Entropy

• Explain why most reactions that occur freely are exothermic

  • Products are more thermodynamically stable than reactants
  • Therefore spontaneous (feasible) [Happens on its own]
• Explain why some spontaneous reactions do not occur

  • Rate of reaction too slow
  • Rate of reaction too high
• Entropy: a measure of the degree of disorder in a system

  • Tells us the number of ways particles can be arranged and their energies can be shared between particles
• Solid = ordered

  Liquid = some disorder

  Gas = chaotic
• Co2 has a higher entropy than CH4 because:

  • Larger Mr
  • More electrons
  • Thus having more energy to distribute
• Why does graphite have a higher entropy than diamond ?

  • Layered structure
  • Delocalised electrons
  • Diamond has a rigid structure
• Why is the entropy 0 when the temperature is 0K ?

  • At 0K particles are stationary
  • Particles are in perfect order
• Explain why the first part of the entropy graph is a line that slops up from the origin

  • As temperature increases, particles start to move/vibrate
  • Disorder increases
• Explain why L2 is longer than L1

  • L2 corresponds to boiling and L1 corresponds to melting
  • Bigger change in disorder for L2 compared with L1
• What do entropy change values tell us ?

  • Positive values - increase in disorder
  • Negative values - decrease in disorder
• Factors affecting entropy

  • Dissolving
  • Dissolving a solid increases entropy
• Factors affecting entropy

  • Number of moles
  • More particles mean more entropy
  • More particles the more they and their energies can be arranged
• Factors affecting entropy

  • Physical state
  • Disorder increases from liquid to gas
• Predict the entropy change for the reaction below:

  CH4(g) + 2o2(g) --> Co2 + 2H2o(g)

  • Entropy change is close to zero as there are equal number of gaseous molecules in the reactant and product side
  • No change in disorder
• When will a substance have an entropy value of 0 ?

  0K
• Describe the movement of particles when a substance has an entropy value of 0 ?

  Particles are stationary
• Describe the arrangement of particles when a substance has an entropy value of 0 ?

  State of perfect order
• • The entropy of an element in its standard state is not 0
  • This is because at 298K the particles still have energy to distribute
• Entropy measured in:

  J K-1 mol-1
• Spontaneous - will proceed on its own without any external influence
• Non-spontaneous - will not proceed on its own without any external influence
• Why is enthalpy unreliable in predicting the feasibility of reactions ?

  • Some endothermic reactions are spontaneous hence feasible
  • e.g. Ice melting, water evaporating
• Equation to calculate entropy change

  A) sum of products - sum of reactants