Unit 9.2: Absolute Entropy and Entropy Changes

Definition of Entropy

• Entropy (S): Measure of chaos or disorder in a system.

Laws of Thermodynamics

• Third Law of Thermodynamics: States that the entropy of a perfect ionic crystal at absolute zero (0 Kelvin) is zero.
  • At absolute zero, there is no disorder; the system is perfectly ordered.
  • Absolute zero has not yet been achieved, so all substances above this temperature have some level of disorder (entropy > 0).

Absolute Entropy Values

• Standard Absolute Entropy: These values are listed in textbooks, typically in an appendix (e.g., Appendix C).
  • They represent the disorder associated with each element or compound at a given temperature (above absolute zero).
  • Pure elements tend to have lower entropy values than large, complex molecules.
  • Larger molecules have more ways to arrange themselves, leading to higher entropy values.
• You are NOT required to memorize these values; they will be provided in exam questions if needed.

Calculating Entropy Changes

• Formula for Change in Entropy (𝚜):
  • $\Delta S = \sum S<em>{products} - \sum S</em>{reactants}$
  • Variables and terms explained:
  • $\Delta S$: change in entropy for a reaction.
  • $\sum$: summation symbol for adding up entropy values based on stoichiometry.
  • Coefficients from the balanced equation indicate how many moles to multiply the entropy values by.
  • This calculation is similar to what you have done in previous units for enthalpy changes.

Example Problem: Calculating Delta S

1. Given Reaction: Find the delta S using provided entropy values.
2. Balanced Equation:
   • Products: P b I2 (1) = 175
   • Two KNO3 (2) = $2 \times 150 = 300$
3. Calculate Total S for Products:
   • Product Entropy: 175 + 300 = 475
4. Calculate Reactants:
   • Leader Nitrate (1) = 250
   • Two KI (2 × 125) = 250
5. Total S of Reactants:
   • Total = 250 + 250 = 500
6. Change in Entropy:
   • $\Delta S = 475 - 500 = -25$ (Joules/Kelvin)
   • Correct answer: -25 Joules/Kelvin.

Prediction of the Sign of Delta S

• When asked to predict the sign based on the balanced equation:
  • Example given involves solid and gaseous reactants transforming into one solid product.
  • Predicted $\Delta S$: Negative, as the reaction becomes more organized.
• Justification:
  • Solids have lower entropy than gases, thus the process reduces overall disorder.

Additional Calculation Example

1. Products minus reactants calculation from values leads to:
   • Final Answer: $\Delta S = -181.1$ Joules per mole Kelvin.

Summary

• Understanding entropy and its calculation is essential in thermodynamics and helps in predicting the nature of chemical reactions. Proper use of tables and equations allows for effective evaluation of changes in entropy during reactions.