22.4 - entropy

• Entropy


  The entropy of a given system is the number of possible arrangements of the particles and their energy in a given system. A measure of how disordered or chaotic a system is.
• What happens to entropy when a system becomes more disordered?


  The entropy increases.
• What does an increase in entropy mean in terms of stability?


  An increase in energy means that the system becomes more energetically stable.
• What can entropy be used to explain?


  Things that occur naturally:

  - a gas spreading through a room.

  - heat from a fire spreading through a room.

  - ice melting in a hot room.
• Symbol of entropy


  S
• What is the natural tendency of energy?


  For it to spread out rather than stay concentrated in one place.
• What does greater entropy mean?


  Th greater the entropy, the greater the dispersal of energy, the greater the disorder.
• Units of entropy


  J K⁻¹ mol⁻¹
• What does a greater entropy value mean?


  The greater the entropy, the greater that energy is spread out per kelvin per mole.
• General entropies in terms of states


  - Solids have the smallest entropies.

  - Liquids have greater entropies.

  - Gases have greatest entropies.
• What is the entropy of all substances at 0 Kelvin?


  Zero
• What is the entropy of all substances above 0 Kelvin?


  Positive entropy?
• Why do all substances have a positive entropy above 0 Kelvin?


  Because energy becomes dispersed amongst the particles.
• What happens if a system changes to become more random (chaotic)?


  Energy can be spread out more - there will be a positive entropy change, ∆S.
• What happens if a system changes to become less random (chaotic)?


  Energy becomes more concentrated - the entropy change, ∆S, will be negative.
• How can you predict whether entropy change in an equation is positive or negative?


  By comparing the physical states and amount of gas molecules on either side of an equation.
• How does entropy change with changes in state?


  It increases during changes of state that give a more random arrangement of particles.

  solid → liquid → gas
• What happens when any substance changes state from solid to liquid to gas?


  Its entropy increases.
• Why does entropy increase when an substance changes state from solid to liquid to gas?


  - Melting and boiling increase the randomness of particles.

  - Energy is spread out more and ∆S is positive.
• Why do reactions that produce gases result in an increase in entropy?


  - Production of a gas increases the disorder of particles.

  - Energy is spread out more and ∆S is positive.
• Standard entropy of a substance


  The entropy of one mole of a substance under standard conditions.
• Standard conditions


  100kPa and 298K.
• Standard entropy units


  J K⁻¹ mol⁻¹
• Standard entropy sign


  Positive
• Equation for standard entropy change


  ∆S° = ∑S° (products) - ∑S°(reactants)