings
  • Example: Water in contact with its vapor in a closed insulated vessel
• State of a system indicates the position of the system and is defined by properties like temperature, volume, pressure, etc.
• Standard state of a system is at 298 K and under 1 bar pressure
• State functions or thermodynamic functions are variables like temperature, pressure, volume, etc. that define the state of a system
• Process is an operation by which a system changes from one equilibrium state to another equilibrium state
• Types of processes:
• Isothermal process:

  • Temperature of the system remains constant
  • Example: Experiment with a reacting mixture in a thermostat
• Adiabatic process:

  • No heat is exchanged between the system and the surroundings
  • Example: Liquefaction of a gas by Joule-Thomson effect
• Differences between isothermal and adiabatic process:
• Isothermal process:

  • Heat is exchanged with the surroundings
  • Temperature remains constant
• Adiabatic process:

  • No heat exchange with the surroundings
  • Temperature does not remain constant
• Reversible process is carried out infinitesimally slowly and can be reversed at any stage
• Irreversible process takes place in one step and cannot be reversed at any stage
• Differences between reversible and irreversible process:
• Reversible process involves infinite steps and requires infinite time
• Irreversible process involves single or a few steps and is a sudden process
• Cyclic process is where the system returns to its initial state after a series of changes
• Isobaric process takes place at constant pressure
• Isochoric process takes place at constant volume
• Intensive property does not depend on the quantity of the substance present in the system
• Extensive property depends on the quantity of the substance present in the system
• Internal energy is the sum of different energies possessed by a system and is a state function
• First law of thermodynamics states that energy can neither be created nor destroyed, but can be converted from one form into another
• Mathematically represented as ΔU = q + w
• Where ΔU is the change in internal energy, q is the amount of energy absorbed or liberated, and w is the work done by the system or on the system
• Derivation of the first law of thermodynamics involves the internal energy of the system at initial and final states, energy absorbed, and work done
• Derivation of the First Law of Thermodynamics:

  • Internal energy of the system at initial state: U1
  • Internal energy of the system at final state: U2
  • System absorbs q kJ of energy
  • Work done on the system to attain the final state: w
  • Total energy of the system in the initial state = U1
  • Total energy of the system in the final state = U1 + q + w = U2
  • ΔU = q + w
  • dU = dq + dw
• Sign conventions used in Thermodynamics:

  • w = +ve if work is done on the system
  • w = -ve if work is done by the system
  • q = +ve if heat is absorbed by the system
  • q = -ve if heat is liberated from the system
• Applications of the First Law of Thermodynamics:

  • Case-1: Isothermal process, dT = 0, ΔU = 0, w = -q
  • Case-2: Adiabatic process, q = 0, w = ΔU
  • Case-3: No work done by the system, w = 0, ΔU = q
  • Case-4: Work done by the system for gas expansion, w = -PΔV, q = ΔU + PΔV
• Work (w):

  • Transferring energy is called work
  • Unit: Joule (J)
  • Mechanical work = force x displacement
  • Electrical work = Charge x potential difference