Enthalpy and entropy PMT flashcards

• What is the definition of entropy?

  A measure of energy dispersal in a system
• What is the symbol for entropy?

  S
• Which state of matter is more disordered, solid or gas?

  Gas
• What is the unit of standard entropy?

  J K^-1 mol^-1
• How does temperature affect entropy?

  Higher temperature increases particle energy and randomness
• What happens to entropy when a solid ionic lattice is dissolved in solution?

  Entropy increases due to more disordered ions
• How does a change in the number of gas molecules affect entropy?

  More gas molecules increase entropy; fewer decrease it
• What is the equation used to calculate entropy change?

  ΔS = S_final - S_initial
• What is the Gibbs’ free energy equation?

  ΔG = ΔH - TΔS
• What do the symbols in the Gibbs’ free energy equation represent?

  • ΔG: Change in Gibbs free energy
  • ΔH: Change in enthalpy
  • T: Temperature in Kelvin
  • ΔS: Change in entropy
• What are the conditions for spontaneous change based on ΔH, ΔS, and ΔG?

  • ΔH negative, ΔS positive: Always negative ΔG (reaction feasible)
  • ΔH negative, ΔS negative: Negative ΔG at low temperatures (feasible)
  • ΔH positive, ΔS negative: Always positive ΔG (reaction never feasible)
  • ΔH positive, ΔS positive: Negative ΔG at high temperatures (feasible)
• For a reaction to occur spontaneously, must ΔG be positive or negative?

  Negative
• What are the limitations of predictions of feasibility made using ΔG?

  • High activation energy may prevent reaction
  • Reaction rate may be very slow
• Enthalpy of solution diagram

  1. Put the ionic lattice on the left dissociating to form the dissolved ions - this is the direct route
  2. Connect the ionic lattice to the gaseous ions by the reverse of lattice enthalpy - the breakdown of the lattice has the opposite enthalpy change to the formation of the lattice
  3. Connect the gaseous ions to the dissolved ions by the hydration enthalpies of each ion. This completes the indirect route
• When will a substance dissolve

  A substance will generally only dissolve if the energy released is roughly the same or greater than the energy taken in. So soluble substances tend to have exothermic enthalpies of solution