Entropy

• Spontaneous processes are those that can proceed without any outside intervention.
• Spontaneous direction is downhill proceeds unaided.
• Non-spontaneous direction is uphill proceeds only if work is done.
• Processes that are spontaneous in one direction are non-spontaneous in the reverse direction.
• In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states but exactly reversing the process.
• Changes are infinitesimally small in a reversible process
• Irreversible processes cannot be undone by exactly reversing the change to the system.
• All spontaneous processes are irreversible.
• All real processes are irreversible.
• Identify whether the process is spontaneous or non-spontaneous

  Silverwares tarnish when exposed to air

  Spontaneous
• Identify whether the process is spontaneous or non-spontaneous

  Table salt dissolves into sodium and chlorine ions in water

  Spontaneous
• Identify whether the process is spontaneous or non-spontaneous

  Water molecules form from hydrogen and oxygen gases

  Spontaneous
• Identify whether the process is spontaneous or non-spontaneousIdentify whether the process is spontaneous or non-spontaneous

  A spoon dipped in boiling water gets hot

  Spontaneous
• Identify whether the process is spontaneous or non-spontaneous

  Wood burns to ash after being set on fire

  Spontaneous
• Entropy is a measure of molecular randomness. A solid with a rigid structure has low entropy. Gases whose molecules are in constant random motion in various directions are described to have very high entropy.
• Like total energy, E, and enthalpy, H, entropy is a state function.

  Therefore,

  ∆S=Sfinal-Sinitial
• Factors that influence entropy

  Temperature

  Number of particles
• Rising temperature, increases the kinetic energy thus, increasing entropy of the system.
• Increase number of particles leads to one disorder in the system and thus increased entropy.
• Entropy on the Molecular Scale

  Implications:

  More particles -> more states-> more entropy

  Higher T -> more energy states -> more entropy

  Less structure (gas vs solid) -> more states -> more entropy
• Molecules exhibit several types of motion

  Translational

  Vibrational

  Rotational
• Movement of the entire molecule from one place to another

  Translational
• Periodic motion of atoms within a molecule

  Vibrational
• Rotation of the molecule on about an axis or rotation about ó bonds (sigma bonds)

  Rotational
• Identify whether the process is spontaneous or non-spontaneous

  Water decomposes into hydrogen and oxygen gases with application of electricity

  non-spontaneous
• In general, entropy increases when

  Gases are formed from liquids and solids

  Liquids or solutions are formed from solids

  The number of gas molecules increase

  The number of moles increases
• The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved Ssolid<Sliquid<Sgas


  Positional entropy
• Predict the sign of the change in entropy for the following processes.

  Freezing water

  Entropy is negative
• Predict the sign of the change in entropy for the following processes.

  Heating liquid water to it's boiling point

  Entropy is positive
• Predict the sign of the change in entropy for the following processes.

  Formation of nitrosyl chloride (NOCl) from nitric oxide and chloride gases

  Entropy is negative
• Predict the sign of the change in entropy for the following processes.

  Sublimation of nephthalene

  Positive
• Predict the sign of the change in entropy for the following processes.

  Melting of lead metal at room temperature

  
  

  Positive
• Predict the sign of the change in entropy for the following processes.

  Decomposition of carbonic acid into water and carbon dioxide.

  Positive
• Predict the sign of the change in entropy for the following processes.

  Formation of dinitrogen tetroxide from nitrogen dioxide

  Negative
• Standard molar entropy (S°) of a substance is the entropy of one mole of a substance at the standard conditions (1 atm and 25°C)
• The second law of thermodynamics: The entropy of the universe does not change for reversible processes and increases for spontaneous processes.
• Reversible (ideal):

  ∆Suniv=∆Ssys+∆Ssurr=0
• Irreversible (real, spontaneous): ∆Suniv=∆Ssys+∆Ssurr>0
• Non-spontaneous ∆Suniv=∆Ssys+∆Ssurr<0
• The second law of thermodynamics: Explains the relations between entropy change and spontaneity of a process. It states that for any spontaneous process, there is a net increase in the total entropy of the system and it's surroundings.